290 



ZOOLOGY 



The outermost layer of the body-wall is an epidermis of 

 columnar cells many of which are ciliated, while others are 

 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 Bimyaria) 

 there are only two layers of muscular fibres, an outer circular and 

 an inner longitudinal ; in the rest {Trimyaria) a third (longitudinal) 

 layer is super-added. Another circular layer of muscular fibres 

 closely encompasses the digestive canal. The interspace enclosed 

 by the outer muscular layers does not comprise any cavity corres- 

 ponding to a true ccelome or body-cavity, except, perhaps, the 



A 



m.p 



3P 



Fig. 229.— Diagrammatic representation of proboscis : (A) in the retracted condition, (B) in the 

 everted condition, g. p. glandular portion of the proboscis ; m. muscle attaching the 

 proboscis to its sheath ; m. p. muscular portion of the proboscis ; p. p. in A, proboscis pore ; 

 p. p. in B represents the position of the proboscis-pore in the retracted condition of the 

 proboscis ; p. s. proboscis sheath. (After Sheldon.) 



cavities of the gonads, the interspaces between the organs being 

 filled with parenchyma (Fig. 234). 



The digestive canal consists 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 proboscis 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 (stomodecum) 

 — but may be more complicated, and divided into various 

 regions. Posteriorly it opens into the intestine. The latter may 



1 When a tail is present the intestine may, or may not, be continued 

 through it. 



