MOLLUSCA. 179 
SUB-KINGDOM V. MOLLUSCA. 
CHAPTER XIX. 
SUB-KINGDOM MOLLuScA.—The Mollusca (Lat. mollis, soft), 
as implied by their scientific name, are mostly soft-bodied 
animals ; but their popular name of “ shell-fish” expresses the 
fact that their soft body is usually protected by an external 
skeleton or “shell,” All the A/ol/usca are furnished with a 
distinct alimentary canal, which is completely shut off from the 
general cavity of the body (fig. 93, a). There is sometimes no 
distinct blood-circulatory apparatus ; but when there is, its cen- 
tral portion (z.¢., the heart) is placed upon the dorsal aspect of 
Fig. 93.—Diagram of a Mollusc. @ Alimentary canal; & Heart; / Foot; 
z Cerebral ganglion ; z’ Pedal ganglion; #” Parieto-splanchnic ganglion. 
the body. The chief peculiarity, however, of the Mollusca is 
found in the nature of the nervous system. In the lower forms 
g 95, 2 @), the nervous system consists essentially of a single 
ganglionic mass, giving off filaments in various directions. In 
the higher MWo//usca (fig. 93, 2}, the nervous system consists of 
three scattered ganglia, united to one another by nervous cords. 
One of these ganglia is placed above the gullet or cesophagus, 
and is known as the “supra-cesophageal” or “ cerebral” gan- 
glion. A second supplies nerves to the great locomotive organ 
