THE PROCTUCHA. 



1G3 



the same fundamental type abound among the polychietous 

 Annelida, the jEchinodermata, and the lloUusca. 



MB 



Fig. 34.—^, young Tefrasfemma.—aa, central 2:anfjlia of the nervous system; bb. cil- 

 iated fossae ; c, aperture through which the proboscis is protrude]; d, anterior 

 portion of proboscis: e, posterior muscular part, tixed to tlie parietes at,/'; {/, in- 

 testine; h, anal aperture; i, water- vessels; k, rhythmically contracting vessels. 

 (After Schulze.) H, anterior extremity of the everted proboscis of 7 etrastemma, 

 exhibiting tlie principal and the reserve stilets. (After Schulze.) 



The lowest JProctiteha, such as 3Ticrostom.um^ have no 

 frontal proboscis (whence they are termed Arhynchia)^ and 

 they differ very little from the lowest lihabdocoela, save in the 

 fact that there is an anus, and that the sexes are distinct. 

 But all the other Proetucha {Rhynchocoela^ or Nemerteans) 

 are provided with a frontal proboscis, which sometimes oc- 

 cupies the greater part of the length of the body (Fig. 34). 

 It has special retractor muscles, and its internal surface is 

 either merely papillose, or may possess a peculiar armature, 



