212 



ZOOLOGY. 



The small mass formed by the first ganglions of this con- 

 nected chain is lodged in the head, and for this reason has 

 been compared to the brain of the vertebrata ; but we find 

 nothing resembling the spinal mar- 

 row, for the rest of the chain of 

 ganglions is situated on the ventral 

 surface of the body under the di- 

 gestive tube (Fig. 163), and the 

 nervous cords uniting them to the 

 ganglions of the head surround the 

 gullet like a collar. All the muscles 

 are attached to the skin, and there 

 is no internal skeleton; but the 

 integuments, by their hardness, 

 form a sort of external skeleton, 

 being arranged in rings more or less 

 moveable on each other. Thus, the 

 annulated or articulated character 

 of these animals may be seen exter- 

 nally; the limbs, in general, are 

 very numerous ; the organs of the 

 senses less numerous and less 

 perfect than in the vertebrata ; the blood is almost always 

 white, and the circulation very incomplete : finally a number 



Fig. 165. Section of a 

 Cuttlefish.* 



Fig. 166. Limnaea Stagnalis; Pond Snail. 



* Ideal section of the body of a'cephalopodous mollusc (a Cuttlefish) : 

 t, arms or tentacula surrounding the head ; b, the mouth ; i, the alimentary 

 canal ; a, the anus ; y, the liver ; c and g, nervous ganglions ; p, branchiae ; 

 s, the heart j v, ink-bag or vesicle ; y, the eyes. 



