INTEGUMENT 1 43 



the cells composing it. It secretes a very thick and firm cuticula. 

 Both epidermis and cuticula vary greatly in thickness in dilTer- 

 ent species of worms, but this variation corresponds approxi- 

 mately with the size of the worm. 



u B 



na. 



Fig, 69. — A, Diagram of Hydra; B, portion of the wall highly magnified; 

 b, bud; cct., ectoderm; ent, entoderm;/, foot;//, flagellum; g.v., gastro-vascular 

 cavity; m, mouth; mes, supporting lamella; m.f., muscle fibre of the ectoderm 

 cells; n, nettling cells; n', same, exploded; nu., nucleus; /, tentacle; v, vacuole. 

 (From Galloway.) 



341. The integument of the crayfish is similar to that of the 

 worm. The chief difference lies in the much greater thickness of 

 the cuticula, which here consists of a peculiar substance called 

 chitin. Chitin is an extremely firm and elastic substance, 



