LOWER INVERTEBRATES WHICH BREATHE IN WATER 409 



Avorms which rove from place to place and the greatly specialized 

 tube-dwellers. It possesses delicate branching gills along part 

 of its upper surface (fig. 539). But in cases where there is a 

 definite dense-walled tube of lime, or horny matter, 

 or sand-grains and the like cemented together, it is 

 clear that the trunk-region is unfavourably situated 

 for breathing, whether by its general surface or by 

 means of special outgrowths. It is not, therefore, 

 surprising to find in such cases that the head bears 

 gills, generally in the shape of a pair of large feather- 

 like plumes covered with cilia (Terebella, Sabella, 

 Serpula, &c.) (fig. 540). Currents of water are set 

 up by the action of the cilia, and these not only con- 

 stantly renew the 

 water necessary for 

 breathing, but also 

 wash away waste- 

 products, and keep 

 up the supply of food 

 (see p. 258). It is 

 interesting to note 

 that these structures 

 present another in- 

 stance of " change of 

 function ", being, in 

 fact, old organs put to 

 a new use, and cor- 

 responding to a pair 

 of little projections 



rum], reduced. 

 To show the fea- 

 thery gills. 



phcato- (p a ]p S ) possessed by 



\* 



Fig. 540. A Tube- Worm (Terebella}, showing 

 gill-plumes on head 



the head of an ordi- 

 nary marine worm 



where they are concerned with touch, and perhaps other senses 



as well. 



LEECHES (DiscoFHORA) 



In most Leeches the soft moist skin is sufficiently effective as 

 a breathing organ without the aid of gills, and it is very richly 

 provided with blood-spaces, many of which are unusually near 

 the surface. There is nothing very extraordinary in this, for 



