xviii INTRODUCTION. 



the lower valve of the shell. Each lobe of the mantle 

 is provided with a spirally coiled arm, which is 

 furnished with filaments (cirri). 



The respiratory organs of the Acephala are gills of a 

 leaf-like form, and consist of a network of blood-vessels ; 

 a continuous stream of water traverses the gills, and a 

 fresh supply of oxygen is thus imparted to the blood. 

 Many of these molluscs are furnished with eye-like 

 protuberances {ocelli) on the mantle, which vary, 

 according to the species, in number, arrangement, and 

 colour. The mantle of the clam {Pecten maximus) is 

 fringed with ocelli of a greenish-blue or purplish 

 colour ; they are arranged in two rows, those in the 

 front row being larger than the others. 



The organ of hearing in the headless molluscs 

 consists of a sac containing small stone-like bodies 

 called otolites. 



With regard to the reproductive system of the 

 Acephala, it has been generally believed that the 

 sexes are in all cases united, and that each individual 

 is capable of fertilizing itself; recent investigations, 

 however, lead to the conclusion that in some instances 

 each animal is either male or female only. Some 

 kinds are oviparous, and others ovoviviparous. 



Cephalic Molluscs. — Cephalic Molluscs are of a higher 

 organism than the Acephala; their nervous system 

 is more fully developed ; they have a distinct head, 

 and usually, tentacles or feelers, on the tips, or some- 

 times at the base of which the eyes are placed ; in 

 some cases, however, the animals are eyeless. Their 

 organs of hearing, like those of the Acephala, consist 

 of stone-like bodies or otolites, which are contained 



