168 



COMPARATIVE ZOOLOGY. 



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 ot 

 such a system. It consists 

 Fio. i36.-NervonB Sys- of a ring arouud the mouti), 



teni (tf Clam: c, cere- ^ 



biai ganglion ; p.ped- made of five ganglia of 



al ganglia ; 7»s, parle- -, . . , . i • ^ • 



tospiauchnicgsngiia; equal sizc. With radiating 

 ^L^^'^-'l-'LT-^^t nerves. The Mollusks are 

 from cerebral to pedal distinsTuished by an irregu- 



ganglia ; j)s', commis- o ./ o 



Bure from cerebral to larly Scattered ncrvous sys- 



parletoBplanchnic , r^^^ ^-^i i .i 



ganglia; oc, (Esopha- tem. I lie (Jiaui 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. iss. — Nervons 

 fish, where the brain is partly enclosed in a pfiinMSiJ/iwra'T!! 

 cartilaginous box (Fig. 151). r^"''' V^^.«'=" 



o \ o J K the cephalic, or 



In the simpler worms there is but a sin- '"""'• gi'ua'i""- 

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

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



