POLYCH.ETA. 



377 



dorsal and ventral trunks may perform the function of hearts, as 

 is also frequently the case in the Oligochceta. Finally the vascular 

 system is in many cases considerably reduced, and, according to 

 Claparede, is entirely wanting in Glycera and Capitella, in which the 

 blood is represented by the perivisceral fluid. 



The generative organs, unlike those of the hermaphrodite Oligo- 

 chceta, are usually placed in different individuals ; and the males and 

 females are sometimes of very different forms. A number of herma- 

 phrodite Polychceta are, however, known ; such principally belong to 

 genera of the Serpulidce, e.g., Spirorbis, Protula. 



The development, unlike that of the Oligochceta, is invariably con- 

 nected with a 

 metamorphosis. 

 Segmentation is, 

 as in the Hirudi- 

 nea, usually un- 

 equal, and even 

 the first two seg- 

 mentation spheres 

 are of unequal 

 size. The smaller 

 (animal) half, 

 which segments 

 more quickly, 

 gives rise to 

 smaller segments, 

 which grow round 

 and envelope the 

 larger segments 

 proceeding from 

 the segmentation 

 of the larger half. In the subsequent development a primitive streak 

 makes its appearance in all embryos of Polychceta, sometimes, how- 

 ever, not until the embryo has begun to lead a free life as larva. 

 The ganglia become differentiated later into the ventral chain. 



In the free-swimming Iarva3 the cilia are rarely distributed over 

 the whole surface of the body (Atrocha*). They are usually confined 

 to special rows (ciliated rings) ; sometimes, as in Loven's larva, there 

 is one row placed in front of the mouth at some distance from the 



* Compare E. Claparede and E. Metschnikoff, " Beitrag* zur Entwickelungs- 

 geschichtc der Chaetopoden," Zeitschr. fur wiss. Zool., Tom. XIX., 1869. 



FIG. 306. Larvse of Polychaeta (after Busch). a, Larva of Nereis 

 F, tentacle; Oc, eyes; PrW, prseoral circle of cilia: 0, 

 mouth; A, anus, b, Mesotrochal, larva of Ouetopterus ; Wp, 

 circle of cilia. 



