The Classification of Animals. 189 



VII. MOLLUSCA. 



The MoUusca are, as their name denotes, soft-bodied 

 animals, which are generally protected by a shell. As a rule, 

 they are entirely unsegmented; but traces of segmentation 

 occur in the divided shell of Chiton, and in the double 

 excretory system of Nautilus. The coelom is always greatly 

 reduced, the copious lacuna between the organs of the body 

 being a portion of the vascular system, which is well developed, 

 with a heart of comparatively complicated structure. As a rule, 

 the ventral surface of the body projects more or less as a 

 muscular " foot," which is the organ of locomotion. Branphise 

 are present, and consist of processes of the body-wall. The 

 nervous system consists typically of a pair of cerebral ganglia, 

 lying above the oesophagus, connected by a pair of commissures, 

 with the pedal ganglia lying in the foot, and with a pair of 

 chlamydo-splanchnic ganglia in the visceral region. The 

 Mollusca have been primarily divided into the Lipocephala 

 (Mollusca without a head), the Lamellibranchiata, or Pelecypoda, 

 such as Anodon; and the Glossophora, with a head and the 

 characteristic radula ; to this group belongs the snail. We 

 shall divide them into four classes — the Amphineura, the 

 Lamellibranchiata, the Cephalophora, and the Cephalopoda — 

 whose characters, and those of their more important sub- 

 divisions, will now be given. 



Class 1. AMPHINEURA. 



To this class belongs the genus Chiton and its allies ; the 

 Chitons are characterized by the dorsal shell consisting of 

 a number of pieces following each other. The animals of this 

 class are bilaterally symmetrical. 



Class 2. LAMELLIBRANCHIATA, or PELECYPODA. 



To this second class belong all the bivalved Mollusca 

 which are without a distinct head and possess no radula. 

 They are bilaterally symmetrical; the generative organs are 



