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 

 fig. 135.-Nervo.us Sys- f a Y [ u & around the mouth, 



tem of Clam: c, cere- ° ■ 



brai ganglion ;j?,ped- made of five ganglia of 



al ganglia ; ps, parie- 

 tosplauchnic ganglia ; 

 o% cerebral commis- 

 sure ; p', commissure 



from cerebral to pedal distinguished by an irregu- 



ganglia;ps', commis- O * ° 



sure from cerebral to larly scattered nervous sys- 



parietosplanchnic . mi i ,1 



ganglia; oe, cesopha- tem. The Clam has three 



sus ' 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 



equal size, with radiating 

 nerves. The Mollusks are 



Fig. 136. — Nervous 

 System of a Cater- 

 pillar (Sphinx li- 

 gustri) : the first 

 is the cephalic, or 

 head, ganglion. 



