CHAPTEE XIV 



MOLLUSOA 



Chaeacteeistics. — Uivsegmented Coelomata, with a primitive 

 bilateral symmetry. Their iody is soft, and is dorsally 

 produced info a fold, the mantle, which usually secretes a 

 shell. The ventral part of the hody forms, as a rule, a 

 muscular process, the foot, which may be modified in various 

 ways, hut whose function is usually to assist in locomotion. 

 Respiration is typically carried on hy a pair of vascular 

 processes, which project from the hody-wall, and are termed 

 the ctenidia. Near the hase of these organs is a modified 

 patch of epithelium, whose function is olfactory, and this has 

 been termed the osphradiAim. The portion of the body-cavity 

 in which the heart lies, the pericardium, communicates directly 

 with the exterior by means of the nephridia. The heart is 

 systemic, and the circulation partly lacunar. The nervous 

 system typically consists of a pair of cerebral ganglia in the 

 head, a pair of pedal ganglia in the foot, and a pair of 

 pleural ganglia in the body. The last pair are united by a 

 long commissure, the visceral nerve cord, which may become 

 twisted. The sense organs comprise the osphradia, otocysts 

 in connection with the pedal ganglia, tactile tentacles on 

 the head, and in many cases eyes. The developement 

 includes a characteristic larva, the Veliger. 

 The phylum Mollusca includes a large number of animals 

 which exhibit the greatest variety of structure and habit. The 

 majority of them are marine, some inhabit fresh water, and 

 many are terrestrial. The group includes the class Cephalopoda, 

 the members of which are the largest, and at the same time 

 the most ferocious of invertebrates. Some members of the 



