86 TRANSPORTATION SYSTEM IN ANIMALS 



Within this system the food is not only digested, but is 

 circulated throughout its entire extent by means of mus- 

 cular contractions of the enclosing wall, thus enabling 

 every cell of the digestive tract to secure a portion of the 

 food supply. The excess diffuses through the intestinal 

 walls, supplies the cells of the immediate neighborhood 

 and still farther on enters numerous liquid-filled cavities 

 between the outlying body cells, that are thus supplied. 

 In these animals, therefore, the transport of food ma- 

 terials falls upon the digestive tract and to a less extent 

 upon the body fluid that is kept in motion merely by the 

 movements of the body. 



Fig. 15. — Common Water Flea (Daphnia) showing heart {h) in 

 the midst of a large blood space traversed by the intestine (d). 



Moderately Complex Transportation Systems. — 

 Above this point there are several interesting types of 

 transportation systems, and of these we shall examine 

 one representing a halfway stage between the flatworms 

 and the higher classes of animals (Fig. 15). It exists in 

 the crabs, insects, spiders and their allies, and accordingly 

 is characteristic of two- thirds of all the known species of 

 animals. In these creatures there is an external body 

 wall, enclosing a cavity spanned by the digestive tract and 

 penetrated by numerous muscles that nevertheless leave 

 a continuous though irregular space of relatively greater 

 volume than in the flatworms. It likewise holds a fluid, 

 but as special cells or corpuscles exist in it, and since it 

 has a fairly definite chemical composition, this fluid is 



