THE PROCTUCHA. 



163 



the same fundamental type abound among the polychaetous 

 Annelida, the Uchinodermata, and the Mollusca. 



i 



wm 



Fig. 34.-^4, young Tetrastemma.—aa, central ganglia of the nervous system; bb. cil- 

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

 portion of proboscis ; e, posterior muscular part, fixed to the 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 7 etrastemma, 

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



The lowest Proctucho, such as Microstomnm, have no 

 frontal proboscis (whence they are termed Arhynchia), and 

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

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

 But all the other Proctucha (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, 



