410 GASTEROPODA. 



and, what is still more remarkable, each auricle would seem to com- 

 municate with the ventricle by two distinct orifices. In Haliotis, 

 Fissurelld) and others of the Scutibranchiate and Cyclobranchiate 

 orders, the resemblance to the arrangement generally met with 

 among the CONCHIFERA is even more striking ; for in such genera 

 not only are there two distinct auricles, but the ventricle embraces 

 the rectum, so that, when superficially examined, it seems to be 

 perforated for the passage of the intestine. 



In Pterotrachea (Jig. 188), the branchiae (e) are placed upon 

 the back, and the blood derived from the tufts composing the 

 branchial apparatus is received into a two-chambered heart (e), 

 whence it is distributed to the body through the aorta, which is at 

 first double, but, after surrounding the visceral sac and supplying 

 the viscera, the two vessels unite to form one large trunk (w), 

 which traverses the body as far as the head. 



(445.) The digestive system of the Gasteropoda, as we might 

 be led to expect from the numerous and widely different forms of 

 the animals belonging to the class under consideration, presents 

 endless diversity of structure; and, did we not strictly refrain 

 from noticing any but the most important modifications, it would 

 be easy to overwhelm the most patient reader with accumulated 

 details. 



The mouth we shall consider as exhibiting four distinct types of 

 organization ; one of which, namely that met with in the Snail 

 and the generality of pulmonated Gasteropoda, has been already 

 described ( 430). 



The second form of mouth, that for instance of Pleurobranchus 

 (fig. 186, a, and of Pterotrachea, fig. 188, >), consists of a simple 

 muscular proboscis, or fleshy tube, which is capable of considerable 

 elongation and contraction : such an oral apparatus is entirely de- 

 void of teeth or any cutting instrument, but is, nevertheless, fully 

 able to seize and force into the stomach such materials as are used 

 for food. 



(446.) A third kind of mouth, by no means so frequently met 

 with as the last, is not a little extraordinary, and forms a more 

 efficient cutting instrument than even that of the snail. We shall 

 offer, as an example of this remarkable organ, that of the Tritonia 

 ffombergii, represented in the annexed figure (fig- 190), whereof 

 Cuvier gives the following graphic description.* In this animal the 

 mouth forms a large, oval, and fleshy mass enclosing the jaws and 



* M6moire sur le Tritonia. 



