Annelida. 167 



in most segments^ there is a pair of segmental organs; 

 eacL. is usually a tightly-coiled glandular canal, opening at one 

 end into the body-cavity by a ciliated funnel, and at the other, 

 ventro-laterally, to the exterior (in Chffitopoda, at the base of the 

 ventral parapodia) ; the outer region, near the external pore, is often 

 swollen into a vesicle. These are the excretory organs 

 (nephridia) of the Annelids,* but they often perform another function 

 in permitting the exit of ova and spermatozoa. The sexual 

 organs are very diversely arranged {see below) ; some Annelids are 

 of separate sexes, some arp hermaphrodite. 



The Annelids in many respei3ts come rather close to the Nemertines, 

 from which they are probably to be derived. If the two lateral nerve-cords 

 of the latter moved ventrally and approached one another, and formed 

 swellings at the origins of the transverse nerves, the chief portions of the Annelid 

 nervous system wotdd be represented. The dorsal vessel of the Nemertines 

 corresponds entirely with that of the Annelids, the lateral vessels of the 

 Nemertines are united to form the Annelid ventral vessel, and in both groups 

 the arch-hke transverse vessels are similar. Many Annelids possess two ciliated 

 grooves coiTesponding with those of the Nemertines. It is also of gi'eat interest 

 that many Annelid larvae are furnished with a provisional exoi-etoi'y apparatus, 

 the so-called "head-kidney," provided, at least in many chsetopod 

 larvse, with closed end-branches, just as is the permanent excretory 

 apparatus of the riatworms, with which it indisputably con-esponds. The 

 segmentation of the body, the peculiar segmental organs, the formation of a 

 body-cavity, etc., are, however, important points, distinguishing the Anaelids 

 from all Flatworms. 



Class 1. Chsetopoda. 



The body is divided by distinct constrictions into a large number 

 of segments. With the exception of the anterior and posterior 

 somites, each usually bears four so-called parapodia, two on 

 each side (Fig. 133). These are short processes of the body -wall of 

 different forms, each bearing a bundle of chitinous bristles (chsetse), 

 which are sunk in deep saccular invaginations of the skin. The 

 chseta, which is a cuticular structure, is secreted by a large cell 

 at the bottom of the invagination. The bundle may be moved by 

 muscles attached to the lower ends of the chseta-sacs. The cheetES 

 are of various and often elegant shapes ; sometimes the outer moiety 

 is jointed upon the shaft-like part; the point, is often hooked, or 

 the end may be pectinate. The chsetee may be so long as to look 

 like long thin hairs, or they may be very short. In very many 

 Cheetopods, there is in each bundle of chsetEe, a peculiarly developed 

 thick and stiff, often dark one, the aciculum, which is implanted 



* In some Chastopods, the epithelial cells of the body-cavity also secrete waste 

 substances, which are probably taken wp by the funnels of the nephridia and carried 

 to the exterior. 



