^8 Natural Hhtori) of MoUuscoiis Animals : — 



formation. It is always easy to inflate or inject these crests; 

 and Cuvier hazards a conjecture that they may be secretory 

 organs for the production of the liquid which fills the peri- 

 cardium. 



But a still more extraordinary peculiarity remains for our 

 notice. The large vessel which carries forward the venous 

 blood to the branchiae, and w^hich may be named either a 

 vena cava, or a branchial artery, since it fulfils the functions 

 of both, after sending off arterial branches to the leaflets of 

 the gills, remains for a certain space smooth and entire ; but 

 one part curves itself to the left, and another to the right, and 

 these two branches assume suddenly anew form and structure; 

 they become, in fact, absolutely confounded with the great 

 general cavity of the body. Their walls are now formed of 

 transverse and oblique muscular ribands, which cross in every 

 direction, but leave between them apertures visible to the 

 naked eye, and permeable to all sorts of injection ; thus 

 establishing a free communication between these vessels and 

 the abdominal cavity, so that the fluids contained in the one 

 can readily permeate into the other. This structure is so ano- 

 malous, that Cuvier was for some time doubtful of the accu- 

 racy of the dissections which seemed to prove it; but at 

 last he fully satisfied himself, and ascertained distinctly that 

 there was no other vessel to carry the blood to the branchiae 

 except the muscular and perforated cavities just described, 

 and into which all the veins of the body open directly or 

 indirectly. It follows, therefore, that the fluids shed into the 

 abdomen can mix directly with the mass of blood, and be 

 carried to the branchiae with it ; and that the veins perform 

 the office of absorbent vessels. This vast communication, 

 says the great naturalist from whom I borrow these anato- 

 mical details, is, doubtless, the first step to that still greater 

 which nature has established in insects, where there is no par- 

 ticular vessels for the nutritive fluid ; and we have already 

 seen a trace of it in the Cephalopoda, where the venae cavae 

 and the abdominal cavity communicate together through the 

 medium of certain spongy glands. 



In the genus Orchidium, a naked mollusque, the venae cavae 

 exhibit a formation in some respects similar to that in Aplysia ; 

 but I pass over this, to notice another sort of deviation from 

 the common, in the ear shell (Haliotis), and in some simple 

 univalves ; as, for example, in Fissurella. The heart in those 

 generals provided with two auricles; one of which receives the 

 vein carrying the purified blood from the right branchiae, and 

 the other that from the left. The auricles open into the 

 ventricle each by a single and generally narrow orifice ; but 



