-Si. 



ZOOLOGY 



SECT. 



J)ro'h af) 



^UuJfc 



prob ~~ 



rein mus 



an 



TIG. 235. Tetrastemma . General view of the 

 internal organs, an. anus ; ac. st. accessory 

 stylet ; cer. g. brain ; cil. gr. ciliated groove o'f 

 cerebral organ ; dors. ves. dorsal vessel ; lot. ne. 

 lateral nerve ; lot. ves. lateral vessel ; neplt. 

 nephridlum ; op. neph. nephridial aperture ; 

 prob 1 . eversible part of proboscis ; prob 2 . non- 

 eversible part of proboscis ; prob. ap. aperture 

 for the protrusion of the proboscis ; retr. mus. 

 retractor muscle of the proboscis ; st. stylet. 

 (From Hatschek's Lehrbvck.) 



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 

 fibres, an outer circular and 

 an inner longitudinal ; in the 

 rest (Trimyaria) a third (longi- 

 tudinal) layer is superadded. 

 Another circular layer of mus- 

 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 parenchvma 

 (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 

 digestive canal is usually a 

 simple tube oesophagus (stomo- 

 dosum) but may be more 

 complicated, and divided into 



1 When a tail is present the 

 intestine may, or may not, be 

 continued through it. 



