STRUCTURE OF POLYPI. 171 



by this simplicity of structure. It is often formed of 

 a double pocket, the one completely enveloping the 

 other. The animal in this case might be well 

 enough described as a sack, closed at one of its ex- 

 tremities, and with its sujierior or opened part folded 

 back upon the bottom. 



The tentaculae are always hollow, and all the 

 cavities communicate one with another. Leaf-like 

 formations, or foldings of the envelope of the body 

 close the internal cavity ; the walls of that cavity 

 reunite at the base of the animal. They contract or 

 they dilate at its pleasure, in order to give free pas- 

 sage to the aliment prepared for nourishment in 

 that first chamber. Matters unfit for nutrition are at 

 the same time ejected, by the only door which has 

 given them entrance. Between the wall of this 

 stomach, and the exterior envelope of the animal's 

 body, a sort of double bottom, imperfectly partitioned 

 off, collects the aliments that have been suitably 

 prepared, and it is there that the eggs of the animal 

 are lodged. 



The spaces left between the foldings of the skin in 

 that second pouch are prolonged into the tentaculae, 

 which the animal can withdraw into itself at will, 

 or spread out like the blossom of a flower. Fig. 29 

 shows the polypus in its various degrees of expansion. 

 The entire polypus is enveloped in a great number of 



