WATER-CANALS. 541 



they will be found to contain more blood than all the rest of the arterial 

 system put together. 



(1430.) Such is the construction of the heart in a great majority of 

 the GASTEROPODA ; but in a few of the lowest orders, namely those most 

 nearly allied to the COISTCHIFERA, slight modifications are met with. 

 Thus, in Chiton (fig. 263), so remarkable from the singularity of its 

 shelly covering, the heart is situated in the middle of the posterior 

 region of the back, and is furnished with two auricles, one appropriated 

 to each lateral series of branchia? ; and, what is still more remarkable, 

 each auricle would seem to communicate with the ventricle by two 

 distinct orifices. In Haliotis, Fissurella, and others of the Scutibran- 

 chiate 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. 



- (1431.) In Pterotrachea (fig. 269), the branchiae (d) 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 (wi), which traverses the body as 

 far as the head. 



(1432.) Independent of the ordinary vascular system, Delle Chiaje 

 discovered the existence in most Gasteropods of a system of water- 

 vessels largely distributed throughout the substance of the foot and 

 other parts of the body. Thus, in the anterior part of the foot of the 

 Muricidae*, there are to be seen certain holes or antra, which are the 

 apertures to as many little cavities lying underneath, and which per- 

 meate the interior substance of the foot. There are, besides, between 

 these cavities slender canals communicating with the same orifices, by 

 means of which the whole are connected and inosculated together. 

 The water entering the body through the siphuncle is thus, at the will 

 of the animal, driven into the substance of the foot, which is in this way 

 rendered turgid and firm ; and when necessary, by a strong pressure, 

 the fluid is ejected, or is spontaneously discharged after death, when the 

 foot becomes flaccid and extenuate. Opinions relative to the use of the 

 water thus freely admitted into the body of the Mollusca are various ; 

 its principal object, however, seems to be to enlarge and moisten the 

 structures over which it is distributed. 



(1433.) 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 . 



* Delle Cliiaje, Anim. senza Vert. d. Nap. ii. p. 204. 



