104 zooiocr 



cuticula. The remarkable thing aloout this front end of the 

 food-canal is that it can be rolled out, carrj-ing the jaws to 

 some distance in front of the head, where they can seize their 

 prey and drag it l^ack into the mouth (Fig. IGO). Behind the 

 pharj'iix comes a short gullet into which glands pour digestive 

 fluids, and then comes the long intestine, extending through the 

 middle of the trunk to the last segment. The food-canal of 

 the polychaetes l^ehind the pharynx is characterized Ijj' great 

 simplicity (Fig. 161). 



Respiration in polychfetes is effected through the skin, 

 particularly' that of the swimming appendages. It is impor- 

 tant to notice that this is the primitive condition out of which 

 the respiratory organs of Crustacea have arisen, but in the 

 latter grou]D, owing to the thick cuticula of the legs and the 

 larger size of the animal, special outgrowths of the appendages 

 (gills) have Ijeen evolved. We see the beginnings of such 

 gills in certain polychietes (Fig. 159). In sessile polychaeta 

 (page 188) respiration is confined to certain appendages which 

 have become greatly enlarged so as to extend beyond the 

 mouth of the tubes in which they live. 



The circulatory system of polvchietes is highly developed. 

 A contracting vessel extends along the whole of the mid-dorsal 

 line and a smaller tulje runs along the inidventral line (Fig. 

 161). These are connected in each segment (Nereis) by a 

 pair of vessels which (in Polychiptes) break up into capillaries 

 in the swimming ajopcndages or gills to facilitate respiration. 

 Blood-vessels also run to the intestine to absorb the digested 

 food. The heart pulsates rhythmically, forcing Idood toward 

 the head. Tlie bhjod contains a ftnv coriiuscles, but the fluid 

 is itself red and cajjable of uniting chemically with 0X3'gen at 

 the skin and jdelding it to the tissues. 



