BIVALVES — THEIR DIGESTIVE ORGANS. 299 



In many of the solitary Ascidiae, the stomach is enve- 

 loped in a large liver,* which pours the bile directly into it 

 through several orifices ; and, in others, the parietes of the 

 intestine are also thickened by a glandulous tissue, which 

 probably secretes some liquor essential to proper digestion : 

 but there is no liver in the social Alcyoneae, or only some 

 obscure traces of it in a few species, as in Diazona violacea ; 

 to the intestine of which, a little underneath the pylorus, are 

 appended some little greenish tubes, simple, bifid, or trifid, 

 which, Savigny conjectures, may be hepatic. f There is 

 also an essential difference in the position of the viscera in 

 the two families ; the Ascidiae have the abdominal viscera 

 applied entirely against one of the sides of the branchial 

 sac, beyond the base of which they do not project ; on the 

 contrary, the abdominal viscera of the Alcyoneae are with- 

 out and under the sac from which they are dependent, 

 and often separated by a distinct pedicle, the terminal por- 

 tion of the intestine being the only part which is connected 

 with the thorax. There are, however, some intermediate 

 species to show that this distinction is one of inconsiderable 

 importance in their economy. 



In the bivalved mollusks, the mouth, in the form of a 

 transverse slit, is placed at the anterior part of the animal, 

 deeply hidden between the foot and the anterior retractor 

 muscle in the Dimyaria, and under a kind of cowl formed 

 by the mantle in the Monomyaria. There is thus no neces- 

 sity that in these animals, the nutritive fluid should pass 

 over the branchiae, but it is probable that the greater part 

 really does so, for the current that enters by the respiratory 

 siphon is driven forward and amid the gills, whence some 

 part of it seems to be directed to the mouth, by the motions 

 of the cilia, and by the movements of the labial palps, of 

 which there are two on each side. These labial appendages 

 are triangular in shape, and very variable in size ; they are 

 scored and ciliated, particularly on the inner surface, in the 

 manner of the branchiae, with which their connection is 

 often very intimate ; and they are almost always very soft, 

 and directed backwards ; but, in the Nucula, they are rigid, 

 and pointed towards the mouth, similating a sort of jaws. 

 There are no salivary glands, for, being destitute of any hard 

 parts about the mouth for comminuting alimentary sub- 

 stances, glands for pouring in a fluid to blend with the food 

 during that operation are not wanted. The oesophagus, 



* In Boltcnia there is no liver. — Savigny's Mem. ii. 88. 

 t Mem. ii. 37. 



