| OCR Text |
Show 1878.] THE A N A T O M Y O F P L O T U S LEVAILLANTI. 681 corresponding to the surface between the limbs of the U, apparently not glandular in nature. All these structures are covered with the tough gastric epithelium, which ceases just above them. The first true gastric cavity is larger than the second, though not much so. In the second the peculiar hairy covering of its pyloric portion is largely developed, and in a different manner from what it is in P. anhinga, where, as I have shown in m y paper on the anatomy of that bird, it forms a kind of sieve to prevent large solid particles from entering the duodenum. In P. levaillanti a more elaborate arrangement obtains ; the hairy epithelium surrounding the pyloric orifice, near the lower margin of the gastric surface of which" it is developed, is produced into a considerable conical hair-covered process, projecting into the second stomach, and evidently acting as a valve to close the pylorus when necessary. In general appearance it much resembles the operculum of the Cheilostomatous Polyzoa, and is very striking at first sight, the hirsute conical plug when retracted, fitting exactly into the equally hirsute conical pyloric end of the second stomach-cavity. All the rest of the second stomach is lined with a non-hirsute epithelium, which ceases abruptly where it meets the hairy surface. I can find no trace of this operculum in Plotus anhinga, upon re-examination. The small intestine measured two feet, and the large three inches ; but they may have been contracted by the inflammation of their surfaces. Two minute caeca were clearly seen, one a little larger than the other. In P. anhinga there is no indication of a second caecum. As in P. anhinga, P. levaillainti possesses but one carotid artery, the left. In their myology the two species agree in every respect, as far as I can see. In P. levaillanti the ambiens is large, grooving the patella, the femoro-caudal is present without an accessorius, as is the semitendinosus. There is a slip from the biceps of the arm, which traverses the patagium ; and the temporal muscles run back beyond the skull, being separated by a median fibrous raphe, which is not ossified into a separate bony style. The great pectoral muscle is formed of two layers. Donitz's bridge is ossified, as in the specimen described by the author after whom it is named: it is developed on the ninth, and not on the eighth cervical vertebra, as I predicted would be the case. The lower larynx is indistinguishable from that of P. anhinga. It is interesting to notice that the Manatee and Dugong have special gastric gland-structures, the method of arrangement of which differs in exactly the same way as does that of the two species of Plotus under consideration, the peculiar flat gland-area found in Halicore and Plotus levaillanti being converted into a glandular cavity in Manatus and Plotus anhinga. PROC. ZOOL. Soc-1878, No. XLIV. 44 |
