THE NEMERTEANS 



455 



.-A. o 



CF 



-V cc 



C 



----- LN 



..-,-- RC 



I 



P 



ffl 



-/ S 



G 



RM 



than the posterior portion, where the in- 

 testinal lobes and reproductive glands 

 modify the brilliancy of the coloring. 

 Some individuals have a cast of reddish 

 brown. 



On the anterior end of the body are 

 usually six black pigment spots, or ocelli, 

 arranged symmetrically in three pairs (o, 

 Fig. 765). Smaller specimens may have 

 but four ocelli, and occasional irregularities 

 occur in which the number may be five, 

 seven, or eight. 



It is to the internal organization of the 

 body, however, that one must look for 

 those characters which are mainly used 

 in the classification of the nemerteans. 

 These structures must be studied in most 

 species by means of serial sections, but, 

 fortunately, the fresh-water forms are so 

 nearly transparent that the principal or- 

 gan systems of the body may be studied 

 in the living animal. It is only necessary 

 to place the worm on a slide with a small 

 drop of water and flatten the body be- 

 neath a cover glass. When such a speci- 

 men is examined under the low powers 

 of a microscope the principal anatomical 

 features are easily made out. 



Particularly characteristic is the probos- 

 cis, a strong muscular organ (p, Fig. 765) 

 contained in the rhynchocoel and en- 

 closed by the proboscis sheath. This 

 organ extends from the anterior extrem- 

 ity nearly to the posterior end of the body. 



1MM 



FIG. 765. Stichostemma rubrum (Leidy). Diagram of living indi- 

 vidual flattened beneath a cover glass, showing internal anatomy: 

 .4, anus; C, pyloric cecum; CF, cephalic furrow; CG, cerebral 

 ganglia; G.gonad; /, intestine; LN, lateral nerve; O,ocellus; P.pro- 

 R, rhynchodaeum; RC, rhynchocoel; RM, retractor muscle of proboscis; S, central stylet and basis. 



/ A 



