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 Holothnria, Annelida, Planarians, Water-Insects, 

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



Fig. 47.— a, Amthas scandms; head, with i- the gill-eavity laid open, and i the conti- 

 guous cavity containing the foliated labyrinthine structure. 6, Tadpole : c, yonng 

 Polypteru from the Nile ; d, emhryo of the Shark. All these have external gills, b?: 



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

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

 the miicous membrane of the intestine serves for respiiution, 

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



