DIGESTIVE ORGANS. 101 



resembling the gizzard of a fowl performs the dat}' of commi- 

 uutiiig the food. It is composed of three calcareous plates (xiv, 

 t3), enormous in relative size in Scaphander Ugnarius, which 

 are strong enough to break the shells of the small mollusks which 

 the animal has swallowed entire. In the Aplysia, which is a 

 vegetable feeder, a number of semicartilaginous plates and spines 

 perform the office of a gizzard. Again, in Cyclostoma, Paludina, 

 Trochus, Strombus, the stomach contains a free chitinous stylet 

 of analagous function, notwithstanding the presence of teeth 

 upon the lingual ribbon. 



The intestine in spiral shells may enter the stomach opposite 

 the entrance of the oesophagus, or, in consequence of the bending 

 of the stomach, it most usually enters not far from the oesoph- 

 agus ; it then bends forwards, terminating in an anus situated 

 not far from the respiratory orifice. In the carnivorous species, 

 Murex, Triton, etc., the intestine is short and direct or nearly so, 

 but in the phytophaga it is elongated and usually forms one or 

 more convolutions (xv, 81). In the rhipidoglossate gastro- 

 pods (Turbo, Trochus, Haliotis) the intestine traverses the heart. 

 The intestine may be distinguished into two portions, the small 

 intestine and the rectum, the latter being usually enlarged in 

 diameter, confined to the anal end and straight portion of the 

 tube, and having longitudinal folds of its inner wall. In the 

 female, the vagina is placed alongside the rectum, and in some 

 univalves there are anal glands opening bj^ the anus. 



The anus is simply a round opening closing by sphincter 

 muscles, situated in the anterior part of the respiratory cavity ; 

 and therefore lying on the right side of the animal when dextral, 

 or on the left side when its shell is sinistral. In certain nudi- 

 branchs it is dorsal, and in some of the naked snails, etc., at the 

 posterior extremity. 



Salivary Glands. Usually a pair of these lie along the oesoph- 

 agus (behind the oesophageal ring), and open into it close to 

 its entrance into the oral mass. These glands may be tubular 

 and long, dilated behind as in Strombus, or the posterior 

 extremity cork-screwed as in Voluta (xv, 82), or they may be 

 short or cylindrical or clavate as in Pleurotoma, Littorina, 

 Trochus, etc. Sometimes, as in Dolium, Cassis and Triton, the 

 elongated glands are in two subdivisions, divided by a deep 

 fissure into a small anterior and a larger posterior portion (ix, 

 96). The two glands may also unite over the dorsal side of the 

 oesophagus, into a single mass, from which, however, separate 

 ducts proceed on either side. When two pairs of salivary glands 

 exist, as in Janthina, the anterior pair open near the buccal 

 orifice. 



Souleyet was not able to discover distinct salivarj^ glands in 

 Turbo, but its oesophagus is enlarged just behind the mouth, and 



