CLASS OF THE CEPHALOPODA. 507 



of two auricles, is found in the course of the arterial blood, 

 transmitting this liquid into all parts of the body, from whence 

 it returns to the organ of respiration by canals more or less 

 incomplete. We sometimes also meet with, at the base of the 

 vessels entering the organ of respiration, contractile venous 

 reservoirs called pulmonary hearts. 



With regard to the arrangement of the organs of respira- 

 tion, it varies too much to allow of us describing it in this 

 place. We shall therefore only say that they have sometimes 

 the form of lungs, at others that of branchiae or gills. 



596. In like manner we cannot say anything generally 

 of the structure of the organs of sense, which, however, are 

 always less complete than in vertebrate animals. Certain 

 mollusca seem to be gifted only with the senses of touch and 

 taste ; but a great number have eyes, whose structure varies, 

 and in many of them there even exists an apparatus for- hear- 

 ing ; but none have yet been found possessing a special organ 

 for smell. 



The mollusca spring from eggs, and never multiply by 

 granulations, as happens in most of the molluscoides, but 

 sometimes these eggs are hatched externally, sometimes in 

 the interior of the body of their parent, which may then be 

 said to be ovoviviparous. 



597. The sub-division of the molluscs properly so called, 

 is composed of four principal groups or classes, called the 

 cephalopoda, gastropoda, pteropoda, and acephala. We shall 

 now make known their more prominent characters. 



CLASS OF THE CEPHALOPODA. 



598. This class is composed of mollusca of an extremely 

 odd form, for their head is placed between the trunk and the 

 feet, or tentacula, serving for locomotion ; and when they 

 walk, it is with the body upwards and the head downwards 

 that they draw themselves along the soil (Fig. 185) ; in fact, 

 it is on the head, around the mouth, that their feet are in- 

 serted; and hence their name of cephalopoda. The trunk of 

 these animals is covered by the mantle, which has the form of 

 a sac, sometimes almost spherical, at others elongated, open 

 in front onl} r , and enclosing all the viscera (Fig. 533, o). The 

 head projects through this opening; it is round, and is gene- 

 rally furnished with two large eyes (Fig. 10), of a structure 

 very analogous to that of the eyes of vertebrate animals. The 



