168 



COMPARATIVE ZOOLOGY. 



The very lowest animals, like the Amceba 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 ot 

 such a system. It consists 

 PIG. 135. Nervous Sys- O f a r j n or around the mouth, 



tern of Clam : c, cere- t ' 



bral ganglion ;p,ped- made of five ganglia of 

 al ganglia ; ps, parie- , . , , . . 



tospianchnic ganglia; equal size, with radiating 

 from cerebral to pedal distinguished by an irregu- 



ganglia;X,commis- 



sure from cerebral to larly scattered nervous sys- 



parietogplanchnic ,, / ~ 1 , ., 



ganglia; oe, oesopha- tern. The Clam has three 

 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- FIG. ise. Nervous 

 fish, where the brain is partly enclosed in a 

 cartilaginous box (Fig. 151). 



In the simpler worms there is but a sin- head - ganglion. 

 gle ganglion or a single pair. The Earth-worm has a pair 

 of brain-ganglia lying above the gullet, and connected by 



