278 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



gans concerned with the distinctive animal life. Animal 

 life and activity has to do specially with action and reac- 

 tion between the animal and its environment. The limbs 

 especially take hold on the environment and are modi- 

 fied by it. The vegetative organs, on the contrary, are 

 little affected by the environment. Segmented structure 

 is peculiarly adapted for diversified modification, showing 

 homology. Now, in articulates the distinctive animal 

 functions are very highly developed in proportion to the 

 vegetative, and these are therefore the most perfectly 

 segmented of all animals. In mollusks, on the contrary, 

 the vegetative functions are very highly developed in 

 proportion to the distinctively animal, and these do not 

 lend themselves readily to homology. The vertebrates 

 seem in this regard to combine the characters of both. 



3. RADIATA. 



In these, including echinoderms and ccelenterates, 

 we again find an entirely different type of structure. We 

 find, indeed, segmentation again, but of an entirely dif- 

 ferent kind. All these animals have an essentially radi- 

 ated structure. Like the plugs of an orange arranged 

 about the central pith, or like the spokes and fellies of 

 a wheel about the central hub, so are the several seg- 

 ments of a radiated animal symmetrically arranged about 

 its central mouth and stomach, and all the organs of the 

 body are repeated in each segment. Thus, in a starfish, 

 for example, the mouth and the stomach are in the cen- 

 ter and surrounded by the nerve centers (oesophageal 

 collar) and by the blood centers. From this center go 

 to each arm or segment a branch of the stomach, a 

 branch of the nervous system from a corresponding gan- 

 glion of the collar, and a great branch of the blood sys- 

 tem. Each arm contains also its own equal part of the 

 respiratory system and the reproductive system. The 



