86 



GENERAL ZOOLOGY 



gested remains are cast out through the mouth. From the 

 endoderm, absorbed food is distributed to the ectoderm by 

 cell-to-cell transfer. The simple digestive tube of Hydra may be 

 regarded as an early step in the evolution of the digestive tract of 



Fig. 51. Fig. 52. 



Fig. 51. — A, Hydra, expanded, diagrammatic; B, longitudinal section of body 

 wall, ec, ectoderm; en, endoderm; g, gastro vascular cavity, shown in black; 

 me, mesoglea; m, mouth; t, tentacles. 



Fig, 52. — Digestive and nervous system of a flatworm, Euplanaria; the diges- 

 tive system shown in solid black, g, ganglionic mass; i, intestine; p, pharynx. 



higher forms. A more complete description of the histology of 

 the hydra is given on page 389. 



The flatworm, Eupla?iaria, may be utilized to illustrate a 

 modification of the same type of canal (Fig. 52). This animal is 

 also a fresh-water form, of common occurrence in ponds. Its 

 body measures about 12 mm. in length, and in shape it is flat- 

 tened and elongated, being blunt at its anterior end and pointed 



