The Anatomy of Pleurotomaria Beyrichii, Hilg. by Martin F

THE ANATOMY OF PLEUROTOMARFA BEYRTOH1I. 215 The Anatomy of Pleurotomaria Beyrichii, Hilg. By Martin F. Woodward, Demonstrator of Zoology, Royal College of Science, London. With Plates 13—16. THE vast antiquity which characterises the genus Pleuro- tomaria—for no one can doubt the identity of the living and fossil shells which are customarily grouped together under this name—has justly endowed this mollusc with great interest for those studying the ancestry of the Prosobran- chia. When, thei-efore, a living example was obtained by Agassiz in 1871, and later in 1879 several specimens of both P. Quoyana and P. Adansoniana were dredged by the United States steamer " Blake," the result of an investigation of the anatomy of these forms was awaited with great interest. Unfortunately, however, the specimens all turned out to be in a bad state of preservation, and although falling into such skilled hands as those of Dr. Dall, it was found impossible to make out much of their anatomy. Dall, however, published * figures and descriptions of the external cha- racters of the animals, of the radulse and of some few points in connection with the pallial complex, the rest of the body being too much decomposed for investigation. During the last few years a further examination of one of 1 " Report on the Mollusca dredged by the United States steamer ' Blake,'" 'Bull. Mas. Comp. Zool., Harvard,' vol. xviii, 1889. 218 MARTIN P. WOODWARD. second specimen, however, showed that P. Beyrichii, like the two other species mentioned above, possessed a fairly stout though somewhat small operculum attached to the foot by a large circular lobe (figs. 3 and 4). We are, however, still unable to determine whether the first specimen had lost its operculum during its free life, or if it had been born without one. Judging from the presence of the opercular lobe, I should be inclined, in spite of its small size, to suggest that the operculum had been present, but accidentally lost either through disease, or mishap, early in life. The operculum (fig. 4) is nearly circular in outline, measuring, in the largest specimen, 14-5 mm. in diameter; in character it is trochiform, consisting of about twenty closely coiled whorls, strongly marked with line of growth. It is apparently composed solely of dark brown horny (chitinous) matter, and for its size is very thick and strong, retaining its thickness quite to the margin. The mouth of the shell from which the operculum was taken measured 40 mm. in transverse and 30 mm. in vertical diameter. Hence it will be seen that the operculnm can be of very little use in closing the aperture, and thus protecting the retracted animal; it may, however, be of some service in protecting the upper surface of the foot from mechanical injury which might be caused by the rubbing of the shell when the animal was fully extended, since under these con- ditions the shell rests, as in the Trochidse and TurbinidEe, directly upon the operculum. Compared with the opercula of P. Quoyana and P. Adan- soniana, the operculum of P. Beyrichii appears to most nearly resemble that of the first-named species, although Dall in his description does not mention what is such a striking feature in P. Beyrichii, the thickness of the opei-- culum. In P. Adansoniana the operculum is very much larger and thinner, and still more closely resembles the opercula of the Trocbidse. The small size of the operculum in two of the three specimens, and its absence in the third, suggests that this organ THE ANATOMY OF PLEUKOTOMARIA BETRICHTI. 219 is of Very little importance to P. Beyrichii, and that pos- sibly it will disappear in the near future. The Foot.—The foot, although contracted in my specimens, is still very large, and is evidently capable of great extension. As is the case with many Prosobranchs its anterior margin is double (6gs. 1, 2, and 7), the upper surface of the foot being separated from the sole by a well-marked transverse groove. We are quite at a loss to account for this structure, which is evidently of great importance since it is pi'esent in so many Gastropoda. The lateral surfaces of the foot are finely rugose, being closely beset with minute papillae (figs. 1 and 2). These papillaa are wantiug on the dorsal surface, which is separated from the lateral surfaces by the paired epipodial folds (ep.). At the anterior extremity of the dorsal surface is situated the opercular lobe (figs. 2 and 3, op. I.); this in its func- tional condition is circular and nearly as large as the operculum. On the right side it is produced out into a little lobe, which is in turn attached to the upper surface of the foot, and marks the growing point of the multispiral operculum. Behind the opercular lobe a median longitudinal groove leads to the posterior end of the foot; on either side of this is a modified area due to the presence of numerous ti'ansverse grooves originating from the median one; some of these are symmetrically arranged, but others are unpaired (fig. 2). This somewhat y-shaped modified area is bounded in front by a couple of longitudinally-placed bands running, back from under the opercular lobe; these, however, only extend for about one third of the length of this area, which is else- where bounded by a groove marking the commencement of the epipodium. A similar modified area was found by Dall in P. Adansoniana, but strangely enough this appears to be quite wanting in P. Quoyana, a point upon which Dall lays some stress. This is a very curious fact, for in other respects, notably in the operculum and in the radula, as we shall see later, P. Beyrichii is more closely related to P. Quoyana than to P. Adansoniana, a relationship which 220 • MARTIN F. WOODWARD. had already been noted by Crosse and Fischer from a study of the shells, and expressed by the institution of the section Perotrochus for the first two species. This peculiar specialised area is also to be met with in the Trochidae (notably in T. [G-ibbula] magus and T. [Callio- stoma] zizyphinus); but though so commonly present, I am unable to offer any suggestion as to its function. The Epipodium.—This structure, which is so characteristic of the niajorit}' of the Diotocardia, or of that sub- division for which Fischer proposed the name Thysanopoda, is not conspicuously developed in P. Beyrichii. It takes the form of a couple of folds, one on either side of the body. They start a short distance behind the head and attain their maximum development in the region of the operculum; whence they extend back in the posterior extremity of the foot, practically meeting in the middle line behiud the median dorsal groove. These folds, which are evidently somewhat contracted in the spirit specimen, are like the rest of the body covered witli minute papillae, and are entirely devoid of those accessory lapets and tentacles so characteristic of the epipodiaof the Trochidae, Haliotidte, and other Thysanopoda. Judging by the figures given by Dall (op. cit., pi. xxx, figs. 1, 4, and 5) of the living animal of P. Adausoniana, the epipodium would be more conspicuous in the living animal in P. Quoyana; Fischer and Bouvier even speak of it as being largely developed. In comparison with the Trochidse and Haliotidse, however, I should rather conclude that the epipodium was feebly developed in Pleurotomaria. I do not think there is any evidence to support the view advanced by Mitsukuri that these lobes partly envelop the shell, although they are apparently closely applied to its base, and I would rather account for the clean nature of the shell by the habitat of the animal being in deep water— seventy to eighty fathoms,—where life, both animal and vegetable, is not so abundant as in the littoral zone inhabited by the Trochidae, whose shells are so generally encrusted with foreign matter. THK ANATOMY OF PLEUROTOMARJA BBYKICHH. 221 The Head.—The head is large and produced into a somewhat cylindrical snout, bent downwards as in the TVo- chidfe. On the under surface, which is abruptly truncated, is situated the mouth, surrounded by a horseshoe-shaped lip, the gap in the lip being placed ventrally. The head and anterior part of the body are much smoother than the foot, being practically devoid of papillae. The tentacles are situated low down on either side of the head. In form they are cylindrical, with roughly pointed extremities ; they are highly mus- cular and evidently much contracted. The only peculiar feature about them is the tendency which they show to be- come branched at their free ends; this is most marked in the left tentacle of the first specimen examined (fig. 1), but the second specimen also showed indications of a similar condition in both tentacles, although to a lesser degree, while the right tentacle of the third specimen shows no less than two accessory branches. , The Byes.—The eyes, which are small and inconspicuous, are situated, each on a slight elevation, at the posterior side of the base of the tentacle. Examination with a lens shows the cornea to be perforated, as in Trochus and most Dioto- cardia. An examination of sections (fig. 20), however, shows that they are simple in constitution; like those of Trochus the central cavity is only partially filled by a vitreous body, the rest of the space being occupied by sea-water. The eyes, as may be supposed, are not specially well pre- served, but one can see that the retina consists of a series of pigmented rods, turned towards the optic cup, and an external layer of ganglionic cells.

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