612 



M L L U S C A. 



Moilufc*. There is a common cavity, a vesicle, penis, vas deferens 



V ""Y"" P '' aud testicle, together with an oviduct and ovarium. 

 These open near the mouth on its ventral margin. 

 There is nothing known with respect to the appearance 

 of the eggs, the period of propagating, or the form of 

 their young. 



All the animals of this class inhabit the sea. Some 

 of those, astheClioandLimacina, frequent thearctic re- 

 gions, and afford the whale a great part of its suste- 

 nance. None of the species of the class have hitherto 

 been detected in the British seas. 



M. Cuvier, in his liegne Animal, II. p. 378, sub-di- 

 vides the genera of this class into two orders. The ani- 

 mals included in the first are furnished with a head dis- 

 tinct from the body, and are arranged under the five 

 following genera ; Clio, Cleodora, Cymbulia, Limaci- 

 na, and Pneumodermon. Those of the second division, 

 which are destitute of a head, constitute only one ge- 

 nus, named Hyalea. 



Clio. i. Clio. In this genus, the body is ovate and elon- 



gated, the tunic membranaceous, and the head divided 

 into two lobes, whose summits are furnished with ten- 

 tacula. The mouth is transverse, with two lateral 

 longitudinal lips. On each side the neck arise two 

 blunt, conical, fin-like expansions with a fine reticulat- 

 ed surface, considered as serving the double purpose of 

 fins and branchiae. The anus and orifice of generation 

 terminate under the base of the right branchia. The 

 viscera do not fill entirely the cavity of the inner bag. 

 The gut makes only one fold. The existence of eyes 

 has not been ascertained. 



The genus Clio was originally instituted by Brown in 

 his Natural History of Jamaica. It was afterwards 

 embraced and modified by Linnaeus and Pallas, in such 

 a manner as ultimately to exclude the species for whose 

 reception Brown originally formed it. It contains two 

 species, the most remarkable of which is the Clio borea- 

 lis. Mr. Scoresby, in his valuable work on the Arctic 

 Regions, lately published, states (vol. i. p. 544.) that it 

 occurs in vast numbers in some situations near Spitz- 

 bergen, but is not found generally throughout the arc- 

 tic sea* In swimming, it brings the tips of the fins 

 almost info contact, first on one side, and then on the 

 other. 



Clecdore. 2. Cleodora. This genus was instituted by Peron 

 for the reception of Brown's species of Clio. The body 

 is covered with a triangular pyramidal tunic, carrying 

 two membranaceous wings, between which is the mouth, 

 furnished with a semicircular lip. 



The species whose characters have been most fully 

 developed is the Cleodora pyramidata. Brown's Jo* 

 maica, p. 386. tab. 43. fig. 1. 



Cymbulia. 3. Cymbulia. The tunic of the species of this genus 

 is trough-shaped, and cartilaginous. The fin-like ex- 

 pansion is single, divided into three lobes, one of which 

 is small, with two tubercles, and a minute fleshy beard. 

 This genus was instituted by Peron, in Annales du Mu- 

 seum, vol. xv. tab. 3. fig. 10, 11. 



Liraacina. * Limacina. This genus is nearly allied to Clio, in 

 the form of the head and wings, but the body of the 

 animal is contained in a tender shell of one turn and a 

 half, flat on one side, with a large umbilicus in the 

 other. When the animal swims, the head and fin-like 

 expansions are protruded from the shell. 



This genus was instituted by Cuvier for the recep- 

 tion of the Clio helicina, an animal first described by 

 Captain Phipps ; and afterwards by Fabricius, under 

 the name of Argonauta artica. According to Mr. 

 .Scoresby, it is found in immense quantities near the 



coast of Spitzbergen, but does not occur out of sight of Mollusta. 



5. Pneumodermon. The body, in the species of this Pneumo- 

 genus, is oval. The neck is narrow, with a fin-like ex- dermon. 

 pansion on each side. The mouth is nearly terminal, 

 furnished on each side with a fleshy lip, and below 



with a fleshy chin. On the summit are two bundles 

 of tentacula, each consisting of a filament, with a tu- 

 bercle at the end pierced by a small hole, and consider- 

 ed as exercising the office of a sucker. Cuvier, in his 

 Memoire stir L'Hayle et Le Pnewnoderme, considered 

 the leaf-like ridges which occur on the caudal extremity 

 of the body, as the branchiae, and even describes thie 

 pulmonary vein which conveys the blood from these 

 to the heart. But, in his Regne Animal, he states it 

 as the opinion of his assistant, M. Blainville, that the 

 fin-like expansions of the neck contain the branchiae 

 on their surface, as in the case of Clio. The rectum 

 and oviduct terminate under the right wing. Cuvier 

 has figured and described the only known species, 

 which he terms Pneumodermon Peronii, the trivial name 

 being in honour of the discoverer, M. Peron. 



6. Hyalea. The animals of this genus, as we have Hyatea. 

 already stated, are destitute of a head. The body is 

 lodged between two plates or valves, united at the 

 base, where they inclose the caudal extremity. The 

 ventral valve is nearly flat, with an uneven margin, 

 narrow anteriorly, but expanding behind, and termina- 

 ting in three projecting points. From the middle point 



four ribs diverge forward, and a muscle arises, which, 

 fixed in the superior viscera, enables the animal to with- 

 draw into the shell. The dorsal valve is shorter than 

 the preceding, the margin fiat and circular, and the 

 middle convex outwardly. In the space between the 

 lateral margin of the two valves, on each side, the 

 branchiae are situated, in a duplicature of the tunic, 

 the sides of which are furnished with filaments. The 

 fleshy neck supports the two membranaceous expan- 

 sions ; between which and the base the mouth is situa- 

 ted, surrounded by two lips, and strengthened within 

 by two fleshy cheeks. The opening of the anus and 

 oviduct are at the base of the right fin. 



The Hyalea tridentata, the best known species of 

 the genus, was first noticed by Forskal, in his Descrip~ 

 tlones Animalium, p. 124, as an Anomia, and inhabit- 

 ing the Mediterranean. 



CLASS III. GASTEROPODA. 



THE animals which are included in this class are Gasteropo- 

 more numerous in regard to species, and exhibit a great- da, 

 er diversity of organization than those belonging to the 

 other great divisions of molluscous animals. They 

 have long occupied the attention of the zoologist, and 

 various methods have been proposed for their artificial 

 and natural arrangement. Their internal structure has 

 likewise been investigated with a considerable degree 

 of success, and the purposes which the different organs 

 serve in the animal economy, have been in a great mea- 

 sure ascertained. 



The gasteropoda present few characters which are 

 common to the whole class. Their body may be re- 

 garded as consisting of a foot, the cloak or tunic, and 

 the head. The foot is a firm muscular expansion, co- 

 vering a part or the whole of the ventral surface of the 

 animal. The viscera and head rests on its central sur- 

 face, while externally it exhibits a soft even disk, capa- 

 ble of acting as a sucker, with which the animal ad- 



