168 



THE INFLUENCE OF INANIMATE SURROUNDINGS. 



appendages of the skin, known as gills or branchiae, and finally 

 the interior of the intestinal canal. In very many Invertebrate 

 animals as in Holothnria, Annelida, Planarians, Water-Insects, 

 and others a constant stream of water enters by the anus, and 



FIG. 47. a, Anabas scandens; head, with t the gill-cavity laid open, and I the conti- 

 guous cavity containing the foliated labyrinthine structure. 6, Tadpole ; c, young 

 Polypteru from the Kile ; d, embryo of the Shark. All these have external gills, br. 



in a few cases, as in Holothuria, a very easily demonstrable 

 stream passes out from it also. Thus, in the simplest condition, 

 the mucous membrane of the intestine serves for respiration, 

 like the skin of an animal ; and in this respect the well-known 



