Chap. VI.] 



GILLS. 



213 



out-pushings of the body (external gills) ; or water may 

 pass through or into the intestine and be pumped out 

 of it again, or the walls of the intestine may give rise 

 to cavities into which air is received ; or, lastly, the 

 body wall may give rise to an air chamber by folding 

 over and becoming attached, for a more or less con- 

 siderable extent, to the surface of the body, as is the 

 case in the snail. 



In a general way we apply the term gill to an 



,cu 



[J lyol-Asc* 



Fig. 91. From a Trarsverse Section of a Leech ; to show cu, cuticle ; 

 v, intra-epithelial blood-vessel ; ep, epithelial cells. (After Ray 

 Lankester. ) 



organ which is fitted to take up the oxygen dissolved 

 in water, and lung- to that which breathes the oxygen 

 of atmospheric air ; but these terms can only be used 

 in a very general sense. 



The simplest form of in-pushing is seen in the 

 Nemertinea, where the so-called side-organs or 

 ciliated furrows are in Carinella annulata (Fig. 92 ; A) 

 mere pits ; this pit in C. inexpectata forms a more 

 complicated groove, which leads into a ciliated duct 

 (Fig. 92 ; B), which ends blindly among the cells of the 

 brain ; in both these species the nervous centres are 

 quite close to the surface (page 398), and the tissue is, 

 as we have just learnt, impregnated with haemoglobin. 

 In Polia, where the brain is more deeply placed, 

 the ciliated duct is longer (Fig. 92 ; c). In the 



