434 



ZOOLOGY 



SECT. 



for the closure of the mouth of the tube in which the annelid 

 lives. In shape the branchiae are sometimes filiform, sometimes 

 compressed and leaf-like, sometimes branched in a tree-like 

 manner, sometimes pinnate. In Serpula (Fig. 349) and its allies 

 each branchia consists of an elongated stem, on which are borne 

 two rows of short filaments. The surface of the branchia 1 is 

 usually ciliated. They are richly supplied with blood-vessels, when 



FIG. 342 Terebella. (After Quatrefagcs. ) 



a blood-vascular system is developed ; in Glycera, in which there 

 are no blood-vessels, each branchia contains a diverticulum of the 

 coelome. 



In the Oligochseta branchiae are rarely present : but in certain 

 worms of the family Tiibijicnlu there are metamerically arranged 

 simple branchiae on the segments of the posterior region, which is 

 the part ordinarily protruded from the tube. 



The body-wall consists of a cuticle, an epidermis, muscular 



