220 Natural History of Molluscous Animals : — 



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

 are also thickened by a gland ulous tissue, which probably 

 secretes some liquor essential to proper digestion : but there 

 is no liver in the social y^lcyoneae, 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. {Mem., vol. ii. p. 37.) 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 ^Icyoneae are without and under the 

 sac from which they are dependent, and often separated by a 

 distinct pedicle, the terminal portion of the intestine being the 

 only part which is connected with the thorax. There are, 

 however, some intermediate species to show that this dis- 

 tinction is one of inconsiderable importance in their economy. 

 The Bivalved Mollusca present some material differences in 

 the structure of their alimentary canal. The mouth is always 

 separate from the branchial aperture, and leads only to the 

 proper stomach ; it is very wide, never cut into starlike seg- 

 ments, nor guarded interiorly with ciliary segments, but, on 

 the outside, is furnished with four compressed lobes, which 

 seem to perform the office of lips rather than of tentacula * ; 

 and serve, by their constant play, to force the nutrient fluids 

 into the mouth ; for these are not sucked in with the current, 

 but swallowed by the muscular efforts of the gullet. These 

 labial appendages are triangular in shape, and very variable 

 in size; they are scored, 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 Niicula, they are rigid, and 

 pointed towards the mouth, simulating a sort of jaws. (Blain- 

 ville, Man., p. 121.) The anal aperture, unlike that of the 

 Tunicata, is situated on the side of the body opposite to the 

 mouth, and opens into a common excrementitious tube, the 

 external orifice of which is often surrounded with a fringe of 

 numerous short tubercles or fleshy filaments. The liver is 

 always present, and closely invests the stomach, into which 

 the bile is poured through several large pores; and, in the 



* Cuvier expresses a different opinion. " Aux cotes de la bouche sont 

 quatre autres feuillets triangulaires, qui sont les extremites des deux levres, 

 et servent de tentacules." (Beg. Anim.^ vol. iii. p. 117.) [" At the sides of 

 the mouth are four triangular leaves, which form the extremities of the 

 two lips, and serve for tentacula."] 



