OF THE ANNELIDES. 499 



the bead, or the anterior extremity of the body, when there 

 is no distinct head ; it is often armed with a protractile pro- 

 boscis (Fig. 526), and with jaws having the form of horny 

 hooks. The intestine is straight, simple, or furnished with 

 caeca, placed on either side ; the anus is at the extremity of 

 the body. 



The blood is almost always red ; sometimes it is green, and 

 at others scarcely coloured ; it circulates in a complex system 

 of vessels, varying in different species ; of these vessels some 

 are contractile, and perform the function of a heart; others 

 those of arteries and veins. 



Fig. 628. A Group of Serpulse. 



The respiration of these animals is generally ae'rian, but 

 sometimes aquatic, and in this case it is performed by means 

 of external branchise, whose form and arrangement vary 

 much ; sometimes they resemble little trees or leaves, and are 

 fixed above the feet on each side of the back, as in the areni- 

 cola (Fig. 56) ; at other times they resemble bunches of 

 feathers, and unite in a corona around the extremity of the 

 body, an arrangement of which we have an example in the 

 serpula (Fig. 528). 



585. Most of the annelides live in the sea, and several 

 K K. 2 



