DESCRIPTIONS OF FOUR DISTOMES. 33 



posteriorly to about the posterior end of the intestinal caeca, as described by Llihe, 

 but in a large worm, in which the caeca are relatively longer, they do not reach the 

 posterior ends of those organs. Two or three yolk-ducts leave each yolk-gland and 

 proceed towards the centre of the body. They join on each side to form a single 

 duct, the transverse yolk-ducts, which meet and form a median yolk-duct. This 

 duct, which proceeds posteriorly, is very short and widens at once to form a small 

 yolk-reservoir 0.06 millimetre long and 0.03 millimetre wide, beyond which it im- 

 mediately joins the ootyp. 



The uterus leaves the ootyp near the centre of the body and proceeds towards 

 its hinder end. This organ is very voluminous and forms a descending and an ascend- 

 ing limb, the former on the left and the latter on the right side of the body. These 

 limbs are both thrown into transverse folds which, posterior to the testes, extend 

 from the centre to the sides of the body. Anterior to the testes the folds do not 

 extend laterally beyond the intestinal cseca. The broad hinder part of the body is 

 entirely filled with these uterine folds. The ascending limb finally passes forward, 

 dorsal of the acetabulum, to the genital pore. No specialized metraterm is present. 

 The uterus is crowded with small dark-colored eggs, the average length of which is 

 0.034 millimetre, and the average width 0.018 millimetre. 



The affinities of R. variabilis were indicated by Liihe (:01, p. 561), who called 

 attention to the similarity of structure that exists between it and Styphlodora 

 solitaria Looss. 



There seems to me little doubt that these two species belong to a common genus, 

 but I would not ascribe them to the genus Styphlodora. The principal points of differ- 

 ence between them are the possession of a receptaculum seminis by S. solitaria, which 

 is wanting in R. variabilis, and the shortness of the median limb of the excretory 

 vesicle in S. solitaria, as compared with that of R. variabilis. As regards the first 

 point of difference, the minute size of the receptaculum in S. solitaria deprives it of 

 importance. As regards the second, Looss ('99, p. 708) was unable from lack of 

 material (he possessed but a single specimen) to determine the exact course of the 

 excretory vesicle in S. solitaria, and 'he mentions the possibility that its median 

 portion may be long instead of short or wanting, as he describes it. He says: " Es hat 

 mir geschienen, als ob ausser diesen Schenkeln noch ein medianer Stamm nach vorn 

 liefe, doch war wegen der Fiillung des Hinterleibes mit den Schlingen des Uterus 

 etwas Genaueres nicht zu erkennen." 



Looss ('99, p. 592) also seems to express a doubt that S. solitaria belongs to the 

 genus Styphlodora. He states that "Dieser Gattung scheint noch zuzuzahlen zu 

 sem St. solitaria." Its generic identity with S. serrata, the type-species of this genus, 



