KESPIEATION BY THE INTESTINE, 



171 



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

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

 the larvse of the Libellulidaj, for instance, leaf-shaped oi-gans 

 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 Holothuridse 

 (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 

 laminae. In most Annelida and many other Invertebrata, no 



Fig, 49. — Part of the stomach of a Holothurian (Stichoptis variegaius) 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 ; c, 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 (Gobitis 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 surface 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.^" 



All these different organs of respiration ^' must act in 



