616 



MOLLUSCA. 



in any other animal. The first liver is situated near 

 the middle of the body on the right side, while the se- 

 cond is situated near the posterior extremity. The 

 ducts enter the cardiac opening of the stomach, each by 

 a separate hole, and seem to occupy the place of the 

 zone of gastric glands observed in birds. The third 

 liver is placed at the posterior end of the gizzard,. into 

 which it pours its contents by a short duct. 



The most remarkable feature of the circulating sys- 

 tem, is the position of the lungs, at the posterior extre- 

 mity of the body, which occasions a corresponding ar- 

 rangement in the connecting organs. The entrance to 

 the pulmonary cavity is immediately above the anus. 

 The vessels in which the blood is aerated are distribu- 

 ted on the roof and sides of the cavity. The pulmonic 

 veins consist of two receptacles, one on each side, ex- 

 tending nearly the length of the body, which may be 

 considered as vena: cavae. These receive the blood by 

 numerous vessels, and convey it directly to the lungs. 

 The aerated blood is conveyed by a systemic vein in- 

 to a large auricle, seated in front of the lungs, of consi- 

 derable size, with the walls fortified on the interior 

 by branched ligaments. The ventricle is placed at its 

 anterior extremity, and separated by two valves. The 

 aorta arises from the opposite side of the ventricle, its 

 main trunk passing on towards the head. 



The male and female organs of generation, although 

 occurring in the same individual, appear to occupy dif- 

 ferent parts of the body. The opening of the male or- 

 gans is at the tentacula, which leads to a cavity termi- 

 nating in two unequal recesses. The anterior is the 

 smallest, and receives the termination of a vessel three 

 or four times longer than the body itself, which takes 

 its rise at the external base of the cavity itself, appa- 

 rently from the cellular substance, and, after a variety 

 of convolutions inj the neighbourhood of the mouth, 

 opens into the recess. The second recess is the largest, 

 and the vessel connected with it is more complicated. 

 Its origin is in a mass which occupies a considerable 

 portion of the abdominal cavity, and which consists of 

 a vessel forming a great number of complicated convo- 

 lutions, liberally supplied with blood vessels. The duct 

 which proceeds from this mass suffers a sudden thick, 

 ening of its walls for a short space, after which it again 

 contracts, and before it terminates in the recess, in a 

 perforated glandular knob, it contains a pedunculated 

 fleshy body with a sharp-pointed corneous extremity, 

 probably capable of being protruded into the recess and 

 cavity. 



The parts which are considered as forming the fe- 

 male organs, or which are connected with the sexual 

 cavity on the right side of the anus, consist of an ova- 

 rium divided into two lobes, each of which may be per- 

 ceived to be again minutely subdivided. The oviduct 

 is tortuous, and passes through a glandular body, which 

 in the other gasteropoda is regarded as the testicle. 

 The pedunculated vesicle gives out two duets, one of 

 which goes to the testicle, the other to the uterus. It is 

 difficult to form even a conjecture regarding the uses of 

 all this complicated sexual apparatus. The subject can 

 only be elucidated by an attentive examination of the 

 -condition of the organs at different seasons of the year, 

 and studying at the same time the habits of the ani- 

 mals. 



The preceding description of the characters of the 

 genus is taken from the anatomical details of Onchi- 

 dium peronii, a species found creeping upon the rock 

 under water in the Mauritius, by M. Peron. The ge- 

 nus was first instituted by Dr. Buchanan in the Lin- 



nean Transactions, vol. v. p. 132., for the reception of M 

 a species which he found in Bengal, on the leaves of Ty. *~> -* 

 pha elephantina. This species, however, if the descrip. 

 tion be accurate, differs essentially from the one descri- 

 bed by Cuvier, and would lead us to infer that a new 

 genus would be necessary for the reception of the spe- 

 cies of the last-mentioned naturalist. " This is not, 

 (says Dr. Buchanan,) like many others of the worm 

 kind, an hermaphrodite animal, for the male and fe- 

 male organs of generation are in distinct individuals. 

 I have not yet perceived any mark to distinguish the 

 sexes while they are in copulation, as, in both, the anus 

 and sexual organs are placed in a perforation (cloaca 

 commitnis) in the under part of the tail, immediately 

 behind the foot ; but during coition the distinction of 

 sexes is very evident, the penis protruding to a great 

 length, considering the size of the animal." Cuvier 

 seems disposed to suspect the accuracy of these state- 

 ments, and as they do not appear to have been the re- 

 sult of dissection, it would be desirable to have the 

 doubts on the subject cleared up by a re-examination 

 of the animal in question. Dr. Buchanan is probably 

 correct in considering the tentacula as occillated. 



TRIBE II. In this tribe the animals are furnished 

 with a distinct external testaceous covering. The spe- 

 cies which it includes were dispersed by Linna-us 

 through the genera Helix, Bulla, Voluta, and Patella, 

 according to the characters furnished by the shell. 

 They are now arranged under the following genera. 



1. LYMNAEUS. The shells of all the species are dex- Lymn*a>. 

 tral. The animal is constructed so nearly on the plan 

 of the snail, that it will be unnecessary to enter into 

 any details of its structure, farther than to take notice 

 of the more remarkable peculiarities. The tentacula 

 are two in number, laneolate and depressed, and inca- 

 pable of being withdrawn. The eyes are seated on the 

 head, at the inner base, and a little in front of the ten- 

 tacula. The mouth is furnished with three jaws, the 

 two lateral ones simple, the upper one like the slug, 

 crescent-shaped, with a notch in the middle. The male 

 and female organs, though intimately connected, inter- 

 nally, have their external orifices separated to a consi- 

 derable distance, the former issuing as usual under the 

 right tentaculum, the latter occurring in a cavity near 

 the pulmonary opening on the side under the border of 

 the cloak. In consequence of this arrangement, the 

 individuals of L. stagnalis have been observed by Geof- 

 froy and Muller to unite together in a chain during 

 coition, the first and last members of the series exerci- 

 sing only one of the sexual functions, the intervening 

 individuals impregnating and receiving impregnation 

 at the same time. Whether this is the constant or on- 

 ly accidental practice of this species, does not appear to 

 be determined. We know that many other species of the 

 genus are mutually impregnated as usual in pairs only. 



The species of this genus are numerous. They re- 

 side in pools, lakes, and rivers. They furnish a favour, 

 ite repast to the different kinds of trouts and water- 

 fowl. The following are natives of Britain. L. stag, 

 nalis, fragilis, palustris, fossarius, octanfractus, detritus, 

 auricularius, putris, glutinosus, and probably Helix lu- 

 tea of Montagu. 



With regard to the Lymneus auricularius, it woukl 

 appear, from the observations of Draparnaud, " Histoire 

 des Mollusqiics," p. 4-9. that it exhibits a very singular 

 structure of the respiratory organs. We shall quote his 

 own words. " L'animal est pourvu de quatre filamens 

 ou tubes qui partent de la partie superiure du cou, pres 

 du manteau ; ce sont des trachees. Ces tubes sont longs, 



3 



