9^ 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



3. Radiata. — We include in these the echinoderms 

 and the coelenterates, as having the same general plan 

 of nervous system. Here again we have the whole plan 

 of structure of the animal different. We have again 

 segments of the body, not, however, repeated in a linear 



Fig. 66. — Diagram of nervous system of a squid : eg, cephalic ganglion ; 

 oeg, oesophageal ganglion ; mg, mantle ganglion. 



series, but in a circle about the stomach, like the seg- 

 ments of an orange about the pit or the spokes of a 

 wheel about the hub. Therefore we find that the nerv- 

 ous system follows the same plan (Fig. 67). 



General Plan. — Take a starfish as an example. There 

 is a ganglion at the base of each arm, and therefore five 



surrounding the mouth, con- 

 nected into a perfect oeso- 

 phageal collar. From these 

 ganglia there go nerves, one 

 to each arm, giving branches 

 to the arm and terminating at 

 the extremity in an eye spot. 

 In the medusae we have the 

 same perfect radiated struc- 

 ture, but the nervous system 

 has as yet no general center. 

 From the several eye spots on 

 the margin of the disk there go nerve cords a little way 

 toward the center of radiation, but they do not meet 

 one another in a common center. It is difficult to see 



Fig. 67. — Diagram of the nerv- 

 ous system of a starfish. 



