68 ZOOLOGY 



as the circulatory system. Pouches and outgrowths from the 

 wall of the mesenteron are of common occurrence. These 

 serve to increase the glandular or secreting surface, the absorbent 

 surface, and also to retain the food longer in contact therewith 

 by retarding its passage through the canal. The removal of 

 the digested food from the canal may be effected by absorption 

 or by the active engulfing of food by the entodermal cells, much 

 as is done by the amoeba. 



c v 



g 



FIG. 31. Stenostoma (after Hertwig). In this Turbellarian the digestive tract (d.t.) is a blind 

 sac. st., boundary of stomodseum and mesenteron; c, cilia; g, ganglion (brain); g' t ganglion of a 

 new individual which is being formed by fission; o, mouth; o', mouth of new individual in process 

 of formation; w, excretory system. 



Questions on the figure. How much of this digestive tract is lined with 

 ectoderm? Which portion with entoderm? Is there a proctodaeum? What are 

 the evidences that the worm is in process of division? Compare this digestive 

 tract with those in Figs. 81, 87, 95, 101. 



94. The Respiratory System and Function. In addition 

 to its other food requirements, all protoplasm, in proportion 

 to its activity, must have free oxygen. This is obtainable from 

 the air or from the oxygen dissolved in water. Oxygen, being 

 a gas, must enter the system in a somewhat different way from 

 that by which fluids and solids are ingested. It is best obtained 

 by absorption through moist, thin-walled membranes. Such 

 surfaces, in connection with which blood vessels are usually 

 found, constitute the respiratory organs. Any exposed sur- 

 face meeting these requirements may serve as such. The 

 general surface of all animals is respiratory in some degree. 

 In the more complex animals, however, special additional 



