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Longitudinal Fission in Actinia Bermudensis Verrilli A

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Longitudinal Fission in Actinia Bermudensis Verrilli A AUTHOR'S ARSTRAIX OF THIS PAPER ISjUED BY THE BIRLIOGRAPHlC SERVICE. MAY 4 LONGITUDINAL FISSION IN ACTINIA BERMUDENSIS VERRILLI A. C. WALTON Bermuda Biological Station for Research EIGHT FIGURES Longitudinal fission has long been recognized as a normal means of non-sexual reproduction among sea anemones. McCrady ( '58)2 observed aboral-oral fission in Actinia cavernosa, and G. C. Davenport ('03) observed similar phenomena in Sagartia luciae. In these cases fission was by constriction, followed by rupture. Carlgren ('93) in Protanthea simplex, and Torrey ('98) in Metridium found an aboral-oral fission by constriction without rupture. Mrs. Thynne ('59) observed a side-to-side fission accompanied by rupture in the case of Cyanthina smithi. Torrey and Mery ('04) show that all three of the above methods are normally found in Sagartia davisi, with the plane of divi- sion running perpendicular to the long axis of the mouth. It is especially in Metridium that the double condition has been observed. Two-headed specimens had been noted as early as 1775 (Dicquemare). Johnston ('47) stated his belief that these conditions were due to the coalescence of two separate individiials; but Gosse ('60) contended that such individuals owed their existence to spontaneous division. Foot ('63) re- corded the discovery of a specimen having two mouths on one oral disc, but expressed no opinion as to the cause. Carlgren ('93), like Gosse, contended that such forms were stages in a 1 Contributions from the Bermuda Biological Station for Research, No. 87. 2 For further literature see the following papers of Parker and Carlgren. Parker, G. H., 1899. Longitudinal fission in Metridium marginatum Milne- Edwards. Bull. MUS. Comp. Zool., vol. 25, pp. 43-55, 3 pls. Carlgren, O., 1909. Studien uber Regenerations- und Regulationserschein- ungen. 11. Erganzende Untersuchungen an Actinien. Kongl. Svenska Veten- skaps Akad. Handlingar, Bd. 43, No. 9, 48 pp., 4 Taf. 43 44 A. C. WALTON process of longitudinal division, and Torrey ('98) actually fol- lowed the changes of certain steps of this process in Metridium. In working upon Metridium marginatum, Parker ('99) defi- nitely showed that there is natural normal longitudinal fission, and that the double mouthed and double headed animals were but stages in such a division. Cases where there was circum- stantial evidence of completed division were also found. The division plane was always through primary ectocoels or primary entocoels. Carlgren ('04) believed that, though aboral-oral fission could take place as the result of injury, oral-aboral fission did not occur, and that the double forms of Metridium studied by Parker and by Torrey were not stages of longitudinal division, but monstrosities developed from partially double embryos, such as are often found in Sagartia and Cribrina species. His further work (Carlgren, '09) only strengthened his belief that fission was due to regeneration after laceration, followed by division, and was not a normal mode of reproduction. The material upon which the present article is based consisted of specimens of Actinia bermudensis Verrill obtained at Bermuda during the summer of 1916. I wish here to thank Dr. E. L. Mark, Director, and Dr. W. J. Crozier, Resident Naturalist, €or the opportunity to study at the Bermuda Biological Station, and also for their personal assistance. Actinia bermudensis Verrill is a small, blood red, shade-loving anemone, living between tide levels, and found only in certain caves along the shores of Bermuda. There are two color phases, perhaps varying enough to be classed as subspecies: one a bright blood red, the other slightly darker and more nearly prune- colored. That the darker colored forms are not merely old in- dividuals is shown by the fact that the embryos of each type are colored exactly like their parents. Mature specimens of either type are from 3 to 4 em. across the base, and from 2.5 to 3.5 em. across the oral disc, while the column is 4 to 5 em. in height. The animals are commonly hexamerous, having two pairs of directives, one pair to each siphonoglyph, and from ninety-six to one hundred and twenty mesenteries arranged in three orders. The column wall and both oral and pedal discs FISSION IN ACTINIA BERMUDENSIS VERRILL 45 are sufficiently transparent to show the attachments of the mesenteries. The inner edges of the complete mesenteries (be- low the level of the stomenteron) as well as those of the incom- plete mesenteries are provided with mesenteric filaments. These at their bases give rise to long, much convoluted acontia. These organs are protruded through the mouth, and not through cinclides. Gonads are present on all of the mesenteries of the first and second orders, except the two pairs of directives; and a few traces of immature gohads were also found on some of the mesenteries of the third order. Only one ostium, the median, is generally present in the mesenteries of Actinia bermudensis. The tentacles, arranged in two very irregular rows, are from eighty-five to one hundred and twenty in number in mature specimens, and at the bases of the outer row are twenty-four irregularly spaced, bright bluish-violet eminences, the acrorhagi. This species of sea anemone was first described by Verrill in 1898, and since that time has been observed and worked upon by several investigators. No mention, however, has been made of specimens of a double nature, and, except for a single speci- men kept in the laboratory for some time as a curiosity by Dr. Crozier, I believe the specimens obtained during the summer of 1916 are the first individuals having either double heads or double mouths that have been carefully examined. The speci- men Dr. Crozier found had a double mouth and more than the ordinary number of tentacles, but' it underwent no visible changes while under observation. While working on the early embryology of Actinia bermuden- sis, I had occasion to examine carefully over a thousand speci- mens of all ages, and all but a very few were of the bright red variety. Among this number of animals were two which first attracted attention by their size and shape. The oral discs were greatly elongated, the long axis being almost twice the length of the shorter, which had normal dimensions. On examination it was found that one specimen (fig. 1) had two separate diglyphic mouths, the long axes of which made a slight angle with each other. In the other specimen (fig. 2) the mouth was in the proc- ess of division. The mouth opening had roughly the shape of 46 A. C. WALTON an arrow head, with three siphonoglyphs, one corresponding to the point of the arrow head, the other two to its barbs, the latter having arisen by a division of one of the original siphonoglyphs. These two specimens, living in the laboratory in a tank of run- ning sea-water, and fed on bits of crushed crab, were kept under observation for almost five weeks. Their development is of especial interest, inasmuch as they showed a slow, but evident, progressive fission longitudinally, which in one case ended in a sudden completion of the process by the ' constriction-and- rupture' method shown by Torrey and Mery ('04) for Metri- Fig. 1 Photograph of Aktinia A, oral view, showing two separate mouths. dium. In Metridium, however, no direct evidence of the fis- sion of the double forms has as yet been recorded. When specimen A (fig. 1) was fully expanded, the long axis of its oral disc measured 6.5 cm., and its pedal disc 7.5 em. As in the case of Metridium, the number of tentacles was greater than normal. The normal number for an adult individual is about one hundred and ten, generally closely corresponding to the number of inter-mesenteric spaces. In this animal, when first found, there were one hundred and eighty-seven tentacles, and two more were formed before fission was completed. The trans- parency of the oral disc allowed the counting of the mesenteries FISSION IN ACTINIA BEBMUDENSIS VERRILL 47 of the first and second orderc with great accuracy. At the ends of the siphonoglyphs (fig. 3) two directives could be made out for each member of the most widely separated pair, while only one directive extended from each of the members of the other pair. A diamond-shaped area lying between the two mouths was entirely destitute of mesenteries. In all, thirty-two complete mesenteries were made out, of which twenty-eight were arranged in pairs, while four-two directives and two non-directives- were unpaired. Slight evidences of longitudinal infolding, es- pecially at the oral end of the column, were apparent. Three weeks later the infolding (fig. 4) was very evident, and the halves of the oral disc had almost completely separated. In place of each of the two unpaired directives there was now a paired directive, also in place of the two unpaired non-directives there were paired non-directives. One of the halves of the ani- mal (right) had formed another pair of complete mesenteries. The opposite half had formed only a single complete non-direc- tive, its mate being incomplete. During the next night rapid fission had begun, and by morning had proceeded about 1 em. down the column. The fission continued rapidly and was com- plete eight hours later, the whole of the final separation taking from twelve to fourteen hours. The opposite walls of the column had infolded until only a narrow strip of material joined the halves. This narrow strip was ruptured by the straining apart of the halves, the split moving in an aboral direction. The ruptured surfaces were immediately rolled inward, and the animals contracted entirely, remaining in this state for five days.
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