THE PROCTUCnA. 



1G3 



the same fundamental type abound among tlie polycbastous 

 Annelida, the Uchinodermata, and the Mollusca, 



mm' 





Fig. 34.—^, young Tetrastemma.—aa, central ^nglia of the nervous system; 55, cil- 

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

 portion of proboscis ; e, posterior muscular part, fixed to ttie parietes at/; g, in- 

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

 (After Schulze.) B, anterior extremity of the everted proboscis of letrastemmay 

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



The lowest Proctucha^ such as 3Iicrostomiim^ have no 

 frontal proboscis (whence they are termed Arhynchid)^ and 

 they differ very little from the lowest Rhahdocoela., save in the 

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

 But all the other Proctucha [Rhynchoccela., 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, 



