COMPARATIVE ZOOLOGY. 
168 
The very lowest animals, like the Amoeba and Infuso* 
ria, have no nerves, although their protoplasm has a gen¬ 
eral sensibility. The Hydra has certain 
cells which are, perhaps, partly nervous 
and partly muscular in function. The 
Jelly-fish has a nervous system, consist¬ 
ing of a net-work of threads and ganglia 
scattered all over its disk. We should 
look for a definite system of ganglia and 
nerves only in those animals which pos¬ 
sess a definite muscular structure, and 
show definitely co-ordinat¬ 
ed muscular movements. 
In the Star-fish we detect 
the first clear specimen of 
such a system. It consists 
Fig. 135.— Nervous Sys- 0 f a r j n g around the mouth, 
tem of Clam: c, cere- ° 
braiganglion;p,ped- made of five ganglia of 
tospianchnicganglia; equal size, with radiating 
nerves. The Mollusks are 
from cerebral to pedal distinguished by an irregu- 
ganglia; ps', commis- ° 
sure from cerebral to larly scattered nervous sys- 
parietosplancfhnic . rr,, 
ganglia; oe, cesopha- tem. I lie Clam lias three 
gus ' main pairs of connected 
ganglia—one near the mouth, one in the 
foot, and the third in the posterior region, 
near the siphons. In the Snail, these are 
united into a ring around the gullet, and 
there are other ganglia scattered through 
the body. The same is true of the Cuttle¬ 
fish, where the brain is partly enclosed in a 
cartilaginous box (Fig. 151). 
In the simpler worms there is but a sin¬ 
gle ganglion or a single pair. The Earth-worm has a pair 
of brain-ganglia lying above the gullet, and connected by 
Fig. 136. — Nervous 
System of a Cater¬ 
pillar (Sphinx U- 
gustri): the first 
is the cephalic, or 
head, ganglion. 
