PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



111 



with water, and the rate of aversion can be 

 controlled by the rate at which moisture is 

 supplied. Hence swelling at this point must 

 be responsible for the progressive eversion 

 of the thread. 



Since nematocysts are discharged by di- 

 rect stimuli, and not as a result of nervous 

 control, they are independent effectors. 



An animal "shot" by nematocysts is im- 

 mediately paralyzed and sometimes killed 

 by a poison that has been called hypnotoxin, 

 which is injected into it through the 

 tube. 



Cnidoblasts are developed from intersti- 

 tial cells (Fig. 52), which appear in nests or 

 clusters in the epidermis of the stomach re- 

 gion. Before the nematocyst is completely 

 developed, the cnidoblast in which it is 

 formed migrates to the part of the body 

 where it is needed. Here the cell matures, 

 developing the cnidocil. The function of the 

 cnidoblast is limited to the formation of the 

 nematocyst, and possibly the cnidocil. Since 

 the tube of the nematocyst cannot be re- 

 turned to the capsule, nor can another nem- 

 atocyst be developed by the cnidoblast, 

 the cnidoblast perishes with the loss of its 

 nematocyst, and a new cnidoblast must be 

 formed from an interstitial cell to replace 

 the one that has been used. 



DifFerentiation of the 

 body regions 



The hypostome 



The muscle layers of the hypostome con- 

 sist of the epidermal muscle fibers which 

 radiate from the mouth and the gastroder- 

 mal fibers which surround it. The hypostome 

 is rich in nervous elements and is the most 

 sensitive region of the body. Cnidoblasts 

 and a few interstitial cells also occur here. 

 The gastrodermal layer of the hypostome is 

 thrown into large deep folds when the 

 mouth is contracted, so that a cross section 

 of the oral cone shows a star-shaped 

 "throat," which has been mistaken for the 



mouth by some authors. The folds contain 

 the mucous gland cells. The secretion of 

 these cells is poured out over the food in 

 swallowing and is a necessary forerunner of 

 gastric digestion. Food introduced directly 

 into the stomach through a pipette, or by 

 an incision, without first coming in contact 

 with these cells, is not digested. Hence di- 

 gestion in the hydra is dependent upon an 

 enzyme system which follows an orderly 

 sequence of events. 



The tentacles 



There is a poorly developed "sphincter" 

 formed by the gastrodermal fibers at the base 

 of the tentacles. The tentacles can be rapidly 

 elongated by pumping fluid from the gastro- 

 dermal cavity into them, and the "sphinc- 

 ter" probably influences the entrance or 

 escape of this fluid. 



Stomach-reproductive and stalk region 



The epidermis of this region is about 

 twice as thick as that of the hypostome, and 

 harbors the great bulk of the interstitial cells. 

 This is the region of nematocyst formation, 

 and of testes, ovaries, and buds. The epithe- 

 liomuscular cells form the supporting cells 

 of testis and ovary. The gastrodcrmis of this 

 region is the chief digestive organ, effecting 

 both extracellular and intracellular digestion. 

 Correlated with this digestive activity is the 

 presence of many enzymatic gland cells; 

 their secretion reduces the food to a broth 

 of fine particles. The particles are then fished 

 out by the nutritive cells with their flagella 

 and taken into food vacuoles, where diges- 

 tion is completed. Thus digestion in the 

 hydra reflects certain features of the process 

 as it occurs in both sponges and protozoans 

 on the one hand, and in the higher meta- 

 zoans on the other. Although the cavity of 

 the stomach and stalk is continuous, the 

 hydra confines large food objects within the 

 stomach by muscular action and does not 

 permit them to enter the stalk. 



Since the stalk is primarily a region of ex- 

 tension and motility, its structure is adapted 



