145 



EXTERNAIv CHARACTERS 



The body is smooth, slender, cyHndrical, and tapers gTadually to- 

 wards the two extremities. The length in alcoholic specimens varied 

 from 9 to 14 mm., the more common length being 10 to 12 mm., but 

 in a series of measurements of mature living' specimens not extremely 

 contracted or elongated, the length was found to vary from 15 to 19 

 mm. The lower range of alcoholic specimens is probably due to the 

 effect of the fixing fluid. The diameter in alcoholic specimens is 

 greatest in the region of the clitellum, where it is 0.446-0.684 mm. 

 The first five to nine intersegmental grooves are very distinct, but the 

 others are rather obscure. The number of somites is variable, but 

 the limits of the variation are quite narrow, counts in thirty-five speci- 

 mens ranging only from 41 to 49. The moderately developed clitellum 

 is on XII-XIII and occurs only on the dorsal and lateral surfaces 

 of the worm. In alcoholic specimens the body is opaque, and light 

 brown except on the clitellum, where the color is much fainter. The 

 living animal is reddish with a slight tinge of yellow. The prostomium 

 is blunt, smooth, and rounded. The setae (PI. IX, Fig. 21) are dis- 

 tinctly sigmoid, and are arranged in fan-shaped bundles. The bun- 

 dles are disposed in four longitudinal rows, two ventral and two 

 lateral. In the ventral rows the number of setae per bundle varies 

 from 4 to 8, being occasionally 9 and in rare instances 10. In the 

 lateral rows the number varies from 4 to 7. On the last few somites 

 the number in both sets of rows varies from 2 to 4. 



INTERNAI, CHARACTERS 



Brain. — The brain (PI. IX, Fig. 22) lies in I and II. The length 

 is about one and one-half times the width. The posterior margin is 

 distinctly emarginate; the anterior concave. The lateral margins vary 

 to some extent, being in some cases nearly parallel, in others slightly 

 divergent caudad. Two pairs of supporting strands extend from the 

 posterior end of the brain to the body wall, while from the anterior 

 end a rather strong muscular strand extends from the mid-ventral 

 region to the wall of the prostomium. 



Blood Vascular System. — Since the blood in this species is colored 

 and the vessels are rather large, remaining distended after death, it 

 has been possible to follow out the course of the chief vessels. 

 Furthermore, owing to the fact that the integument of the body is 

 semitrans|)arent, permitting the blood-vessels to stand out promi- 

 nently, it was possible to study the vascular system in the living form 

 and thus to verifv the observations made on the alcoholic material. 



