CHAPTER XIV 
MOLLUSCA 
CHARACTERISTICS.— Unsegmented Coelomata, with a primitive 
bilateral symmetry. Their body is soft, and is dorsally 
produced into a fold, the mantle, which usually secretes a 
shell. The ventral part of the body forms, as a rule, a 
muscular process, the foot, which may be modified in various 
ways, but whose function is usually to assist in locomotion. 
Respiration is typically carrved on by a pair of vascular 
processes, which project from the body-wall, and are termed 
the ctenidia. Near the base of these organs is a modified 
patch of epithelium, whose function is olfactory, and this has 
been termed the osphradium. 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 wnited 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 characteristie 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 
