MOLLUSCA — CHITONS 207 



which passes nearly all of the blood just before entering the 

 auricles on its way back from circulation throughout the body. 



The excretory system consists primarily of renal sacs or kid- 

 neys, — tubes which in most mollusks connect the body cavity 

 with the exterior through the mantle cavity. Nearly all of 

 the venous blood traverses the kidneys on its way to the gills 

 and is by them purified of the nitrogenous waste products of 

 metabolism. 



The nervous system consists essentially of a nerve ring sur- 

 rounding the cesophagus, giving off four nerve cords, — ■ a 

 pair to the dorsal region of the body and a pair to the ventral 

 region. 



The sexes are usually separate. The union of spermatozoa 

 and ova is followed by laying of eggs, usually in a gelatinous or 

 leathery matrix. 



Most mollusks (Cephalopoda excepted) in their development 

 pass through a larval stage, the veliger, in which the tiny, free- 

 swimming form possesses a thickened retractile rim in front of 

 the mouth, which bears a circlet of cilia and is called the velum. 

 A shell gland on the dorsal surface secretes the shell, character- 

 istic of most Mollusca. 



Derivation of name. — Latin molluscum, a soft-bodied animal. 



The Mollusca are divided into the following classes ; the 

 names are mainly references to the form assumed by the foot : 



Page 



A. Amphineura 207 



B. Pelecypoda 208 



C. Gastropoda 234 



D. Scaphopoda 250 



E. Cephalopoda 251 



CLASS A, AMPHINEURA (CHITONS) 



The Amphineura are distinguished by their bilaterally sym- 

 metrical body, with mouth and anus at opposite ends. They 

 possess a foot adapted for creeping. The mantle surrounds 



