62 ZOOLOGY. 



secrete materials which act upon foods in such a way as to 

 render them capable of being absorbed through the entodermic 

 cells into the body cavity, or that special portion of it known 

 as the circulatory system. Pouches and outgrowths from the 



c 



FIG. 29. Stenostoma (after Hertwig). In this Turbellarian the digestive tract (d.t.) 

 is a blind sac. st., boundary of stomodaeum and mesenteron; c, cilia; g, ganglion 

 (brain); g', 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. 79, 85, 93, 99. 



wall of the mesenteron are of common occurrence. These 

 serve to increase the glandular or secreting surface, the ab- 

 sorbent 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 ento- 

 dermal cells, much as is done by the amoeba. 



91. 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 



