286 



ZOOLOGY 



SECT. 



cilgr 

 cer.g 



tat-, ne 



prob 



neph 



unicellular glands, some of 

 which are arranged in groups ; 

 these secrete the mucus with 

 which the surface is usually 

 covered, and which may form 

 a gelatinous tube. Beneath 

 the epidermis is a basement 

 membrane, very thin in most 

 cases, followed by the muscular 

 layers. In some Nemerteans 

 (whence called Dimyaria) there 

 are only two layers of muscular 



dors.ves fibres, an outer circular and 

 an inner longitudinal ; in the 

 rest (Trimyaria) a third (longi- 

 tudinal) layer is superadded. 

 Another circular layer of mus- 



op. neph cular fibres closely encompasses 

 the digestive canal. The inter- 

 space enclosed by the outer 

 muscular layers does not com- 

 prise any cavity corresponding 

 to a true coelome or body- 

 cavity, except, perhaps, the 

 cavities of the gonads, the 

 interspaces between the organs 

 being filled with parenchyma 

 (Fig. 239). 



The digestive canal con- 

 sists of a tube which extends 

 throughout the length of the 

 body from the mouth situated 

 near the anterior extremity on 

 the ventral side to the anus 

 at the posterior extremity. 1 

 The mouth is usually placed 

 some distance behind the pro- 

 boscis pore, but may be shifted 

 forwards so as to lie close to 

 the latter, or to be incorporated 

 with it. The first part of the 



FIQ. 235. Tetrastemma. General view of the digestive Canal is Usually a 



J8H? m-raSf; Sti. && ^ S v s e 0r o y f simple tabe-oop%iM (stomo- 

 y^S^SS^S&^SSfi^t ^H"** may be more 



nephridium; op. neph. nephridial aperture; Complicated, and divided into 

 prob 1 . eversible part of proboscis ; prob*. non- 



eversible part of proboscis ; prob. ap. aperture * When a tail is present the 



for the protrusion of the proboscis ; retr. mus. intpqtina mav or rnav not hf 

 retractor muscle of the proboscis at stylet Des ne may. or may tot, I 



(From Hatschek's Lehrbuch.) continued through it. 



retr. mus 



