4IO 



ANIMAL RESPIRATION 



breathing organs of all kinds must of necessity be closely related 

 to the blood-system. 



There is, however, a marine leech (^Branckellion), living as a 

 parasite upon the Torpedo, which possesses gill-tufts reminding 

 one of an arrangement common among marine bristle-worms. 



SIPHON-WORMS (Gephyrea) AND WHEEL-ANIMAL- 

 CULES (Rotifera) 



Siphon-Worms do not as a rule possess special gills, but 

 breathe by means of the skin. In forms like the Bristle-Tail 

 i^Echiurus) and Bonellia (see p. 150), where there is a projecting 

 proboscis in front of the mouth, it is probable that 

 this organ is concerned with breathing as well as with 

 feeding. In the sand -swallowing Siphon -Worm 

 iySipitnculus) (see p. 259) the mouth is surrounded by 

 a horse-shoe-shaped fold with a folded edge, and it 

 is likely that breathing is one of the uses of this fold. 

 Quite a different arrangement is found in an allied 

 form iyPriapulus) in the shape of a deeply-lobed 

 appendage at the hinder end of the body, and which 

 is almost certainly to be regarded as a breathing 

 organ (fig. 541). Siphon- Worms also breathe in part 

 by means of the digestive tube. 



The minute Wheel - Animalcules, like many 

 other small animals, do not require anything ela- 

 borate by way of gills, and breathe through the 

 general surface of the body. The ciliated wheel-organ at the 

 front end sets up currents in the surrounding water, which no 

 doubt, as in so many other cases, not only bring food but also the 

 oxygen necessary for respiration. In those Wheel- Animalcules 

 which live in cups or tubes the wheel-organ is particularly large and 

 often complicated, reminding us of the large complex gills which 

 are borne upon the heads of certain tube-worms (see p. 409). 



Fig. 541. — Priapu- 

 lustreduced). resp. , 

 Breathing organ at 

 hinder end of body. 



MOSS-POLYPES (Polyzoa) AND LAMP-SHELLS 



(Brachiopoda) 



The members of both these groups have already been adduced 

 as instances of animals which feed by means of ciliary currents 



