378 



COMPARATIVE ZOOLOGY 



a definite muscular structure, and show definitely coordi- 

 nated muscular movements. In the starfish we detect 

 the first clear specimen of such a sys- 

 tem. It consists of a ring around the 

 mouth, made of five ganglia of equal 

 size, with radiating nerves. The mol- 

 lusks are distinguished by an irregu- 

 larly scattered nervous system. 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 syphons. 

 In the snail, these are 

 united into a ring around 

 the gullet, and there are 



FIG. 332. Nervous Sys- - ,. , 



tem of ciam; , , cere- other ganglia scattered 



bral ganglion ; /, pedal t h rOU gh the body. The 

 ganglia ; ps, pane- J 



tosplanchnic ganglia ; 

 c' , cerebral commis- 

 sure ; p', commissure 

 from cerebral to pedal 

 ganglia ; ps' , commis- 

 sure from cerebral to 

 parietosplanchnic 

 ganglia ; es, esopha- 

 gus. 



same is true of the cuttle- 

 fish, where the brain is 

 partly inclosed in a carti- 

 laginous box (Fig. 348). 

 In the simpler worms 

 there is but a single 

 ganglion or a single pair. The earthworm 

 has a pair of brain ganglia lying above the 

 gullet, and connected by two cords with a 

 ventral chain of ganglia one pair, appar- 

 ently a single ganglion, for each segment, 

 In the lower arthropods, such as Crustacea, 

 centipedes, and larval insects, the arrange- 

 ment is substantially the same. In higher 

 insects and Crustacea, many of the ganglia are fused to- 

 gether in the head and thorax, indicating a concentration 

 of organs for sensation and locomotion. 



pillar (Sphinx li- 

 gustrt): the first 

 is the cephalic, or 

 head, ganglion. 



