168 



THE INFLUENCE OF INANIMATE SUEEOUNDINGS. 



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

 the interior of the intestinal canal. In very many Invertebrate 

 animals — as in Holothuria, Annelida, Planarians, Water-Insects, 

 and others— a constant stream of water enters by the amis, and 



Fig. 47.— a, Anabas scandens', head, with A the gill-cavity laid open, and/ the conti- 

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

 Polyptern from the Hile ; (2, embryo of the Shark. AIL these have external gihs, br. 



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

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

 the mucous membrane of the intestine serves for respuation, 

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



