MOLLUSCA. 203 



the liver does not possess a distinct portal system, as in 

 Vertebrates. (Fig. 102.) 



FIG. 102. Anatomy of Turbo Pica : /. Foot. o. Operculum. t. Proboscis, ta. Tenta- 

 cula. y. Eyes. m. Mantle opened longitudinally, to show the disposition of the respira- 

 tory cavity, f. Anterior border of the mantle, which, in its natural position, covers the 

 back of the animal, leaving a wide slit by which the water enters the branchial cavity. 

 b. Gills, vb. Branchial vein, returning to the heart, c. ab. Branchial artery, a. Anus, 

 z. Intestine, e. Stomach and liver, ov. Oviduct. On the upper side of the neck are seen 

 the cephalic ganglion, and the salivary glands ; and at d. is shown a fringed membrane, 

 which forms the lower border of the left side of the opening that leads to tne respiratory 

 cavities. 



The univalve shell is generally a coiled tube, wound 

 round a central axis, or columella; the nucleus, or earli- 

 est part of the shell being at the apex, and the portion 

 last formed being the open mouth at the lower part, 01 

 base. The direction of the coil may be concentric, form- 

 ing a discoidal shell, as Planorbis, but it is generally a 

 true spiral. The mouth, or aperture, of the shell is en- 

 tire in most vegetable-feeding Gasteropods, and notched 

 or produced into a canal for the siphons in the carniv- 



