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the mesoglcea and parallel with the body. When these 

 contract the body is shortened. 



The endoderm cells have similar processes ; but, they 

 are circular in position. When these contract the diameter 

 of the body is reduced and it lengthens out. 



Creeping is accomplished by using the tentacles, by the 

 mall pseudopodia of their ectoderm cells. 



Explain the Structure and Action of the Hydra Stinging 

 Cell. 



Embedded in the ectoderm there are cnidoblast cells. 

 Each has a small projecting barb, the cnidocil, and contains 

 a sac called the nematocyst which has inside it fluid and 

 a long stinging thread coiled up. 



When, for instance, a water-flea comes in contact with 

 the barb, cnidocil, or touch point, the cnidoblast suddenly 

 contracts and the pressure on the nematocyst causes the 

 contained stinging thread to be instantly shot forth into 

 the body of the lea with a poisonous paralysing effect. 



How does Hydra feed ? 



It captures its prey by means of its tentacles. The ten- 

 tacles are tightly turned in, and the food is thus brought 

 to the mouth and passed in. Within the coalenteron portions 

 of it are digested by a ferment secreted by those endoderm 

 cells near the mouth which are specialised gland cells. 



The ordinary endoderm cells simply absorb the digested 

 substance ; and by means of their pseudopodia they en- 

 gulf the more resistent portions, digesting these in food 

 vacuoles. Digestion in Hydra is therefore partly inter- 

 cellular, partly intracellular. 



The residue or waste is passed out at the mouth. This 

 is accomplished by the activities of the flagella, which 

 sustain the circulation of the watery fluid within the 

 coelenteron. 



What is the Function of the Interstitial Cells ? 



Some of them are specialised for reproduction and pro- 

 duce the male and female gametes. Others move towards 



