387 



and an inner layer of longitudinal fibres. The latter are connected 

 with the former and also amongst themselves by net-like anastomoses. 

 These dermal muscles cause the folds of the cuticle. Internally to 

 the longitudinal muscles there is another layer of circular muscles. 

 In the Chcetifera two hooked setre are present near the genital 

 opening (fig. 311); these assist locomotion. There may also be 

 present one or two 

 circles of setre at the 

 posterior end of the 

 body (Echiurus). 

 In the Chcetifera 



A 



(fig. 311), the ante- 

 rior part of the body 

 is elongated to form 

 a kind of proboscis, 

 which projects im- 

 movably and cor- 

 responds to the 

 prseoral lobe (prfe- 

 stomium) of the 

 Annelida. The 

 mouth is placed 

 ventrally at the 

 base of the probos- 

 cis. In the Achveta 

 (Sipuncididce) this 

 proboscis is want- 

 ing ; the mouth is 

 placed at the ex- 

 tremity of the an- 

 terior region of the 

 body, which is sur- 

 rounded with cili- 

 ated tentacles and FIG. 312. Sipuneulus nudm, laid open from the side (after IV. 



Keferstein). Ta, Tentacles ; O, cerebral ganglion ; VO, ven- 

 Can be retracted by tra i nerve cord ; D, intestine ; A, anus ; BD brown tubes 



means of retractor (ventral glands), 

 muscles (fig. 312). 



Alimentary canal. The mouth opens into a pharynx, which is 

 sometimes furnished with teeth ; this is followed by a ciliated intes- 

 tinal canal, which is usually longer than the body and disposed in 

 coils in the body cavity. The terminal portion of the intestine is 



