CLASSIFICATION DEVELOPMENT 



113 



similar to those of the Protonemertini. The mouth lies in front 

 of the brain. The proboscis is armed. At the junction of the 

 fore- and mid-gut a diverticulum is given off which projects for- 

 wards beneath the fore-gut and ends blindly in front. 



Order IV. Heteronemertini ( = Schizonemertea, and the 

 genera Eupolia and Valencinia, placed provisionally by Hubrecht 

 in the Palaeonemertea). The body-wall consists of the following 

 layers : epidermis, thick cutis, and an outer and an inner longi- 

 tudinal muscle layer separated from one another by a circular 

 muscle layer. The brain and lateral nerve-cords lie between the 

 outer longitudinal and the circular muscle layers. The mouth 

 lies behind the brain. The proboscis is unarmed. 



Development of the Nemertinea. The development of the 

 Palaeonemertea is at present not known : in the Schizonemertea 

 a larval stage occurs ; while in the Hoplonemertea the egg 

 develops directly without undergoing any metamorphosis. 



There are two forms of larva characteristic of the Schizo- 

 nemertea, known respec- 

 tively as Pilidium and 

 the Type of Desor. The 

 Pilidium is hatched 

 early and leads a free- 

 swimming existence, 



whereas the Type of 

 Desor, though in many 

 respects resembling it, 

 never passes through the 

 free-swimming phase. 



The Pilidium (Pig. 

 60) is a helmet-shaped 

 larva bearing a tuft or 

 spike dorsally, and pro- 

 longed downwards later- 

 ally into a pair of lobes. 

 The whole larva is 

 covered with cilia, there 

 being a specially strong 



band round its ventral surface. The dorsal spike is composed 

 of a bunch of strongly developed cilia or of a long tiagellum. 

 The alimentary canal consists of a sac constricted into oesopha. 



VOL. 11 1 



FlG> 60. Diagram of a Pilidium larva. (After 

 Salensky.) c, Tuft of cilia ; m, muscle-fibres ; 

 mo, mouth, seen through one of the lateral lobes ; 

 n, nerve -fibres; n.r, nerve -ring; /<;/. nerve 

 ganglion ; oes, oesophagus ; st, stomach. 



