M O L L U S C A. 



617 



Moiiuie*. blanes et tres-transparens, et on ne les distingue bien 



x ^, qu'a la loupe. Leur surface est conune rugueuse, et 

 leur extremity est un peu renflee. Us sont retractile?. 

 L 'animal les fait sortir a volonte, un, deux, trois en- 

 semble : il les agite et les contoume sans cesse en di- 

 vers sens : ce qui fait qu'on les prendroit pour tie petits 

 vers. Je preiiime que par ce mouvement ces organes 

 separent de 1'eau 1'air que y'est contenu et 1'absorbent. 

 Cet animal est tres sujet, ainsi que les autres gasteropo- 

 des fluviatiles, a etre infeste par le nais vermicular is qui 

 se loge ordinairement entre le cou et le manteau, au 

 dessoin de? tent icules, et s 'agite sans cesse d'un mouve- 

 ment vertniculaire." Little doubt, we think, can be en- 

 tertained that this naturalist had been deceived by 

 some of the parasitical leeches which infest the aquatic 

 pulmonifera, and that instead of breathing by means 

 of tubular gills, the animsl of the L. auricularius pos- 

 sesses, like those which it resembles in other charac- 

 ters, a pulmonary cavity. 



ffcyta, !' J '"vv. The shell of the animah of this genus 



nearly resembles that of the preceding, with these differ- 

 ences, however, that the whorls are sinistral, and the pil- 

 lar lip is destitute of a fold. The external appearance of 

 the anitnil is similar to Lymneus, but the margin of the 

 cloak it looe, divided into lobes, and capable of being 

 relectr I over the surface of the shell near the mouth. 

 The Ilalia fomtinalit of British conchologists, is regard- 

 ed as the type of the genus which was instituted by 

 Dranarnaud. 



Ipleza. U I. APLIX A. The shell of this genus is likewise si- 



nistral, but it is more produced th in the former, and 

 poueoes a fold on the pillar lip. The cloak of the ani- 

 mal is incapable of being reflected upon the shell, and 

 its margin U destitute of lobes. This genus ( from *. 

 and *!<<< lacinia) has been instituted for the reception 

 of the Eulla hypttnrum and rirulu of British authors. 



>ooiula. IV. CoNOvti.A. This genus was instituted by La- 

 mark for the reception of several species of fluviatile 

 ru/uiii as the aurit-mida. The shape of the shell is 

 conical, the spire forming the base. The pillar is fur- 

 nished with ridges. The form of the inhabitant is un- 

 known. 



"jn^r , . V. PLAXORBH. Independent of the character deri- 

 ved from the spires of the shell, revolving in nearly 

 the same horizontal plane, the animal is furnished with 

 tenUcula, which, unlike those of the preceding genera, 

 arc long and filiform, with the eyes, however, placed as 

 u* ual on their internal base. It is tinistral genus, the 

 anus, pulmonary cavity, and sexual organs, being on 

 the left, and the heart on the right side. Some of the 

 species, particularly /'. content, pour forth, when irri- 

 tated, a purple fluid from the tides between the foot 

 and the margin of the cloak. In intern*! structure, the 

 animal* of this genus resemble those of the genus Lym- 

 neus. The British species are nine in number, and re- 

 side in 'hollow ponds and ditches, among aquatic 

 plants, on which they feed. 



\fgrntn- \ ' SroMBKTiXA. We instituted this genus several 

 years ago for the reception of the \iiiiilni lacuttrit of 

 Ltghtfoot, first described and figured in Phil. Tram. vol. 

 Ixxvi. p. I(>O. tab. i. fig. 1 8. The shell externally bears 

 a clone resemblance to a Plannrbis, but, internally, it is 

 divided into distinct chambers by solid transverse sep- 

 ta, which communicate with ich other by a triradia- 

 ted aperture. The animal possesses the same filiform 

 tentacula with the eye* at their base, which distinguish 

 the pecies of I'lanorbi*, but it is uncertain whether the 

 openings on the tide be dextral or sinistral. 

 vou xiv. PART n. 



VII. ANCVLUS. The shell in this genus is a short Mollusc*. 

 compressed cone, nearly resembling the common Lim- ^~JJ~"~ 

 pet. The tentacula are short, compressed, a little trun- 

 cated, with eyes at the internal base. The foot is short 

 and elliptical. This genus was instituted by Geoffrey, 

 and includes the Patella lacustrit and oblonga of British 

 writers. 



SUBDIVISION II. GASTEROPODA BRANCHIFERA. 



The animals of this subdivision are chiefly inhabi- Gaiteropo- 

 tants of the ocean j a few, however, are found in fresh- da branchi- 

 water lakes and streams. They admit of distribution cra - 

 into the following orders. 



I. ORDER. Nudibranchia. 



This order chiefly contains those animals which Lin- Nudibraa- 

 neus included in his genus Doris. Since the writings chia. 

 of the illustrious Swede, many new species have been 

 discovered, rendering the construction of several new- 

 genera necessary. 



All the animals of this order are destitute of a shell 

 or corneous plate on the back. The hend is furnished 

 with tentacula. The branchiae are external, and consist 

 of simple or corrlpound filaments arising from the cloak 

 on the sides or back. In many species, the back is co- 

 vered with perforated papillae, which probably give rise 

 to mucous secretion. All the species are hermaphro- 

 dite, with reciprocal impregnation. The external open- 

 ing of generation is situated at the right side. In two 

 genera, Doris and Polycera, the anus is situated on the 

 back, and surrounded by a disk fringed by the bran- 

 chiae. In the other genera the anus is situated on the 

 right side. 



1. DORIS. This genus, as now restricted, contains Doris. 

 such animals as have the anus on the back towards the 

 tail, surrounded only with a fringe of plumose bran- 

 chiae. The cloak is more or less covered with papilla?, 

 which the animal has the power of contracting at plea- 

 sure. It is divided from the foo: by a distinct duplica- 

 ture. Towards the anterior margin are the two su- 

 perior tenUcula, which are likewise retractile. Each, 

 at the base, is surrounded with a short sheath, support- 

 ed on a slender stem, with an enlarged compound pli. 

 catcd summit. The neck is short, and above the mouth 

 is a small projecting membrane connected at each side 

 with the inferior tentacula. These tentacula are in ge- 

 neral small, and in some cases it is difficult to detect 

 them. 



The mouth i in the form of a short trunk, leading 

 to fleshy lips, within which the tongue is placed. It is 

 covemi with minute reflected hairs, and, from its mo- 

 tion, appear* to be destined exclusively for deglutition. 

 The gullet is a simple membraneous tube, terminating 

 in a stomach presenting on the interior a few longitu- 

 dinal folds. It is furnished with a small ciivum, whose 

 extremity receives the bile from the liver. The stomach 

 likewise receives the secretion of another gland, which 

 is not connected with the liver. It is in the form of a 

 small bag, whose inner surface is covered with nume- 

 rous papilla;. The intestine is lodged in a groove on 

 the surface of the liver, and proceeds directly to the 

 anus. 



The liver itself is divided into two lobes, and 

 gives rise to numerous biliary ducts, which proceed 

 to the stomach. But it likewise gives rise to a duct 

 which proceeds to a small bag plaited on the inside, 

 and afterwards opens on the surface at a small hole 

 near the anus. It yet remains to be determined, whe. 

 4l 



