472 



ZOOLOGY 



Tcrehellido' (Fig. 373) the branchiae are situated on the dorsal sur- 

 faces of some of the anterior segments. In the Scqndnhr (Fig. 

 366) they form two incomplete lateral circlets of elongated 

 appendages situated at the anterior end of the body, apparently 

 representing modified palpi, and sometimes supported by a carti- 

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

 opercuhi.m (op.), often armed with calcareous plates and spines, for 

 the closure of the mouth of the tube in which the annelid lives. In 





Fi'i. 371.— Heads of various Folycheeta (liiagrammfitic). A, Polynoid ; B, Syllid ; C, ^cpMliy^ : 

 D, Btiiilce; /;, Ph.yUodoct: \ l'\ Tro'fihoiila. a, pro.stomium ; c, cirri of Ijody segments; (1, 

 Ijeristonnal cirri (tentacles) ; c'^, cirrus of first body-segment ; c'*, cirrus of second body-scg- 

 inent ; cV, point of attachment (jf elytron ; p, palp ; s, nuclml organ ; t, tentacle ; /, pcri- 

 stomiuni : //, 111, IV, segments. (From the Cambridge Natural History.) 



shape the branchiae are sometimes filiform, sometimes compressed 

 and leaf-like, sometimes branched in a tree-like manner, some- 

 times pinnate. In SerpxUa (Figs. 366 and 383) 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. 



