462 



ZOOLOGY 



SECT. 



situated at the anterior end of the body, apparently repre- 

 senting modified palpi, and sometimes supported by a carti- 

 laginous skeleton ; one of them is enlarged to form a stopper or 

 operculum (op.), often armed with calcareous plates and spines, 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, some- 



FIG. 379. Terebella. (After Quatrefages.) 



times pinnate. In Serpula (Figs. 372 and 390) and its allies each 

 branchia consists of an elongated stem on which are borne two 

 rows of short filaments. The surface of the branchiae is usually 

 ciliated. They are richly supplied with blood-vessels when 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 



