678 



VERMES— CLASS GEPHYREA. 



The species fall into two well-marked orders known as the Chsetitera and 

 the Achjeta. The Chretifera, or bristle-bearing Gephyrea, show strong indica- 

 tions of affinity with the Chietopoda, and are by some authors considered to 

 belong to that class. In the young the body is partially segmented, and 

 in the adult is furnished below with a pair of hooked bristles. The alimen- 

 tary canal terminates at the hinder end of the body, and the proboscis, which 

 is ciliated and often of enormous length as compared with the size of the 

 animal, appears to be nothing but an enlargement of the prostomium or region 

 in front of the mouth, the latter being situated at its base. The best-known 

 members of the group are Bonellia, Echiurus, and Thalassema. They are to 



be found at the bottom of the sea or beneath 

 stones in pools, and appear to be gifted with 

 but little activity. 



The Achasta have no bristles, as their name 

 indicates ; there is no sign of segmentation 

 at any period of life ; the alimentary canal is 

 twisted upon itself, and terminates in the 

 front half of the body ; and the proboscis 

 carries the mouth at its tip, and can be re- 

 tracted inside the body by muscles specially 

 set apart for the purpose. In some of these 

 worms — in those, for example, known as 

 Sipunculus, Phascolosoma, and Phymosoma — 

 the tip of the proboscis is furnished with a 

 circlet of tentacles surrounding the mouth. 

 These animals live on the sea-bottom in de- 

 serted univalve shells, closing up the mouth 

 of the shell with sand grains cemented to- 

 gether with slime, but leaving an aperture 

 just large enough to allow the extensible 

 proboscis to be thrust in and out with ease. 

 A British representative of this group is 

 Sipuncuhis bernhardxis. A still more curious 

 form is Priapulus, found in deep water in the 

 North Atlantic. This worm burrows in the 

 sand, and is furnished with a stout probocis 

 armed with horny teeth, but without the 

 circlet of tentacles observable in Sipmu:tihi,s. 

 In both groups of the Gephyrea occur ciliated larval forms which present 

 a close resemblance to the Trochophore of the Polychseta. 



Fig. 7.— Gephteban Worms. 



A, Bonellia viridia (with proboscis 



distendel). 



B, Priapulus caud%tus. 



C, Echiurus, 



THE THREAD-WORMS. 



Class Nematohelminthes. 



As implied by their name, the worms of this class are mostly long, slender, 

 and more or less thread-like. The body is usually pointed at both ends, and 

 it presents at most but slight external indications of segmentation, and none 

 of that serial repetition of internal organs which is so characteristic of the 

 highly organised Annelida. The skin, moreover, instead of being furnished 

 with bristles or the delicate hairs known as cilia, is protected by a smooth 

 and relatively tough cuticle. In all the typical members there is a distinct 



