MOLLUSCA 283 



612. The cerebral, visceral, pedal, pleural and buccal ganglia are all 

 grouped in the region of the buccal mass. Other ganglia occur at the bases 

 of the arms and in other parts of the body. The sense organs consist of 

 a pair of eyes, a pair of statocysts and a pair of chemical sense organs below 

 the eyes. The eyes are usually very highly developed and have a remark- 

 able resemblance to the vertebrate eye, though fundamentally very 

 different. 



613. An internal skeleton of cartilage supports and protects the eyes 

 and central nervous system. Other cartilages are found at the bases of 

 the arms, at the edge of the mantle and in the funnel and in the fin. The 

 rudimentary shell in most cases serves as a supporting structure. It 

 may be either horny or calcareous. 



Fig. 168. — The pearly Nautilus, a tetrabranch Cephalopod. e, Eye; h, hood; 

 s, siphon; se^ septa forming the chambers of the shell; 5^, siphuncle; t, tentacles. 

 (From Galloway after Nicholson.) 



614. The sexes are separate and dimorphic. The single gonad lies in 

 the end of the visceral sack and its products are emptied into the coelomic 

 cavity. A pair of complicated ducts with associated glands lead from the 

 body cavity to the mantle cavity. One of these ducts is frequently 

 rudimentary or wanting. Development is direct. 



615. The cephalopods are carnivorous, using their arms for catching 

 their prey. When on the bottom the arms are also used for locomotion. 

 By means of the mechanism already described a strong swimming stroke 

 which carries the animal backward is performed. Some species are 

 habitually swimming, others keep close to the bottom. 



