SYNOPSIS OF THE CLASSES. 



SUB-KINGDOM MOLLUSCA. 



SOFT bodied animals, usually provided with an exo-skeleton ; the 

 intestinal canal bounded by its own proper walls, and completely shut 

 off from the perivisceral cavity. 



DIVISION A ODONTOPHORA. 



Nervous system of three principal pairs of ganglia ; heart well 

 developed, never composed of less than two chambers ; tongue with 

 lingual teeth. 



Cephalopoda. Mouth surrounded by a circle of eight or more arms, p. 1. 



Pteropoda. Swimming by means of two fins on each side of the mouth, p. 5. 



Heteropoda. Swimming by means of a raft-like foot, p. 6. 



Gasteropoda. Head distinct, creeping by means of a foot ; shell never 

 bivalve, p. 7. 



DIVISION B. ACEPHALA. 



Nervous system of three principal pairs of ganglia ; heart of two 

 or three chambers ; no head nor lingual teeth. 



Lamellibranchiata. Body protected by a bivalve shell ; mantle divided into 

 two lobes, p. 58. 



DIVISION C. MOLLUSCOIDA. 



Nervous system of a single ganglion, or of a principal pair with 

 accessory ganglia ; heart imperfect or wanting. 



Brachiopoda. Simple ; body protected by a bivalve shell, p. 85. 

 PolyZOa. Composite, body protected by a horny or calcareous cell, p. 87. 

 TlUlicata. Simple or composite ; body protected by a leathery tunic, p. 104. 

 ii 



