The Digesfive System - 293 



Fig. 16-4. Hydra, longitudinal section. 



. ti TENTACLE 



l& MOUTH 



STINGING CELL 



ECTODERMAL EPITHELUM 



MESOGLEA 

 ENDOOERMAL EPITHELIUM 



GASTROVASCULAR CAVITY 

 FOOT 



happen to swim into contact with one of the 

 waving tentacles. Then the tentacles push 

 the immobile prey into the mouth, which can 

 be opened very wide when necessary. 



The ingested food of Hydra tends to ac- 

 cumulate in the saccular enteron, which is 

 also called the gastrovascular cavity. Diges- 

 tion begins in the gastrovascular cavity, but 

 is completed in the separate cells that line 

 the cavity and form gastric vacuoles on an 

 individual basis. Some of the cells of the 

 endermal epithelium are unicellular glands, 

 which secrete enzymes into the gastrovascular 

 cavity. These enzymes appear to act prima- 

 rily upon the connective tissues of the in- 

 gested organism, and gradually the body of 

 the prey disintegrates into many small pieces. 

 Then the epithelial cells, which are flagel- 

 lated, begin to form food vacuoles, and the 

 fragments of the food organism are ingested 

 for a second time. In the individual food 

 vacuoles, digestion is completed. The com- 

 plex protein, carbohydrate, and lipid com- 

 ponents of the food are hydrolyzed, forming 

 absorbable end products, which diffuse to 

 the neighboring cells of the hydra. Non- 

 digestible remnants of the food mass are 

 passed from the vacuoles back into the gastro- 



vascular cavity and out into the environment 

 via the mouth. 



The saccular type of enteron, in which the 

 mouth opening serves for both ingestion and 

 egestion, has a fairly wide distribution among 

 primitive kinds of animals. It is possessed 

 not only by Hydra and other Coelenterata 

 (p. 630), but also by the flatworms or Platy- 

 helminthes (p. 635). Among flatworms, how- 

 ever, the gastrovascular cavity is not a simple 

 sac, but a branched system of blind pockets 

 all connected by main channels which lead 

 inward from the mouth (Fig. 16-5). 



The branched type of gastrovascular cavity 

 is well exemplified by Planaria, one of the 

 commonest of the free-living flatworms. 

 These graceful little animals (Fig. 16-6) have 

 a flexible ribbonlike form. The ciliated epi- 

 thelium, which covers the undersurface of 

 the body externally, enables the flatworm to 

 glide through the water, with the flat ventral 

 surface in contact with rocks and other ob- 

 jects on the bottom of the pond. The anterior 

 end, or head, is broader than the tapered tail 

 and the dorsal surface has a darker color 

 than the ventral. A pair of light-sensitive 

 eyespots is located on the dorsal surface near 

 the anterior end of the head; and on each 



