258 THE PROTOCHORDATA. 



place from the tip backwards by the in-roIHng of its walls. 

 According to the graphic description of Hubrecht, it is 

 retracted "in the same way as the tip of a glove finger 

 would be if it were pulled backwards by a thread situated 

 in the axis and attached to the tip." 



When at rest within the body the proboscis lies freely 

 within a hollow cylinder, the wall of which is thick and 

 muscular, and constitutes t\i& proboscis-sheath (Fig. 123). 



Fig. 123. — Diagrammatic transverse section through the middle of the body 

 of a N'emertine. (After LANG, Text-book of Comp. A?iat.) 



bjn. Basement-membrane, c.vi. Circular muscles. d.Ji, Dorsal or " medullary " 

 nerve, d.v. Dorsal blood-vessel, g. Gonads, int. Intestine, l.m. Longitudinal 

 muscles, l.n. Lateral nerves, l.v. Lateral blood-vessel, p. Proboscis. p.s. Pro- 

 boscis-sheath. 



Sometimes beneath the ectodermal epithelium of the 

 Nemertine proboscis there is a continuous sheath of nerve- 

 fibres, comparable to the nervous plexus in the proboscis 

 of Balanoglossus. 



Partly, therefore, on account of its structure, and partly 

 on account of its topographical relations when extruded, 

 we are led to suppose that a certain homology exists 



