BIVALVE S, 
17 
veniently used. A much higher magnifying 
power is required to observe the movements of 
each cilium. 
The nervous system consists of three pairs of 
white nervous centres, called ganglia , from which 
the nerves radiate. Two ganglionic masses (the 
cerebral) are situated on the sides of the mouth, 
and are united to form a ring around the gullet ; 
they are connected with two others (pedal) in the 
foot, and with those (the branchial) placed a little 
below the posterior adductor muscle near the 
anal orifice. This distribution of the nervous 
centres is eminently characteristic of the sub- 
kingdom Mollusca. 
Organs of Sense,- —The organs of hearing are 
two little sacs, situated on a pair of nerves 
arising from the ganglia of the foot, and contain 
a fluid in which is suspended a small calcareous 
body or otolith. The organs of sight are in the 
form of black dots, or ocelli , along the margin of 
the mantle. 
Muscular Organs .-—The ventral portion of 
the body is prolonged to form a fleshy tongue¬ 
shaped foot (/), and has some analogy with the 
Gasteropods: this locomotive organ is large, 
flexible, and muscular, and capable of being 
protruded beyond the shell. Change of position 
is effected by the insertion of the foot beneath 
the shell, and, by being employed as a lever, the 
c 
