142 



ANIMALS 



to maintain the integrity of the body. The delicate peUicle 

 is therefore sufficient for the amoeba, although for larger forms 

 it would be entirely inadequate. 



339. The body wall of hydra consists of two layers of cells, 

 an outer ectoderm and an inner entoderm. The cells of the 



. Fig. 68. — Amoeba Proteus. Na, A cluster of algal cells which is being engulfed 

 by the protoplasm of the amoeba; Cf, contractile vacuole; N, nucleus. (From 

 Marshall, after Doflein). 



ectoderm are practically naked protoplasm on the exposed 

 surfaces, but this protoplasm is so dense that it serves as an 

 integument. In the closely related hydroids the ectoderm 

 secretes a thin membrane known as the perisarc or cuticula. 

 It is extremely tough and well adapted for protection and 

 support. 



340. The epidermis of worms corresponds to the ectoderm 

 of hydra, but is much thicker because of the columnar form of 



