OBSERVATIONS ON GROWTH IN EUCIDARIS TRIBULOIDES (LAMARCK), WITH SPECIAL REFERENCE TO THE ORIGIN OF THE ORAL PRIMARY SPINES1 BERTHA M. CUTRESS institute of Marine Biology University of Puerto Rico, Mayagiiez
ABSTRACT Observations of 7 specimens of Eucidaris tribuloides maintained in an aquarium during one year, along with study of a large series of preserved specimens, revealed that oral primary spines in this species develop by regeneration, almost always from a break through the proximal portion of the collar of an old ambital primary spine. Replacement of aboral by oral primaries was found to follow a constant sequence in the living specimens: first in IA 3a or 3b, then in the compar- able plate in the same column of IA 2 and 4, then in IA Land 5. The same sequence was found in the development of new interambulacral plates aborally and resorption of plates orally. In preserved specimens, this se- quence was the most common, although a 3-2, 4-1, 5 sequence was occas- ionally found. The axis of symmetry is discussed in relation to each sequence. Information is also given on rates of growth of the corona and growth and regeneration of spines in the species.
INTRODUCTION In the course of a study of Recent and fossil cidaroids, still in progress, series of several Recent species were examined to determine the changes with age in the primary spines. It was observed that although the adults had moderately to extremely specialized spines (oral primaries) on inter- ambulacral plates at the oral edge of the corona, very young specimens lacked them. No direct reference was found in the literature to this lack of oral primaries in young cidaroids and, therefore, none as to how or when these spines arise. There is one quite incidental reference: Mortensen (1928: 397, 404) noted that in Eucidaris tribuloides of 4 mm horizontal diameter (h.d.), the primaries on plates nearest the peristome appeared to cor- respond, respectively, to spines number 4-5 and 5-6 in E. metularia. It is not clear to what size E. metularia he referred: if he meant the very young specimens (0.5-3.5 mm) described in his earlier paper (1927: 378), the number 4-6 spines would be non-flattened embryonic spines which he believed were lost, along with their plates, soon after metamorphosis;
IContribution from the Institute of Marine Biology, University of Puerto Rico. Mayagiiez. Work done at the U.S. National Museum, Washington, D.C. and supported in part by grants from the Jersey Production Research Company, Tulsa, Oklahoma and the National Science Foundation (G-15902). 798 Bulletin of Marine Science [15(4) if he referred to larger specimens of E. metularia, the 4-6 spines would be ambitals. In either case, he apparently was not referring to oral primaries. However, Mortensen seems not to have attached any special significance to his observations. During growth of echinoids, new plates are added to the corona at the aboral margin. At the same time, in cidaroids and certain other regular echinoids, as noted by Loven (1874, 1892), Jackson (1912) and others, interambulacral plates are resorbed at the oral margin of the corona. In line with this, cidaroid oral primary spines could form only by either a continual transformation of ambital-type primaries to the oral type or a loss of ambital primaries and replacement, through regeneration, by oral primaries. Close study of several series of preserved cidaroids having extremely specialized oral primaries (e.g., Acanthocidaris hastigera) did not reveal conclusively the mode of formation of these spines. There certainly was no evidence of transformation of ambital to oral spines; only in large specimens which already had their full complement of orals were spines found which were of a transitional type. On the other hand, the number of regenerating spines in these series seemed far too small to account for the increase in number of oral-type primaries. To resolve the problem, living specimens of Eucidaris tribuloides were obtained. By recording the type of primary spine on each interambulacral plate when the specimens were first received, and by subsequent careful checking of the changes in the spines on the same plates at intervals over a year's time while the specimens were maintained in an aquarium, it was possible to determine the origin of the oral primaries. In the living specimens, it was found that oral-type spines develop by regeneration, almost always from a break through the proximal portion of the collar of an old ambital spine. No evidence whatever was found for any other mode of origin of the orals. The oral spines were found to develop in a definite sequence in the five interambulacra, appearing first in IA 3, then in IA 2, and 4 then IA 1 and 5. This sequence. which is the same as that observed for development of new aboral in- terambulacral plates in the living specimens, is incompatible with the axis of symmetry proposed by Loven (1874) for the cidaroids. In addition to details of these observations and discussion pertaining to them, information is given here on the increase in horizontal diameter, change in number of interambulacral plates with growth, rate of develop- ment of spines on new plates at the aboral margin of the corona, and rate of regeneration of the various types of spines. Also noted and discussed is a marked change in growth pattern in aboral most spines of several specimens, which occurred after transfer from the original tank to a larger one durin!! the sixth month. Data from the living specimens have been checked and augmented with 1965] Cutress: Growth in Eucidaris tribuloides 799 observations on preserved specimens in collections of the U.S. National Museum, Washington, D.C. and the Museum of Comparative Zoology, Cambridge, Massachusetts.
ACKNOWLEDGEMENTS This work was undertaken during the course of another study which was initiated and supported in early phases by the Jersey Production Research Company, Tulsa, Oklahoma, and later by the National Science Foundation. I am grateful to both organizations for their financial help. I am indebted to Dr. John E. Randall, Institute of Marine Biology, Universi~y of Puerto Rico, Mayagtiez, Puerto Rico, and to Dr. Frederick M. Bayer, Institute of Marine Science, University of Miami, Florida, for the living specimens used in this investigation. Sincere gratitude is expressed also to officials of the Smithsonian In- stitution, Washington, D. C. for administering the two grants and for providing working space, access to collections and library, and other invaluable aid. Dr. Porter M. Kier and Mr. Charles E. Cutress of the U.S. National Museum contributed much to the work with suggestions and encouragement; they and Dr. David L. Pawson, also of the U.S. National Museum, critically read the manuscript and made valuable sug- gestions. Special thanks are due Dr. Elisabeth Deichmann, Dr. H. B. Fell, and others at the Museum of Comparative Zoology, Cambridge, Massachusetts, for courtesies extended during visits there.
PREVIOUS WORK Although much has been written on early development of echinoids, there is far less on growth after metamorphosis and very little of this concerns cidaroids. Prouho (1887: 248-264) studied development of test and spines in Dorocidaris papillata [= Cidaris cidaris]. Loven (1892: 5-10, 17-24) followed the development of test and spines in Goniocidaris [= Austro- cidaris] cana/iculata specimens of 1.45-1.7 mm h.d., exclusive of spines, and in 26-62 mm Cidaris papillata [= cidaris]. A. Agassiz (1904: 1-34; Pi. 5, 7, 11-13) described and illustrated structures of the test in several ,growth stages (none smaller than 8 mm) in Dorocidaris [= Hesperocidar- is] panamensis, Porocidaris [= Histocidaris] cobosi, Porocidaris r =Aporo- cidaris] milleri, and Centrocidaris doederleini. He also considered (op. cit.: 9, 15,21; Pi. 4, Fig. 1-2; Pi. 12, Fig. 6-7; Pi. 13, Fig. 3) aspects of development and resorption of interambulacral plates. Jackson (1912: 77; Pi. 3, Fig. 1-2) described and figured beginning stages in development of new aboral spines in Eucidaris tribuloides and 800 Bulletin of Marine Science [15(4) (op. cit.: 51, 62, etc.) discussed various aspects of test development. Mor- tensen (1927: 372-380) traced early postmetamorphic development of test and spines in Eucidaris metularia, using specimens of 0.5-3.5 mm h.d. The post-embryonic development of several brood-protecting cidaroids has been described briefly: in Austrocidaris canaliculata by A. Agassiz (1881: 45, PI. 2), in Rhynchocidaris triplopora and Notocidaris gaus- sensis by Mortensen (1909: 10-11; PI. 11, etc.), and in Goniocidaris umbraculum and Aporocidaris milleri by Mortensen (1927: 383-387). Since these studies were based on preserved material, they include no information on rates of development or sequence of appearance of plates and spines. Literature on regeneration in cidaroids is as scant as that for other phases of development. Borig (1933: 640) discussed regeneration in Eucidaris clavata, but it was of an anomalous, branched spine.
MATERIAL AND METHODS Source and Maintenance of Specimens.- Four living Eucidaris tribu- loides, collected at Crashboat Basin, Aguadilla, Puerto Rico, were re- ceived 8 October 1963. Three additional specimens were obtained from Virginia Key, Miami, Florida on 27 October 1963. For the first six months, the seven specimens were maintained in a la-gallon aquarium containing artificial sea water made of commercial Neptune salts and ordinary tap water. It was then necessary to transfer them to a 25-gallon aquarium containing sea water made of Rilla salts and tap water. Several small blocks of "living" coral rock, obtained from a Washington aquarium supply house, also were placed in the tanks in an attempt to achieve balanced aquaria. Water in each aquarium was circulated through a glass-wool filter in an attached small overflow tank. The only aeration in the lO-gaIlon aquarium was from the splash of the circulating water. In the large tank, additional air was pumped in through two small porous stones. Both aquaria were covered, but there was some evaporation. Tap water was added at intervals of several weeks to maintain a specific gravity of 1.028. The temperature in the rooms in which the aquaria were maintained varied from about 70°-80°F (21°-27°C). The small aquarium was in front of an uncovered window, exposed to strong light and afternoon sun. Algae grew luxuriantly on the surface of the coral rocks, sides of the tank, and spines of the echinoids. The large tank was in front of a window covered by a Venetian blind which was usually partially closed. No direct sunlight ever reached this aquarium. Much of the algae died soon after the transfer. The larger masses were taken from the tank as they loosened; the rest was caught in the filter which was replaced whenever it appeared clogged. 1965] Cutress: Growth in Eucidaris tribuloides 801 The echinoids at first accepted small pieces of frozen shrimp but after the first month grazed instead on the coral rock. Large patches of the rock were not only scoured clean of algae but quite distinctly scored by the echinoids. Fecal pellets during the first month contained remnants of shrimp and algae and afterward contained small chunks of the white calcareous rock along with occasional strands of algae. After 10 months, the large aquarium was replaced by the original small tank, but the water, number of coral rocks, position of the tank, light and temperature were unchanged. Method of Observation.-When the echinoids were first received, each was tagged with a different colored thread tied to one spine at the ambitus in IA32 The threads were replaced periodically. As each specimen was first examined under a dissecting microscope, a chart was started which was similar to Tables 1-3 except that all five interambulacra were included. The type of primary spine on each inter- ambulacral plate was thus carefully recorded. In nine subsequent obser- vation periods during the following year, addition or loss of plates, change in the type of spine on each plate, and changes in dimensions of develop- ing and regenerating spines were recorded for each specimen. The 10 observation periods were as follows: (1) 9-10 October 1963;
Loven's axis 3 3 a a IV b L 111 Iii iV
b j; a ,t 2 2 b it a ~ L
V II II \/ ,. ;1 b b J b :-, a it b
FIGURE 1. Aboral (left) and oral (right) surfaces of a test of Eucidaris tribu- {aides Lamarck, with areas numbered according to Loven; a and b columns of ambulacra not indicated. USNM 27843, Jamaica; X 1.3. 2Throughout this paper, "interambulacrum" and "interambulacra," when used with a specific interambulacral number, will be abbreviated to TA. 802 Bulletin of Marine Science [15(4)
FIGURE 2. Primary spines of Eucidaris tribuioides Lamarck. a-c, oral primaries from aquarium specimen 4 (Puerto Rico): a, from IA 2b, number 2 position; b, oralmost from IA 2b; c, oralmost from IA 2a. d-i, ambital primaries: d-e, from USNM 21202, Puerto Rico; I-g, from USNM 32652, Bahamas; h-i, from USNM 14309, Florida. j-i, Styli/era-infested ambitals from USNM 10734, ALBATROSS station 2407. m-n, aboralmost ambitals from aquarium specimen 4 (Puerto Rico): m, from IA Sa; n, from IA 4a; both taken from specimen on 15 October 1964. o-s, aboral most ambitals from aquarium specimen 2 (Puerto Rico): p, from IA 2b; q, from IA 3b; both taken 15 October 1964; 0, from IA 5b; r, from IA Ib; s, from IA 4b; all three taken June "1965. a-c, X 3.8; d-s, X 1.5. 1965] eutress: Growth in Eucidaris tribuloides 803 (2) 6-26 November 1963; (3) 16-30 December 1963; (4) 20-23 January 1964;(5) 8-9 February 1964; (6) 1-3 March 1964; (7) 30 March 1964; (8) 1-2 June 1964; (9) 8-10 September 1964; and (10) 15 October 1964. Periods 1-4, 5-7, 8, and 9-10 correspond closely to three month intervals. Because of space limitations, only five of the 10 observations were included in Tables 1-3: the initial observation (1), the end of the first quarter (4), end of the second quarter (7), end of the third quarter (8), and end of the fourth quarter (10). Also to conserve space, only IA 3, 4 and 5 were included in these tables since IA 2 was very similar to IA 4 and IA 1 to 5. Designation of Plates.-The method of numbering the interambulacra for these observations was that proposed by Loven (1874). When the echinoid is viewed from the aboral surface, the interambulacrum which contains the madreporite is numbered 2. Proceeding counterlockwise, the adjoining ambulacrum is III, the next interambulacrum 3, etc. (Fig. 1). In Eucidaris, as in most cidaroids, each ambulacrum and each inter- ambulacrum comprises two columns of plates. These are designated a and b, proceeding counterclockwise (e.g., 2a, 2b, IlIa, IIIb, 3a, 3b, etc.) according to the Loven system. Designation of Types of Spines on Interambulacral Plates.- The terms used here for the spines are generally those used by Mortensen (1928). In cidaroids, the primary spines which are most typical of the species are found on interambulacral plates at or near the ambitus. In Eucidaris tri- bu/oides, these "ambital" primaries are typically (Fig. 2d-g) cylindrical, stubby, and robust (ca. 25-30 mm long and 3-4 mm wide in specimens of 30-42 mm h.d.) although they may be longer (Fig. 2h-i) or more slender. The collar is shorter (typically ca. 0.5 mm) than in ambital primaries of most other cidaroid genera. The shaft is ornamented with moderately low, rounded or peaked nodules in 14-20, but mostly 18, more or less regular longitudinal series and by stout hairs which in old spines have anastomosed into a coarse meshwork. The tip of a typical ambital primary is broadly obtuse, encircled at the edge by elongate, ridge-like nodules which give it a fluted appearance, and with a central rosette-like prominence on the end. Both the rosette and fluted edges often have been chipped and worn away in old spines. On newly formed interambulacral plates near the aboral edge of the corona, primary spines which will eventually be of ambital type are stilI in developmental stages (FiQ:.3h-p) and so have not acquired all the charac- teristics of spines at the ambitus. These are designated below as "developing ambital" spines. In large -specimens, the primaries on interambulacral olates below the ambitus, on that part of the corona which narrows and curves under the 804 Bulletin of Marine Science [15(4)
FIGURE 3. Regenerative and developmental stages of primary spines of Eucidaris tribuloides Lamarck. a-d, stages of regeneration of oral from broken ambital primary: a, break in proximal portion of collar just capped over; b, truncate cap; c, cone; d, spine-shaped with beginning cortex (nodules). e-g. stages of re- generation of ambital from broken ambital primary: e, truncate cap; f. blunt cone; g, spine-shaped but no cortex. hop, stages of development of new ambital primaries at aboral edge of corona: h, round cap; i, truncate cap; j, low cone; k, high cone; l spine-shaped but no cortex; moo, spines with developing nodules but no hair coat, all from the same specimen, from IA 4b, 2b and 3b, respec- tively; p, spine with developing hair coat. All from specimens of USNM 2 I202, Puerto Rico. X 3.3. 1965] Cutress: Growth in Eucidaris tribuloides 805 echinoid toward the peristome, are more or less different from those at the ambitus (Fig. 2a-c). In some cidaroid genera (e.g., Acanthocidaris), these "oral" primaries are strikingly different, being much flattened, very decidedly curved, long-collared, and marginally serrate. In Eucidaris, the distinction between oral and ambital spines is not as great. The orals have nodular ornamentation, at least on their upper (aboral) surfaces, more or less similar to that of the ambital primaries; they are not curved and have no distinct marginal serrations. The collars of the primaries on plates nearest the peristome (hereafter oralmost) are usually longer than those of ambitals (Fig. 2c), but orals nearer the ambitus (Fig. 2a) have collars almost the same length as those of the ambitals. The primary spines at the "interface" between oral and ambital areas, in fact, are quite similar to ambital-type spines. Mortensen (1928: 403) noted the oral primaries pass "very gradually into the ambital spines." However, although the distinction is not great, oral-type spines in Eucidaris can be recognized by their flatness, comparatively smooth lower (oral) surface, and less well developed (as compared to ambitals) nodular ornamentation. RESULTS Origin and Increase in Number of Oral-type Primary Spines.-The seven living Eucidaris tribuloides which were kept under observation for a year were about one-fourth to one-third the maximum size (60 mm) recorded for the species and already had oral primary spines on one to three plates per interambulacral column (Tables 1-3). . During the year of observation, numerous primary spines were lost. Occasionally the tubercle was entirely denuded, but usually the old spine had broken through the proximal portion of the collar. In the few cases of loss of an entire spine, a new spine regenerated from the denuded tubercle. In most cases, regeneration proceeded from the break in the collar (as in Fig. 3a-d or e-g). Almost all the loss and replacement of spines occurred on plates in positions 1-3 from the oral edge of the corona. Of the 30 plates per specimen in these positions (three each in the 10 IA columns), about one-fourth to two-thirds were lost or broken during the year (17 each in specimens 1 and 2, 8 in specimen 3, 15 in specimen 4, 19 in specimen 5, 9 in specimen 6, and 20 in specimen 7) .. Few spines were lost in other positions on the corona (five in specimen 1, four in specimen 2 plus three removed for photographing, none in specimen 3, none in specimen 4 except for three removed for photograph- ing, one in specimen 5, two in specimen 6, and three in specimen 7). All of these were in positions 1-3 from the aboral margin of the corona. The extent of loss of a primary spine did not seem to be related to position on the corona. In the living specimens, denuding of the tubercle 806 Bulletin of Marine Science (15(4)
<: 00
8 8 < 0 80 8 ":' 00 Ine::• 1.0
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N < 0 0 .( Cl
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l"'l l"'lE' < 0 0 <: -z0 1965] Cutress: Growth in Eucidaris tribuloides 807 8 8 «008 < < 08 0 \0 t- 8 8 < < < 0808 t- V) V) 8 t-8 «008 < < 680 .q- !xv) N ..t: < o 8 t-E < < 0 08 o'8 o .q- ~": .q- r-;-8 «0800 o o ~V) N I < < 0 o .....Q., «< 00 N -, < < 0 o P.. o o ..... o< < < < < < < o 0 < < o o N .-.•..• <~o:l -'-' E .-'" 808 Bulletin of Marine Science [15(4) C'l C'l r-E p.,. ~•... ~ < < < 0 0 0 ~ < E< ....•... 0 o Cl:q o:l ::l c:r ..d •.. C'l, C'lI ' .....<" zci 1965] Cutress: Growth in Eucidaris tribuloides 809 -<: 000 -<: -<: 0 0 -<: 0 0 0 -<: -<: 0 0 0 8 ...., 8 I -<: 0 0 0 -<: -<: ll< "'! 0 0 ..... \0 8 8 < 0 8 0 0 -<: < 08 0 0 \0 00 ...., I o .....ll< \08 1rl8 -<: 680 o -<: 68 0 0 ~\O ~Vl < -<: 0 o < < 0 0 •....• N N r -<: < 0 o r:i.. -<: 6 0 ll< ..... ~ ..... o o. -< -<: 0 o 810 Bulletin of Marine Science [15(4) N N N, - , I d.. 0 ll..- •... ~ -< < < < ll.. -< < < 0 0 ~•... ~ 0 0: - ;l cr ..c .....• 8 .....• I 8 8 , ~ <:l -< II 1965) Cutress: Growth in Eucidaris tribuloides 811 o E E <.,., 0 0 E E «EOOE o co trl E -< < :l E 0 r-- o t-E -< 6 E 0 cGr-- -< -< 0 0 -..t: -< o Q o o I o 0 ~- 0 o o ....• 0 I o 0 -< .....~ 812 Bulletin of Marine Science [15(4) could usually be traced to prior loosening of the spine during examination; this occurred infrequently. Although regenerated spines on plates above the ambitus were always of the same type as the lost or broken spines which they replaced (all ambitals), this was not true on plates near the peristome. On number 1-3 plates, counting from the oral edge of the corona, or sometimes 4 (if there was a plate partway through the dissolution process in position 1), the regenerated primaries were almost always of oral type even if the original spine had been of ambital type. The only exceptions (two) in the seven living specimens were at the "interface" between oral and ambital primaries, a lost ambital in these cases being replaced first by another ambital then, after another loss, by an oral spine. In specimen 1, for instance, there was an ambital spine on plate 3 from the peristome in IA 5a until period 5, a regenerating ambital in periods 6 and 7, a completed new ambital 8 mm long in period 8, a regenerating oral 5 mm long in period 9, and a complete or nearly complete oral 5.5 mm long in period 10. The same occurred in specimen 5 on the number 3 plate in IA Ib between periods 5 and 10. Had specimen 7 (Table 3) been maintained longer than the year of observation, it would have been expected that eventually the regenerated ambital-type spines present on several of the number 3 plates (in IA 3a, 3b, 40, 4b at the end of the third quarter and in IA 5b at the end of the fourth quarter) likewise would have been lost and orals regenerated in their place. In the living Eucidaris, oral-type spines formed only by regeneration after loss (almost always by breakage) of ambital-type spines. There was no evidence whatever of any other method of origin. There was no gradual decrease in dimensions of any of the ambitals, hence no resorptive transformation from ambitals to orals. The only changes in any of the spines, aside from damage, were due to growth. The change from ambital to oral primaries usually occurred in a definite sequence in comparable plates of the five interambulacra. This sequence was the same as that elaborated below for the appearance of new plates abo rally and disappearance of plates orally. Oral spines usually appeared first in IA 3, then in IA 2 and 4, and finally in IA 1 and 5. Even in accidental loss of an ambital primary, if the plate were "ready" (next in sequence), the regenerated spine would be an oral. However, this sequence of regenerative replacement of ambital by oral spines was not quite as firm as that of appearance and disappearance of plates. Since the range in size of the living specimens represented only a small segment of the full growth span of the species, and since these specimens were in a very artificial environment during the year of observation, a series of 32 preserved specimens was examined to determine accurately the amount and pattern of regeneration. These specimens, in collections at the 1965] Cutress: Growth in Eucidaris tribuloides 813 U.S. National Museum and the Museum of Comparative Zoology, ranged from 4-50 mm in h.d. and were from 19 different collection sites. The total number of spine-bearing interambulacral plates on each of these specimens was counted. This total excluded new aboral plates on which primary spines had not yet begun to develop and oralmost plates .-----~ on plates in number 1 ana 2 positions •...... _...~ on plates in number 3 and 4 I>0sitions 80 - on plates in other positions 70 , , ~60 , Z , Q. , l/) ," > II: « ::!: 50 1 pair 3 pairs II: 2 pairs 0.. oral o~al oral spines spones spines CI z ~ 40 T ,T II: w " Z " T " ;: W " CI ': .' w " :: II: " ,'" , ::> 30 , , : .. ,': l/) , ,I w , CI , , r····~ « ,, I- , Z , ~ 20 , II: , w , } 0.. " " :1 10 - I \ '. " :: ,I "~ \ ,, ~····\\·..1./ ,, .. o 4 8 12 16 36 40 44 48 52 HORIZONTAL DIAMETER OF CORONA IN MM FIGURE 4. Percentages of regenerating primary spines on interambulacral plates in various positions on the corona in preserved series of Eucidaris tribuloides Lamarck. 814 Bulletin of Marine Science [15(4) which had lost their spines and were undergoing dissolution, but included denuded plates on which regeneration of lost spines was still possible (there were very few of these). The plates with regenerating primaries were then counted on each specimen, and this count was divided into three categories: (1) the plates in number 1 and 2 position, counting from the oral margin of the corona, (2) those in number 3 and 4 positions, and (3) those in all other positions. Treating each specimen separately, the counts in each category (plate position) were divided by the total number of spine-bearing plates to determine the percentage regeneration for the various positions. These percentages were then plotted against h.d. of the specimens (Fig. 4). If there were two or more specimens of the same h.d. but with different percentages, an average was graphed. As can be seen from Figure 4, until the specimens were 16 mm in h.d., at which stage a full cycle of two pairs of oral primaries per interambula- crum was complete, regenerative activity was limited almost entirely to plates in number 1 and 2 positions. Although it was not possible to show this on the graph, most of this activity was in number 2 position. Between 16 and 26 mm, when a full cycle of three pairs of orals was first noted, the regeneration was mostly in positions 3 and 4, particularly position 3. Regeneration on plates in other positions was zero in specimens smaller than 20 mm; in larger specimens, it was distinctly less than that in positions 3 and 4 until well after the time when three pairs of oral primaries (the maximum for the species) were found in every interambulacrum, and when there was a general increase in regenerative activity over the whole corona. Two peaks of regeneration were noted btween 6 and 16 mm h.d. and two again between 16 and 26 mm. Although it is tempting to view these peaks as activity associated with the appearance of orals in first one interambulacral column and then the other during completion of a full cycle of oral~, it would be necessary to have more specimens to determine just how constant these four peaks might be. Increase in Horizontal Diameter of the Corona.- The seven living Euci- daris grew slowly during the year of observation. Tabulated below are the sizes in mm during the 10 observation periods which were listed under "methods of observation." The last figure after a colon is the total increase (in mm) for each specimen. The first four are the specimens from Puerto Rico, the other three from Florida. HORIZONTAL DIAMETER (MM) Spec. ]: ]4.5 15.0 15.0 15.0 15.0 ]5.0 15.0 16.0 17.0 17.8 3.3 " 2: 16.0 16.0 16.0 16.0 17.0 17.0 17.0 17.0 18.3 18.3 2.3 " 3: J8.018.5 19.5 20.0 20.5 21.0 21.0 21.0 21.0 21.5 3.5 " 4: 18.5 18.5 19.0 19.5 20.5 20.5 21.0 21.5 22.0 22.0 3.5 " 5: 15.0 16.0 16.5 17.0 17.5 17.5 18.5 18.5 19.5 4.5 " 6: 19.0 19.0 19.5 19.5 ]9.5 19.5 20.0 20.0 20.3 1.3 " 7: 19.5 20.5 21.0 21.0 21.0 21.0 21.5 21.5 22.5 3.5 1965] Cutress: Growth in Eucidaris tribuloides 815 There was virtually no difference in average monthly increase for the specimens from each locality: 0.27 mm/month for the Puerto Rico specimens, '0.26 mm/month for those from Florida. There is little correlation, on the basis of the year's data, between per cent increase in h.d. and original size of the specimens, although the small- est specimens from each locality had a slightly greater per cent increase. Change in Number of Interambulacral Plates with Growth.-The living specimens had six to seven interambulacral plates per column at the begin- ning of the year and six to eight at the end (Table 4). While plates were added to the aboral margin, others were being resorbed orally. For instance, specimen 2 added plates in the b columns of alI five interambulacra between the third and fourth observation intervals. However, by the fifth, oral plates had already been lost in the same b columns of three of the interambulacra and of the remaining two by the sixth interval, bringing the total plates back to seven. In the series of 32 preserved specimens of E. tribuloides, the interam- bulacral plates increased in number from four or five per column at 4 mm h.d. to 11 or 12 at 50 mm (Fig. 5). It is estimated that during this growth span, about four to six pairs of oral plates would have disappeared per interambulacrum, one pair between 4 and 12 mm, another by the time the specimens were 18-20 mm, one to two pairs between 20 and 30 mm, and one or more between 30 and 50 mm. The increase in plates follows a "stair-step" pattern when graphed against h.d. (Fig. 5), but a line drawn through the high points in the steps for this series is almost straight. However, Mortensen (1928: 402) has given values for some E. tribuloides specimens (e.g., 9/10 IA plates for 43 mm h.d.; 8/9 for 44 mm; 9/10 for 47 mm; 11/12 for 53 mm; and 9/10 for 57 mm) which, if entered on the graph, would give a more uneven progression. Sequence of Development and Resorption of Interambulacral Plates.- One of the most interesting phenomena observed in the living E. tribuloides was the almost constant sequence in development of new interambulacral plates (and their associated spines) at the aboral margin of the corona and resorption of plates orally. A new aboral plate always developed first in IA 3 (Tables 1-4). The next plates arose in comparable positions in IA 2 and 4, not always simultaneously. Although a plate sometimes began in IA 4 ahead of IA 2 (e.g., Table 4, specimen 4 in the seventh observation period), usually IA 2 was slightly ahead (e.g., Table 4, specimen 5, o~servation period 4). To complete the cycle, new aboral plates appeared next in IA 1 and 5. Usually the plate in IA 5 lagged somewhat behind that in IA 1. It might appear from Table 4 that in specimen 7 new plates appeared simultaneously in IA 2a and 3a (period 4). However, it must be remem- 816 Bulletin of Marine Science [15(4) ------1- o 0 c c o 0000 00000 00000 0000000000 Ol) I:: ";::~ Q I:: 8 ~ '0 u -( ...... •.. •.. -0 oocoe 00000 00 0,0 00 00 00000 o 000 000 IX> 00 00 00 IX> 818 Bulletin of Marine Science [15(4) bered that the periods were usually at least a month apart. Comparison of the size of the plates and spines in such cases showed the developing plate in IA 3 always to be distinctly more advanced than that in IA 2. In the preserved specimens examined, the 3, 2-4, 1-5 sequence was the most common. However, occasionally the new plate in IA 2 was nearly equal to or, more rarely, larger than that in IA 3. In these cases, the new plate in IA 1 was nearly equal to or larger than that in IA 4. Without exception, new aboral plates in the living Euddaris developed in the same column of each of the five interambulacra until a full cycle of new plates was complete. That is, if a new aboral plate developed in IA 3a, it would be followed by new plates in IA 2a, 4a, la, and 5a to complete the cycle. The next cycle would develop in the b columns of all five interambulacra. The same was found in all preserved Eucidaris examined. In preserved material, care must be taken to determine which are com- parable plates, allowing for the fact that plates may not have begun to form in some interambulacra. As would be expected, resorption of plates at the oral margin of the corona followed the same sequence as aboral development. For example, in specimen 5 (Table 4), a new plate had disappeared completely in IA 3a z :E ....•::::l 0 Co) ....•10 ..J< ::::l , llJ , , :E r -~-----..•- ...4.- "" < ' a:: , ' w , I- I l ~ ---~---.-..--.- ..•. a:: w 0.. f/) w I- :3 5 0.. a:: w llJ :E ::::l Z 0 4 8 12 16 20 24 28 32 36 40 44 48 52 HORIZONTAL DIAMETER OF CORONA IN MM FIGURE 5. Increase in number of plates per interambulacral column in preserved series of Eucidaris tribuloides Lamarck. 1965] Cutress: Growth in Eucidaris tribuloides 819 by the third period, in IA 2a and 4a by the fourth, in IA la by the fifth, and in IA 5a by the ninth period. In the same specimen, another cycle of resorption began in the tenth period with the disappearance of a new plate in IA 3b. Rate of Development of Primary Spines on New Aboral Plates.-In the aquarium-maintained Eucidaris, it required almost a year for spines to develop on new aboral plates. Typical was the spine on the aboralmost (initially) plate in IA 5b of specimen 3 (Table 2) which developed through the following stages (as illustrated in Fig. 3a-p): a small, low round cap on the developing tubercle when first observed (period 1); a truncate cap 1.9 mm high after three months; a smooth cone 7.5 mm high after six months; a spine 22 mm long on which nodules were just beginning to show at nine months (and after a change from the small to large aquarium); and an ambital primary 22.5 mm long and quite finished in appearance at the end of the year. In most of the specimens, the developing aboral spines which were beyond the cone stage but had not yet begun to form nodules showed a very pronounced increase in growth rate after six months, when the echinoids were changed from the small to large tank. Those which already had started to form nodules did not show as much, if any, increase. This susceptibility of some but not all the developing spines to greatly accelerated growth was especially notable in specimen 2 (Table 1 and Fig. 20-s) and rather strikingly illustrated the sequence, among the five interambulacra, in the development of aboral spines and plates. In this specimen, the aboralmost spine in IA 5b grew at an unusually rapid rate, lengthening from 7 to 24 mm after the change in tanks. It had been the least developed of the spines in this position in the five interambulacra. In IA Ib, tbe corresponding spine increased substantially but not as much, from 7 to 11 mm. By contrast, the spine in IA 4b, although also 7 mm long, was a little further along in development (baving nodules) before the change, and grew only to 8 mm afterward. The spine in IA 2b changed from 8 to 9 mm. The comparable spine in IA 3b, which bad been 9 mm long and even more advanced in development before tbe change in tanks, had not increased in length at all by the end of the third quarter, although it did grow to 11 mm by the end of the year. To a slight extent in the above mentioned spine in IA 5b of specimen 2 and to a greater extent in some of the aboral most spines in a columns of specimen 4, a decrease in diameter of new growth accompanied the very sudden increase in length which occurred between the second and third quarters. This produced a constriction in the shaft, giving some of the spines an appearance (Fig. 2n) very similar to that of some spines (Fig. 2j) described by Rathbun (1886: 610) and cited by Mortensen (1928: 405) as being infested with a gall-forming Stylifera. There was no infesta- 820 Bulletin of Marine Science [15(4) tion of the spines of the living Eucidaris; a change in growth pattern had produced the same result. The wide proximal portion of the shaft in these spines was exactly the same length as the spines had been at the end of the first 6 months, just before the change in tanks. The spines in IA 2b, 3b, and 4b of specimen 2 (Fig. 2p, q, s), which remained stubby after the change in tanks, had had, from early stages, a rather triangular tip, one side being slightly excavate. Although these spines did not lengthen, they did widen and formed a slight to marked distal cleft on the side of the tip which had been excavate. They came to resemble to some extent the most severely affected of the gall-bearing spines (Fig. 2k-l) described by Rathbun. After the change in tank, there was some acceleration in growth of aboral spines which were in very early stages of development, but this did not seem to be as great as in the spines which had begun to assume a spine shape. The actual increase was more difficult to assess due to the difficulty in getting accurate measurements of spines in low, early stages of development in the living specimens. Rate of Regeneration of Spines on Various Parts of the Corona.-On plates near the oral margin of the corona, regenerative replacement of ambital by oral primary spines proceeded rapidly, a new oral usually being formed in four to five months, although the time ranged from three to 8 months. For example, in specimen 7 (Table 3), on the third plate up from the oral margin in IA 4b, an ambital spine had broken through the proximal portion of the collar sometime between the fourth and fifth observation periods. At the fifth, the break was merely capped over (as in Fig. 3a). A little less than one month later (period 6), a low cone 2.6 mm high was found on the break (slightly more developed than Fig. 3b). Not quite two months after the break (period 7), the new spine was 6.5 mm long with faint nodules but no cortex (as in Fig. 3d). After a little over four months (period 8), the spine was 8 mm long and appeared to be completed. Specimen 2 afforded an example of regeneration of an oral after complete loss of an ambital. On the third plate up from the oral margin in IA 2, the denuded tubercle had only just been covered with epithelium at the first observation period. (It is certain from the type of spines still present on comparable plates in the other four interambulacra that the lost spine had been an ambital.) By the second period, a month later, an oral spine 5 mm long had formed but the cortex was not yet present. After a little over two months (period 3), the spine was 7.5 mm long with a cortex. At three months (period 4), a complete appearing oral primary 8 mm long was present. Oral spines on the plates at the oral margin of the corona sometimes were loosened so much during examination that they were completely lost before the next examination period. In IA 4b of specimen 2 (Table 1), 1965] Cutress: Growth in Eucidaris tribuloides 821 one such primary, lost after the third period, had been replaced by another oral spine 2.2 mm long, with enough cortex to distinguish a collar, after one month (period 4); it was 3.5 mm long with cortex quite well differentiated after not quite two months (period 5) and 4 mm long and complete three months after the loss (period 7). Occasionally, on plates near the "interface" between oral and ambita1 primaries (usually in plate position 3 or 4 from the oral margin of the corona), another ambital would regenerate from a break in the proximal collar of the old ambital primary, developing at about the same rate as oral primaries. For example, on the third plate up in LA 3a of specimen 7 (Table 3), a truncate cap 1.1 mm high (like Fig. 3e) had formed on such a break between period 5 and 6. Not quite two months after the spine had broken (period 7), there was a smooth cone (like Fig. 3f) 5.5 mm long. After four months (period 8), a new ambital 11 mm long, its maximum length, was present (like Fig. 3p). Examples of regeneration of ambital spines from broken ambitals on plates well above the ambitus were rare in the 7 living specimens. In IA 2b of specimen 2 at the initial observation, the sixth plate up from the oral margin (second plate down aborally) bore an old ambita1 broken just above the collar. After one month (period 2), this plate had a cone 6 mm high which appeared to have developed from the tubercle; thus, the remnant of the old spine had been cast off or otherwise completely lost. After two months (period 3), there was an ambita1 spine 8.5 mm long on which nodules were beginning to develop (a little more advanced than Fig. 3g). This spine was 10.5 mm long with quite well developed nodules and cortex after three months (period 4), and 11.5 mm long and complete in appearance after four months (period 5). After five months (period 7), the spine was 12 mm long and was becoming covered with algae, indicating that its epithelium was probably already disappearing. DISCUSSION The distinction between oral and ambital primary spines in cidaroids has been noted by previous authors (e.g., Prouho, 1887; Mortensen, ]928). Also the dissolution of plates at the oral margin of the corona, which occurs in cidaroids and all other regular echinoids except arbaciids and echinothurids, has been described in some detail by Loven (1892: 21-24) and mentioned by others (e.g., Jackson, 1912: 53, 65; Deutler, ]926: 163-164; Gordon, 1926: 295; 1929: 303; Mortensen, 1927: 373). Yet none of these authors noted that the oral primary spines arise during postmetamorphic development and concurrently with the disappearance of oralmost plates. Mortensen (1927: 378; Fig. 9), in an illustration of the primary spines in very young (less than 1 mm h.d.) Eucidaris metularia, showed a flattened spine resembling an oral primary. However, this was a very 822 Bulletin of Marine Science [/5(4) juvenile type spine borne on the unpaired plate which is present at the oral edge of each interambulacrum at metamorphosis; both plate and spine had disappeared when the test reached 1 mm h.d. On paired plates aboral to the unpaired one, he found at metamorphosis unflattened, juvenile spines which were lost by the 3.5 mm h.d. stage. New plates, added at the aboral edge of the corona as the cidaroid grew, carried "adult" type ambitals. This would indicate that in 3.5 mm E. metularia, only ambital primaries would be found on oral plates, yet Mortensen did not specify this. In his monograph, Mortensen (1928: 397, 404) hinted that only ambitals are found on 3 and 4 mm E. thouarsii and E. tribuloides, respectively, but again did not elaborate. Because attention apparently has not centered previously on the lack of oral primaries in young cidaroids, the process of formation of the orals has heretofore been neglected. As detailed in this paper, at the same time that plates are disappearing at the peristome in Eucidaris tribuloides, there is continual loss, usually by breakage through the proximal portion of the collar, of ambital primaries on interambulacral plates below the ambitus followed by regeneration to a flattened, smoother, oral spine. The origin of the oral primaries in other cidaroids is presumed to follow a process much the same as that in Eucidaris. It would require controlled experiments under more natural conditions than those available for this study to establish the factors responsible for breakage of the primary ambital spines and replacement by orals, but some possibilities can be explored. In Eucidaris, the fact that the spines break so commonly through the proximal portion of the collar indicates that some factor inherent in the spines may influence the point of breakage. Further, the change from ambital to oral primaries occurs in such a regular sequence (first in IA 3, then IA 2 and 4, then IA 1 and 5) it seems likely that the breakage is associated with an aspect of growth of the corona which shows the same pattern. This would eliminate predation, which would occur at random, such as that noted by Mortensen (1928: 397) for Eucidaris thouarsii. Environ- mental conditions affecting the entire echinoid also can be discounted. Loss of spines as reported here, then, would be in no way comparable to that reported in Paracentrotus lividus by Krizenecky. (1917: 650), in Echinus esculentus by Chadwick (1929: 761), and in Psammechinus miliaris by Hobson (1930: 168). In each of these, there was complete de- nuding of the primary tubercles over all or most of the corona due to un- favorable environmental conditions, the loss attributed by Krizenecky to autotomy. The breakage of spines in Eucidaris tribuloides is similar to that reported 1965] Cutress: Growth in Eucidaris tribuloides 823 by Prouho (1887: 259) in Dorocidaris papillata [=Cidaris cidaris] where loss of a large portion of a spine resulted, according to Prouho, in formation of a transverse membrane immediately above the milled ring, the portion of spine distal to the membrane then dying and falling off and a new spine regenerating from the membrane. However, in the aquarium- maintained Eucidaris, although some of the broken spines had the break covered by a membrane, this apparently had formed after the break and was the beginning stage of repair; other spines had perfectly clean, apparently newer breaks with no membrane covering. Also, sections of ambital spines which, because of sequence in the replacement cycle, would be expected to be next to undergo breakage showed no internal membrane. Mortensen (1928: 498) noted a somewhat similar type of spine loss in Chondrocidaris brevispina where "it appears that the spine is liable to break at the limit between collar and shaft, the latter then regenerating from the middle of the surface of the fracture; a very characteristic appearance is thereby produced." It may be that in Eucidaris, and presumably other cidaroids, the breakage of the spines is associated in some way with the resorption of interambulacral plates at the oral margin of the corona. This resorption fulfills the requirement of following the same sequence as that noted in the breakage and replacement of spines. . Other authors (Loven, 1892: 24; Deutler, 1926: 154) have noted, concurrent with resorption, a progressive displacement of plates toward the oral margin of the corona and a change in shape of the plates. In Euci- dads tribuloides, the downward displacement of plates is very evident. In a young (7.5 nun h.d.) specimen, for instance, there are five or six plates per interambulacral column, and two or three of these lie below the ambitus. In 14 nun tribuloides, with six or seven plates per column, three or four lie below the ambitus, although at least one plate has disappeared at the oral margin of each column. At 33 mm, with eight or nine plates per column, there are still four plates per column below the ambitus, although another one or two may have disappeared. In Eucidaris, the oral portion of the corona, even in adults, is not greatly flattened and not incurved, but the oralmost two or three interambulacral plates per column are mostly underneath the echinoid as it rests on the substrate. The spines on plates nearest the peristome are used in locomotion. Possibly movement of the spines within or over the substrate causes the moderately long, straight ambital primaries to break. There is loss of epithelium on the shaft (Jackson, 1912: 51; Mortensen, 1928: 27) and apparently a withdrawal of stroma to some extent within the spine as evidenced by the lack of regeneration of tips of old spines (Prouho, 1887: 259). This might increase the brittleness of the spines, making them more susceptible to breakage. The strong muscles which bind the spines to the 824 Bulletin of Marine Science [15(4) tubercles extend to the milled ring, and any purely mechanical damage would be expected above the milled ring. Probably such an explanation is too simple. In some other cidaroids, ambital primaries are used in locomotion without breakage. Mortensen (1?28: 28) reported that in Stereocidaris tubifera the spines so used show only wear to the tips. Moreover, the difference in relative position of comparable plates in the five interambulacra would seem to be too slight to allow for such a regular sequence of loss and replacement of spines as a result of fortuitous mechanical damage. It is possible, then, that the breakage is autotomous and triggered by internal changes in the plates or spines. In this case, could these internal changes be induced by external environmental conditions or by factors associated with growth of the plates and spines? Probably we can eliminate environment (e.g., amount of light reaching individual plates and spines), at least as the primary cause, for the same reason that makes simple mechanical breakage unlikely: the very sligbt differences in relative position of comparable plates. As for the spines, only Prouho (1887: 259) has suggested any change, other than loss of epithelium, which might be associated with breakage. It is likely that Prouho misinterpreted the presence of the membrane. At any rate, no abscission membrane was found in Eucidaris spines prior to breakage. There remains the possibility of changes in tbe plates themselves. Deutler (1926: 157), in his study of growth rings, found no evidence of internal resorption of calcite within the oral plates as suggested by Jackson (1912: 51) but did find loss of skeletal material at the oral and aboral margins as well as along the median suture between two interambulacral columns. At the same time, the tubercles on oral plates may become slightly eccentric with respect to the upper and lower margins of the areoles. Deutler (1926: 173) suggested this is due to addition of calcite, not just to unequal resorption at the periphery of the plate. In Eucidaris, and some other cidaroids examined, the portion of the areole below the tubercle may become slightly wider than that above and usually more incurved. Perhaps such changes in the plates or resultant changes in relation of muscles to the spines may be sufficient to trigger breakage. The real cause remains unknown. There is another unresolved question: why do different type spines (oral primaries) regenerate from the broken ambital primaries? In Eucidaris, the oral primaries are similar enough to the ambitals that the weight of the echinoid upon the spines as well as movement over and through the substrate might conceivably be sufficient to alter the size, shape, and smoothness of the spines during their development. But in cidaroids with exquisitely specialized oral primaries, such as Acanthocidaris, this would not seem an adequate explanation (presuming, of course, that the 1965] Cutress: Growth in Eucidaris tribuloides 825 mechanism of change in type of primaries is the same for all cidaroids). And again, the slight differences in position of comparable plates in the five interambulacra would seem to be too slight to allow a regular sequence of change from ambital to oral primaries, such as that noted in Eucidaris, based simply on pressure factors. Considering not only the regular sequence and manner of spine breakage but also the nature of the regenerated spines, the entire process of change from ambital to oral spines may best be attributed primarily to genetic control. It is not certain how much the artificial environment affected the overall growth of the seven aquarium-maintained Eucidaris tribuloides during the year of observation. There are no studies on normal growth of the species available for comparison. Some investigators (Shearer et aI., 1914; Lewis, 1958) have found no appreciable difference in growth between echinoids maintained artificially and those observed in nature. Others (Swan, 1961; Moore, Jutare, Bauer & Jones, 1963; Moore, Jutare, Jones, McPherson & Roper, 1963), have reported slower or faster growth in other species. Growth studies of a few non-cidarid species have been made: in Echinus miliaris by Elmhirst (1922) and Moore (1935); Salmacis bicolor by Aiyar (1935); Psammechinus miliaris by Bull (1938); Tripneustes ventricosus by Lewis (1958) and McPherson (1965); Diadema antillarum by Randall et at. (1964); A'rbacia punctulata by Brown (1956); Stronqylocentrotus droebachiensis by Grieg (1928) and Swan (1961); and Mellita sexiesper- !orata by Crozier (1920). In these it was found that the increase in size over a one year period depended on the ultimate size and longevity of the echinoid. In general, growth was most rapid in the earliest stages. As Loven (1892: 17), Jackson (1912: 52), DeutIer (1926: 126), Hsia (1948: 29) and others have noted, growth of the test is due not only to increase in size of extant plates but addition of new plates at the aboral margin. The growth due to addition of plates is moderated considerably by loss of plates orally. However, in E. tribuloides the increase in number of interambulacral plates follows very closely a straight line progression (Fig. 5). Hsia (1948: 28) also found a straight line relationship between number of coronal plates and h.d. of the test in Temnopleurus toreumaticus, T. hardwickii, and Strongylocentrotus [=Hemicentrotus] pulcherrimus except in old specimens. Swan (1962: 1213) illustrated the same for two species of Strongylocentrotus; however, in an earlier paper (1958: 510), his graphs show a slight curve for smallest specimens of S. droebachiensis but essentially a straight line for larger ones. The sequence (IA 3, IA 2 and 4, IA 1 and 5), observed in the formation of new interambulacral plates aborally and resorption of old plates orally in all the living and most of the preserved E. tribuloides examined, has been 826 Bulletin of Marine Science [15(4) illustrated but not specifically noted for other species. For instance, A. Agassiz (1904: Text Fig. 26-34; PI. 5, 7, 11-13) apparently represented quite accurately the size of plates in comparable positions in the five interambulacra of Dorocidaris [=Hesperocidaris] panamensis, Porocidaris [= Histocida'ris] cobosi, Porocidaris [= Aporocidaris] milleri, and Centro- cidaris doederleini and thereby illustrated, albeit unwittingly, the 3, 2-4, 1-5 sequence. However, the plate numbers in the text figures should not be relied on completely. In Figure 33, the incomplete oral plate (indicated as plate 2) in IA 5a is actually comparable to plate 1 in IA la, 2a and 4a; only in IA 3a has plate 1 already disappeared. Likewise, in Figure 34, the oralmost plate in IA 5a is almost certainly comparable to plate 1 in IA la and 2a and should be similarly numbered; only in IA 3a and 4a has plate 1 completely disappeared. It is interesting that the illustrations mentioned above indicate a difference among species in time of appearance of new plates in IA 2 and IA 4. For instance, in D. panamensis (Text Fig. 26, 28-29 in A. Agassiz: 19'04), new plates seem to appear first in IA 4. In P. cobosi (op. cit.: Text Fig. 31), the new plate in IA 2 arose first as was usually the case in the specimens of E. tribuloides observed during the present study. In all the E. tribuloides examined, new plates developed and old plates disappeared in the same column, a or b, of all five interambulacra during anyone cycle. Loven (1892: 23) observed the same to be generally true in Cidaris papillata [=cidaris] although he "found a few exceptions and there may possibly exist some interchange in this respect." Deutler's (1926: 136) growth zone studies in Echinus confirmed Loven's concept. The sequence in development of interambulacral plates in E. tribuloides is just the reverse of the sequence found by Jackson (1912: 92, 97) for insertion of the ocular plates of this species. In very young tribuloides, all oculars are exsert. Usually at least three of these plates quickly be- come insert. Typically, the oculars of the bivium are insert first, V followed by I; then IV becomes insert. In 58 per cent of his 849 specimens, only these three oculars were insert. Insertion had progressed beyond this in 37 per cent; II usually came in next, although more rarely III might precede it. In 25 per cent of his specimens, all five oculars were insert. The sequence and completeness of insertion is not the same in all species of cidaroids. For instance, a V, I, IV, III, II sequence is common in Cidaris afJinis. Jackson (1912) saw the sequence of ocular insertion as a further in- dication of the axis of symmetry (through ambulacrum III and IA 5) suggested by Loven (1874: 20) for the cidaroids in general. Since in E. tribuloides the oculars insert most commonly in a V, I, IV, II, III sequence, and the interambulacral plates usually form in sequence 3, 2, 4, 1, 5, we might assume that the process of insertion prevents or slows the formation of plates, giving plates in IA 3 precedence. This 1965] Cutress: Growth in Eucidaris tribuloides 827 might be an adequate explanation if oculars were associated closely with entire interambulacra. But morphologically, at least, there is no such relationship. New coronal plates originate at the bases of the ocular plates, and there is evidence (Jackson, 1912: 86; Kier, 1956: 971) that the oculars are necessary for production of the plates. Each ocular is in contact with an ambulacrum and the adjoining half interambulacrum to either side. This and the manner of insertion of the muscles of the lantern led Jackson (1912: 62) to view the corona as made up of five areas, each com- prised of an ocular and its related ambulacrum and half interambulacra. For the same reason, de Saint-Seine (1958: 1031) suggested suppressing terms "interradius" and "interambulacrum" and using "radius" for an ambulacrum plus the two adjoining half interambulacra (the latter to be called "margina"). Now, if the axis of symmetry were through ambulacrum III and IA 5, as Loven and Jackson supposed, and if ocular III influences equally the half interambulacrum on either side of ambulacrum III, it would be ex- pected that new interambulacral plates would appear first in either of these two columns, in IA 2b or IA 3a. Instead, in all of the living and most of the preserved specimens of E. tribuloides examined, the first formed plates were in IA 3a or 3b. There was, furthermore, a resemblance in stage of development between IA 2, as a whole unit, and IA 4 as a unit, likewise between IA 1 and 5. In only a few specimens was the new aboral plate in IA 2 equal to or larger than th:lt in IA 3; likewise, the new plate in TA 1 was only occasionally equal to or larger than in IA 4. These observations indicate that the two columns of each interambula- crum are more closely allied than would be supposed from their asso- ciation with different oculars. It suggests that the term "interambulacrum" should be maintained on equal footing with "ambulacrum." Furthermore, it would seem that the line of symmetry in E. tribuloides is usually through IA 3 and ambulacrum J. Not all previous investigators have found Loven's 1II-5 axis applicable to the regular echinoids. Von Ubisch (1913: Fig. 2; 1927: Fig 16) diagrammed the arrangement of the plates on and bordering the peristome for a very young Strongylocentrotus droebachiensis according to Loven's formulae: la, IIa, IIlb, IVa, Vb for large ambulacral plates at the oral edge of the corona and for large buccal plates, and Ib, lIb, IlIa, IVb, Va for small ambulacral plates and for the small buccal plates, the latter pierced by the peristomial tube feet. As von Ubisch indicated, if Loven's 111-5 axis is then applied to the diagram, only the plates of radii I and V are symmetrical. But if the axis were considered through ambulacrum II and IA 4, then only radius III is asymmetrical. Von Ubisch referred to the 1I-4 axis as the "primordialebene." 828 Bulletin of Marine Science [15(4) Although von Ubisch did not indicate it, 3-1 axis applied to his S. droebachiensis diagram would give as much symmetry as a 11-4 axis, only radius I then being asymmetrical. According to Mortensen (1927: 372), the buccal plates in Eucidaris metularia also follow Loven's la ... Vb and Ib ... Va formulae, and it can be supposed that the same is true of Eucidaris tribuloides. Therefore, in these cidaroids at least, a 3-1 axis would seem as likely as any other. It must be noted, before ending the discussion of symmetry, that ob- servations made by Lucas (1953) on the orientation of calcite in the genital plates do not confirm either von Ubisch's 11-4 axis for Strongylo- centrotus or a 3-1 axis for the cidaroids. Lucas found that in Strongylocentrotus [=Paracentrotus] lividus, Ar- bacia aequituberculata, and five fossil regular echinoids, the calcite in three of the genital plates (2, 3, 5) had a crystallographic axis perpendic- ular to the surface of the plate, while in the other two (1, 4), it was parallel. This gives a 11I-5 axis of symmetry, Loven's axis. In Dorocidaris and two fossil regulars, he found the crystallographic axis was perpendicular in only two of the plates (3, 5); in the other three (1, 2, 4), it was parallel. The axis of symmetry in this case was 11-4, von Ubisch's "primordialebene," demonstrated by him in Strongylocen- trotus. Apparently then, not all parts of anyone regular echinoid show the same symmetry. Also, the axis of symmetry probably is not the same for all regular echinoids, perhaps not even for all cidaroids. There are no other records of rates of development of new or regen- erated spines in cidaroids with which to compare observations on devel- opment in the seven living E. tribuloides. Prouho (1887: 253) and Loven (1892: 20) described the process of formation of new primary spines in Cidaris cidaris, and Prouho (op. cit.: 259) outlined the steps in spine regeneration in that species. However, neither related development to time. Some information on the rate of regeneration of spines has been given by Swan (1952: 30) for Strongylocentrotus and by Brown (1956: 38) and Jackson (1939: 656) for Arbacia punctulata. Swan removed primary spines from living specimens of three Strongylocentrotus species and then recorded the number of wedges and size of the regenerated spines after 60 days. At that time, most of the new spines had not reached the stage of development of the original spines, and the extent of regeneration (based on length) varied greatly, from 6 to 140 per cent. Swan concluded that "the rate of regeneration is inversely related within a species to the size and age of the specimen." His data also show that regeneration (at least as far as length was concerned) was more rapid in aboral and oral spines than in spines at the ambitus. Brown and Jackson found when spines about 1965] Cutress: Growth in Eucidaris tribuloides 829 12 mm long were removed in Arbacia, they regenerated at a rate of a little over 1 mm a week and were almost completely regenerated in two months. Mortensen (1928: 405) indicated variability due to environment in spines of E. tribuloides by noting that long-spined specimens are found in deeper water than short-spined. He did not speculate on what, if any, factors associated with depth might be responsible. In the seven living Eucidaris, long primaries developed on new aboral plates in several of the otherwise short-spined specimens after they were moved from one tank to another. There was at the same time an accelera- tion in growth of other spines. Depth, density of water, and temperature were approximately the same in the two tanks, but several other things did vary considerably. The second tank was 2.5 times as large as the original one with a correspondingly greater volume of water per echinoid, and there was more aeration. McDaniel (1961) found that in rearing echinoids artificially, a minimum of five gallons per specimen gave the best conditions. The larger tank was exposed to far less light than the first; similarly, there would be less light in deeper water under natural conditions. There was a difference in composition of the artificial sea water in the two tanks but how much is not known since the formulae for the commercial salts used are not available. There were undoubtedly subtle differences in pH and chemical content from metabolism of the organisms contained in the tank. Wilson and Armstrong (1961: 5) have noted some changes of this nature which are possible. Even the dying of the mats of algae on the rocks and spines after the change to the large tank may have been a factor. As Atz (1964: 5) pointed out, there is evidence (he cited several references) that even small amounts of organic matter in sea water can exert an important in- fluence on living animals. SUMMARY 1. Study of the primary spines in growth series of several cidaroids revealed that the specialized oral primaries, which are characteristic of adult cidaroids, are lacking in very young specimens. No information was found in the literature as to how or when the oral primaries develop. 2. By following the changes in the primary spines of seven aquarium- maintained specimens of Eucidaris tribuloides during a period of one year, it was determined that in this species oral primaries develop by regenera- tion, almost always from a break through the proximal portion of the collar of an old ambital primary spine. The general pattern of regenerative activity and increase in number of oral spines has been outlined and pos- sible causes of breakage and change in type of spines explored. 3. Increase in horizontal diameter of the seven living specimens of E. 830 Bulletin of Marine Science [15(4) tribuloides ranged from 1.3 to 4.5 mm during the year of observation. The smallest specimens from each locality showed the largest per cent increase. 4. The increase in number of interambulacral plates was shown to follow a "stair-step" progression, old plates being resorbed orally as new plates are added aborally. However, when the results are graphed, a line drawn through the high points in the steps is almost straight for E. tribuloides. 5. In all the living and most of the preserved specimens of E. tribuloides examined, development of new interambulacral plates aborally and re- sorption of old plates orally followed a constant sequence: first in IA 3, then in IA 2 and 4, then IA 1 and 5. The replacement of ambital by oral primary spines followed the same sequence. Only occasionally was a 3-2, 4-1, 5 sequence noted. 6. Without exception, in all specimens examined, new aboral plates (and spines) of comparable age were found either all in the a columns of the five interambulacra or all in the b columns. 7. The 3, 2-4, 1-5 sequence of development and resorption of plates indicates the axis of symmetry in E. tribuloides is usually through IA 3 and ambulacrum V rather than through ambulacrum III and IA 5, as suggested by Loven. The sequence is the reverse of that noted by Jackson for insertion of oculars in the species. The relation of the oculars to the interambulacra is discussed in connection with the axis of symmetry. 8. Other axes of symmetry, based on orientation of the calcite in the genital plates and on arrangement of oralmost ambulacral plates and buccal plates, are discussed. 9. The rate of development of new aboral spines has been noted as well as rates of regeneration of primary spines on various parts of the corona. 10. After the seven living specimens of E. tribuloides were moved from a small to a large tank at the end of the first six months, there was a distinct acceleration in growth of new aboral spines. Through the change in growth pattern, some of these spines came to resemble spines described by Rathbun and Mortensen as being infested with gall-forming Stylifera. Possible factors responsible for the change in growth are discussed. SUMARIO OBSERVACIONES EN EL CRECIMIENTO DE Eucidaris tribuloides (LAMARCK), CON ESPECIAL REFERENCIA AL ORIGEN DE LAS ESPINAS PRIM ARIAS ORALES 1. EI estudio de las espinas primarias en una serie de crecimientos de varios cidaroideos revelo que las primarias orales especializadas, caracte- rfsticas de cidaroideos adultos, faltan en ejemplares muy j6venes. No se 1965] Cutress: Growth in Eucidaris tribuloides 831 encontro informacion en la ]iteratura de como y em'indo se desarro]]an las primarias orales. 2. Observando los cambios en las espinas primarias de siete ejem- plares vivos de Eucidaris tribuloides mantenidos en un acuario durante un ano, se determino que en esta especie las primarias orales se desarrollan por regeneracion, casi siempre a partir de una hendidura a traves de ]a porcion proximal del collar de una espina primaria ambital vieja. Se ha delineado el patron general de ]a actividad regenerativa y del aumento en el numero de las espinas orales y se han exp]orado las causas posibles de ]a rotura y cambio de tipo de las espinas. 3. El aumento en el diametro horizontal de siete ejemplares vivos de E. tribuloides fue de 1.3 a 4.5 mm durante el ano de observaci6n. Los ejemp]ares mas pequenos de cada localidad mostraron e] mayor par ciento de aumento. 4. El aumento en el numero de placas interambulacra]es se mostr6 que seguia una progrcsi6n escalonada, las placas viejas fueron reabsorbidas oralmente a medida que nuevas p]acas eran aiiadidas abora]mente. Sin embargo, cuando los resultados son llevados al grafico, una linea a traves de los puntos altos en los escalones es casi recta para E. tribuloides. 5. En todos los ejemp]ares vivos y en la mayorfa de los preservados de E. tribuloides examinados, el desarrollo de nuevas placas interambulacrales abora]mente y la reabsorci6n de viejas placas ora]mente, mantuvieron una secuencia constante: primero en IA 3, despues en IA 2 y 4, despues en IA 1 y 5. EI reemplazo de espinas ambitales par primarias orales sigui6 ]a misma secuencia. S610ocasionalmente se not6 una secuencia 3-2, 4-1, 5. 6. Sin excepcion, en todos los ejemplares examinados, nuevas placas aborales (y espinas) de edad comparable fueron encontradas todas en las columnas a de los cinco interambulacros 0 todas en las columnas b. 7. Las secuencias de desarrollo 3, 2-4, 1-5 y la reabsorci6n de las placas indica que el eje de simetrfa en E. tribuloides es generalmente a traves de IA 3 y el ambUlacro V en vez de a traves del ambUlacro III y IA 5, como sugiri6 Loven. La secuencia es el reverse de la notada por Jackson para la inserci6n de oculares en las especies. Se discute la relaci6n de los oculares con los ambulacros en conexi6n con el eje de simetda. 8. Se discuten otros ejes de simetria, basados en la orientaci6n de ]a ca1cita en las placas genitales y en la disposici6n de las placas ambulacrales mas orales y las placas bucales. 9. La proporcion del desarrollo de nuevas espinas aborales ha sido notada asf como las proporciones de regeneraci6n de las espinas primarias en varias partes de la corona. 10. 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