624 



M O L L U S C A, 



. foot behind being furnished with a membranaceous 

 appendage. 



The body is oblong becoming a little narrower in 

 front. Below, the foot is broad, thin, and waved on 

 the margin, expanded on each side behind, and capa- 

 ble of being turned upwards. At the posterior part 

 of the foot, there is a broad membranaceous appen- 

 dage, separated from it by a groove, part of which 

 folds upwards, and a part is spread over bodies, like 

 the foot. It assists in closing the mouth of the shell, 

 and in its position and use is analogous to the opercu- 

 lum, in the following order. Above the foot, in front, 

 but separated from it by a groove, is a flat fleshy ex- 

 pansion, which Cuvier terms the tentacular disk, con- 

 sidering it as formed by the union of the inferior and 

 superior tentacula. In the centre of the disk, in the 

 Bulla hydatis, (Linn. Trans. Vol. ix. Tab. 6. f. 1.) 

 Montagu observed two eyes. Between this portion 

 of the back and the posterior extremity, is the dorsal 

 plate or shell, forming the genus Bulla of conchologists. 

 This shell is covered by the integuments in some spe- 

 cies, while in others it is exposed. But in all, the part 

 containing it is partially concealed by the animal, by 

 means of the reflected margins of the foot, and its ap- 

 pendage. Along the right side of the body there is 

 a groove formed by the foot, and its appendage on 

 one side, and the dorsal plate and tentacular disk on 

 the other. The branchiae are situated in a cavity un- 

 der the shell or dorsal plate, and resemble those of the 

 Aplysia. Behind the gills, in the lateral groove, is 

 the anus, and in front of these, the orifice of the united 

 organs of generation. The penis is removed as in the 

 Aplysia, and connected by a similar slit. 



The mouth is, as usual, in front, above the foot and 

 beneath the tentacular disk, both cf which serve as 

 lips. The cheeks are strengthened on each side by a 

 corneous plate. The tongue is well developed in some, 

 as the B. ampulla, while in the B. aperta it is reduced 

 to a small tubercle. The gullet is large, and in the B. 

 ligniaria makes two folds before entering the gizzard. 

 This last organ is fortified by three testaceous plates, 

 convex and rough on the inner surface, and attached to 

 strong muscular walls. These plates exhibit in the 

 different species considerable varieties of form and 

 markings. The intestine, before terminating in the 

 anus, makes several convolutions in the substance of 

 the liver. The salivary glands exhibit considerable 

 differences. In the B. ampulla, they are long and nar- 

 row, and their inferior extremity fixed to the gizzard. 

 In the B. aperta and //gnaria, they are short, with the 

 extremity free. In the B. hydalis they are long, un- 

 equal, and the extremity of the one, belonging to the 

 left side, is forked. The liver forms a part of the con- 

 tents enclosed in the spire of the shell. It envelopes 

 the intestine, and empties the bile into its pyloric 

 extremity. 



The auricle and ventricle appear to occupy the same 

 relative position as in the Aplysia, but the structure of 

 the arteries is unknown. The organs of generation 

 have also so near a resemblance as to forbid a detailed 

 description. Some species are said to eject a coloured 

 fluid like the Apljsia, from the lid of the branchicc. 

 A gland is observed in the Bulla lignaria, similar to 

 the Aplysia, in which it is probable the fluid is pre- 

 pared. 



The species of this genus have not been sufficiently 

 investigated in a living state. When preserved in spi- 

 rits, it is impossible to form a correct idea of their true 

 appearance, as exhibited when alive in sea- water,, since 



they usually exist as a shapeless mass. Cuvier has Moiiusct. 

 given delineations of such preserved species, but they ""^~Y~^* 

 bear no resemblance to the figures of Montagu, of the 

 same species, taken from living objects. 



M. Lamarck is inclined to subdivide this genus into 

 two, distinguishing those in which the shell is con- 

 cealed by the term Bulloea, from such as have the shell 

 in part exposed, which he retains in the genus Bulla. 

 The Bulla aperta of British conchologists is an exam- 

 pie of the former, and B. lignaria of the latter. The 

 shells of the genus Bulloea are thin and white those 

 of Bulla stronger, more opake, and covered with an 

 epidermis, which, after the death of the animal, ia 

 easily detached. 



VI. DORIDIUM. In the animals of this genus, there Doridium. 

 is no dorsal plate or shell, although there is a cavity 

 with a spiral turn in the cloak. The branchiae and ac- 

 companying organs are nearer the posterior extremity 

 than the species of the preceding genera. 



The digestive system is more simple than in the pre. 

 ceding genus. There is here no appearance of the 

 spinous tongus, the gullet is short, and the stomach is 

 membranaceous. 



The characters of this genus were first developed by 

 Cuvier, but the name which we have adopted is that 

 of Mekel. Only one species, D. carnosum, is known 

 as a native of the Mediterranean. 



The preceding review of the genera of this order 

 indicates the existence of at least three natural fami- 

 lies. The pleurobranchia appears to stand apart, and 

 to have few characters in common with the others, 

 which are all intimately related. But under the head 

 Aplysiae, the genera Aplysia, Dolabella, and Notarchus 

 may be included, while the Acerae will comprehend 

 Bulla, Bulloea, and Doridium. 



IV. ORDER. Cervicibranchia. 



WE have been induced to institute this order, for the Cervici- 

 reception of the genus Valvata of Muller. The species branchia. 

 which it includes are inhabitants of fresh water, and 

 in their general appearance resemble the aquatic pul- 

 moniferous gasteropoda. They are distinguished, how- 

 ever, from all the other gasteropoda, by the position of 

 the branchiae, and the number of tentacula. 



The branchiae appear in the form of a feather, with 

 a central stem, and a row of compound branches on 

 each side, decreasing in size from the base to the extre- 

 mity, which is free. 



This plume is placed on the neck, near the middle, 

 and a short way behind the anterior tentacula. Near 

 this plume, but on the right side, is a single simple 

 filament, like a tentaculum. The anterior tentacula 

 occupy the usual position, and are setaceous. The 

 eyes are placed at their base behind. 



The animal is furnished with a spiral shell, into 

 which it is capable of withdrawing the body. The 

 foot is protected by a spirally-striated, operculnm, 

 which shuts up the mouth of the shell upon the retreat 

 of the inhabitant. 



The species are all of a small size, and their internal 

 structure has never been subjected to a rigorous ex- 

 amination. Two species are natives of Britain. 



1. Valwtla crislata. This is the Helix cristate of 

 Montagu. The shell resembles a planorbis in its de- 

 pressed form, the whorls revolving nearly on the same 

 plane, so that the last-formed one embraces the others. 

 The aperture is circular and simple. It is found in 

 ditches on aquatic plants in England. 



