SIPUNCULOIDEA. 



535 



Te 



A 



D 



There is no trace of a preoral lobe. The anus is approximated to 

 the mouth, being placed on the dorsal surface about one-third of 

 the length of the body from the anterior end. The body is devoid 

 of setae, but the introvert is often covered by rows of hornj hooks 

 or by small, scale-like papillae which are directed backwards and 

 overlap one another. There is no trace of segmentation, but the 

 skin is often thrown 

 into ridges, both 

 transverse and lon- 

 gitudinal. There 

 are no special 

 organs of sense. 

 The body -wall is 

 highly muscular, 

 and consists of an 

 external cuticle, a 

 single - layered epi- 

 dermis, a cutis, a 

 layer of circular 

 muscles, a thin 

 layer of oblique 

 muscles, and a thick 

 layer of longitudinal 

 muscles which are 

 often arranged in 

 bundles ; lastly 

 there is a layer of 

 flat coelomic epi- 

 thelial cells bearing- 

 isolated cilia. The 

 cutis contains nu- 

 merous unicellular 

 or multicellular 

 glandular organs 

 which open to the 

 exterior by pores ; 

 these structures have often a marked nervous supply and have 

 been mistaken for sense-organs. 



The alimentary canal is a thin-walled tube in which three regions 

 may be distinguished the anterior portion (oesophagus) straight and 

 uncoiled, the middle portion (intestine) coiled and bent on itself, and 



FIG. 433. Sipunculus nudus, laid open from the side (after 

 W. Keferstein). Te tentacles; G cerebral ganglion; VG 

 ventral nerve cord ; D intestine ; A anus ; BD brown tubes 

 (nephridia). 



