BY THE INTESTINE. 



171 



Less variety is found among the internal gills, which some- 

 times are situated in the intestinal canal of water-animals. 1 u 

 the larvse of the Libellulida?, for instance, leaf-shaped organs 

 are found inside the rectum, which apparently serve for respira- 

 tion. I myself have described a system of foliated processes 

 on the mucous membrane of the stomach of the Holothuridre 

 (fig. 49) which have all the attributes of true gills as an 

 extensive surface, delicate membrane, and abundant blood- 

 vessels, with a constant renewal of the water that bathes the 

 lamina?. In most Annelida and many other Invertebrata, no 



FlS. 49. Part of the stomach of a Holothtirian (.Wcft^pw rarieoaiut) split open length- 

 wise and laid flat, a, the dorsal furrow between the two series of gill-foliations ; b, 

 the broad tumid ventral surface which divides them ; <, the foliaceous gills. 



doubt a regular current of constantly renewed water passes 

 through the intestine, which nevertheless bears no special gills ; 

 the more or less extensive folds of the mucous membrane 

 here take the place of the absent organs. It may here be inci- 

 dentally mentioned that even a fish (Cobitis fossilis, a species of 

 Loach, fig. 50) breathes through the intestines ; but in this case 

 the conditions are slightly different, inasmuch as it takes in air- 

 bubbles at the suiface of the water through its mouth, and 

 swallows them, so that here the air comes into direct contact 

 with the respiratory surface of the intestine. 70 



All the^e different organs of respiration 71 must act in 



