171 



albidus Henle ; but here distinct differences occur in length, number 

 of somites, and spermathecee. 



EXTERNAI^ CHARACTERS 



Tweiity-se\'en alcoholic specimens have a length varying from 2.5 

 to 4.5 mm. They had been carefully killed and fixed, and since mi- 

 croscopical examination of the whole series showed no conspicuous 

 contraction it is safe to consider these measurements as fairly ac- 

 curate. The number of somites is rather constant, varying within the 

 limits of 25 and 27. The greatest diameter, which is in the region of 

 the clitellum, is 0.144-0. 188 mm. The body is smooth, cylindrical, and 

 tapers very gradually caudad from the clitellumi. The clitellum is only 

 moderately developed and occurs on XII and XIII. It is interrupted 

 on the mid-ventral surface. The intersegmental grooves are most dis- 

 tinct in the anterior region ; posteriorly they tend to become obsolete. 

 The prostomium is blunt and rounded. The setse are sufficiently de- 

 scribed in the definition. 



INTERNAL CHARACTERS 



Lymphocytes. — The lymphocytes are scattered throughout the 

 greater part of the coelom, but occur only in small numbers. They are 

 nucleated, ovoid, and have a decided affinity for stains. 



Chloragog Cells. — The elongated club-shaped chloragog cells are 

 very highly developed. They first appear in V, and from that point 

 caudad they almost completely cover the digestive tract, filling the 

 greater part of the coelom. The cytoplasmic contents are distinctly 

 reticular and sometimes appear to be alveolar in structure. 



Brain.— The brain (PI. XII, Fig. 52) lies chiefly in I and II. The 

 length varies slightly in the different specimens, but the ratio of the 

 length to the greatest diameter appears to be almost uniformly 11:6. 

 The posterior margin is approximately straight, although in some 

 specimens there is a slight convexity. The lateral margins converge 

 anteriorly, the smallest width being just posterior to the origin of the 

 commissural trunks. The anterior margin is concave and slightly V- 

 shaped. A neurilemma surrounds the brain and appears to be thick- 

 est about the posterior margin. Two pairs of strands connect the 

 brain with the body wall, one pair arising from the lateral margins 

 near the region of the greatest diameter, the other arising near the 

 region of junction of the lateral margins with the posterior one. In 

 transverse section the brain is ovoid in shape. Close examination of 

 cleared specimens showed the presence of an elliptical area in the re- 



