OBSERVATIONS ON THE ANATOMY OF MIKlfDOTROCIIUS AMABILIS BAYER' VERA FRETfER University of Reading, England ABSTRACT The gross anatomy of the hoJotype specimen of Mikadotrochus amabilis Bayer, 1963, is described insofar as the state of preservation permits. A brief description of the external features is given to point out differences between this species and those previously described, especially Mikado- trochus beyrichi (Hilgendorf). INTRODUCTION The most comprehensive description of the anatomy of a member of the gastropod family Pleurotomariidae is given by Woodward (1901) for Mikadotrochus beyrichi (Hilgendorf). With three preserved speci- mens he was able to examine the most important systems of the body and compare this species with what was already known of the anatomy of Entemnotrochus adansoni{Jnl!s (Crosse & Fischer) and Perotrochus quoyanus (Fisher & Bernardi); owing to poor preservation of the speci- mens our knowledge of these two species is confined to external features, radula, some details in connection with the pallial complex and for P. quoyanus the nervous system (Dall, 1889: Bouvier & Fischer, 1899). The single specimen of a new species, Mikadotrochlls amabilis Bayer, 1963 (described below), was trawled southeast of Sombrero Key, Florida. It was fixed in alcohol and retracted into the shell. When removed it was found that the stomach was damaged so that nothing but its contents could be justifiably examined, and the posterior part of the right kidney was torn. The rest of the animal was moderately well preserved for work on gross anatomy. A brief description of the external features will be given to emphasize certain differences between this and other species which have already been described, especially M. beyrichi (Woodward, 1901); the value of some of these remarks will be known only when more specimens of pleurotomarians become available so that the living animal can be studied and the extent of variation within a species revealed. IContribution No. 524 from The Marine Laboratory, Institute of Marine Science, University of Miami. The material described herein was collected during deep water operations of R/V GERDA supported by grant G-20355 from the National Science Foundation. 1964] Fretter: Anatomy of Mikadotrochus 173 EXPLANATION OF LETTERING a, anal papilla j, jaw aa, anterior aorta la, left auricle ahv, afferent brachial vein lav, left afferent branchial vessel ah, accessory lobe of hypobranchial lh, left hypobranchial gland gland lk, left kidney h, bolus of food held by dorsal n, supraoesophageal nerve in blood folds of oesophagus sinus ha, bulbus aortae ad, odontophore bbs, basibranchial sinus of, dorsal fold of oesophagus bf, dorsal folds of buccal cavity og, oesophageal gland bw, body wall opened mid-dorsally as, osphradium overlying branchial c, columellar muscle ganglion ca, ctenidial axis ov, oesophageal valve ee, cerebral commissure p, papillae cm, cut edge of mantle skirt showing pc, pericardium blood spaces pa, posterior aorta cpv, cephalopedal vein pg, opening of anterior pedal gland ct, ctenidium r, rectum d, morphologically mid-dorsal wall ra, right auricle of oesophagus rav, right afferent branchial vessel e, eye rk, right kidney ebv, efferent branchial vein rkp, urogenital papilla of right kidney ep, left epipodium rs, rectal sinus ev, efferent branchial vessel sn, snout f, foot sg, salivary gland h, heart sm, strands of muscle and he, head connective tissue hv, hypobranchial vein (Fig. 2) or t, tentacle vessels (Fig. 4) ve, ventricle if. inner lip vh, visceral hump DESCRIPTION The relatively large head, resting on the propodium, has a truncated snout (Fig. 1, sn). The outer lip which surrounds the oral area is in- terrupted mid ventrally and its inner surface is densely papillated. It encloses a deep membranous frill, the inner lip (it), which also bears papillae and is brown. This inner lip enveloped a mass of detrital material, which the mollusc selects as food, and this suggests that it may be used to hold particles which will be gathered into the buccal cavity later by the radula. It is reminiscent of the frill associated with the outer lip of some members of the Acmaeidae and Lepetidae (Fretter & Graham, 1962) which has been shown by Thiem (1917) to be sensory. Although the lip is not mentioned by Woodward (1901) in M. beyrichi, he refers to, and figures, a number of "small flattened papillae" immediately in front of the jaws, suggesting a structure similar to, though much smaller than, the inner lip of M. amabilis. There is no reference to anything comparable in the descriptions of E. adansonianus and P. quoyanus. 174 Bulletin of Marine Science of the Gulf and Caribbean [14(1) The cylindrical tentacles, lying low on the head, are contracted. They do not branch at the tip as in M. beyrichi. The eye, on an eyestalk at the base of each, is partly hidden by the lateral lobe of the mantle skirt. The mantle skirt or pallial fold encircles the body. It is very shallow posteriorly and except in this region is fringed with rows of papillae (p). These increase in length and number in the region of the pallial slit. The 17 mm FIGURE 1. Mikadotrochus amabilis. Animal removed from shell and seen from the left side. The damaged visceral mass is not shown. (For explanation of lettering of all figures, see list on page 174.) slit is conspicuous though its margins were retracted from the shell slit in the intact animal; on either side projects the tip of a ctenidium. The foot is large and pointed posteriorly. A transverse furrow across the broad anterior limit of the sole is overhung by the thin edge of the propodium. This has been mentioned in all species and presumably in all, as in P. amabilis, the furrow marks the opening of an anterior pedal gland (pg), though the gland was not known to previous authors. The surface of the anterior part of the sole, or mesopodium, is relatively smooth and has a deep, median longitudinal groove, whilst that of the more posterior parts is contracted to give transverse furrows. The lateral 1964] Fretter: Anatomy of Mikadotrochus 175 surfaces of the foot are minutely papillose and reddish in color, re- sembling the shell. The papillae extend over the epipodial folds. These are rather shallow anteriorly but deepen posteriorly so that they arch over the surface of the metapodium. The edges of the folds are fringed with papillae. There are no epipodial tentacles or sense organs. The right epipodium is better developed than the left and extends forward to near the level of the anterior limit of the foot. The left (ep), arising about half way along the side of the foot, resembles the condition de- scribed by Dall (1889) for P. quoyanus. In E. adansonianus it is even shorter, but appears to be as long as the right in M. beyrichi. The opercular lobe of the meta podium is small and its posterior edge is some distance from the posterior tip of the foot. Spanning the distance between them and originating beneath the lobe is a deep median groove bordered on each side by a prominent tumid edge which, with the con- traction of the foot, is folded to give the appearance of fairly regular crenations. This edge is bounded laterally by a more or less straight, thickened ridge. When the edges of the groove are forced apart two or three shallow ridges are seen along its length, though undoubtedly the surface would be smooth in the relaxed state. A similarly modified metapodial area is present in E. adansonianus (Dall, 1889) and M. beyrichi (Woodward, 1901) but does not appear to be developed in P. quoyanus (Dall, 1889). The opercular groove is a slit on the right side of the foot and pro- duces a polygyrous operculum of spiral pattern. The oldest part of the operculum shows a large number of turns which gradually increase in breadth as in trochids. In the peripheral younger part, the turns are considerably broader, suggesting that at a certain age there was a rapid increase in the extent of the groove. This increase is greater than sug- gested by Woodward's figure of the operculum of M. beyrichi. As in this species the operculum is not big enough to close the opening to the shell through which the animal had retracted so that a fully retracted animal lies high up the body whorl; when extended it would presumably protect the metapodium against mechanical injury which might be caused by the shell rubbing against it. Anterior to the opercular lobe the metapodial surface is smooth. Here the posterior part of the double columellar muscle passes dorsally to a right and left attachment on the shell; the right muscle is the larger. A double columellar muscle may be present in other pleurotomarians though no mention has been made of it by previous authors. The pallial complex of M. beyrichi has been admirably described and that of M. amabilis appears to agree in general layout (Fig. 2). There are, however, further details to be added, especially in connection with the vascular system, and some different interpretations of the structures. 176 Bulletin of Marine Science of the Gulf and Caribbean [14(1) hv sn r rkp lav 17 mm FIGURE 2. M. amabilis. Dissection to display the contents of the mantle cavity. The ctenidial axes, especially the left one, are considerably contracted. The mantle cavity is deep, narrowing posteriorly, and the gills do not extend into the posterior third. Each ctenidium, except for the free anterior tip, is attached to the mantle skirt by an afferent membrane, and through this runs the efferent branchial vessel (ev). The afferent vessel can be seen along the afferent edge of the ctenidial axis. There is no afferent membrane.