364 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 2 



Dactylostomum differs from the so-called "Opecoelinae" only in the ab- 

 sence of an anus. The character of the terminal sex ducts and the charac- 

 ter of the acetabulum are exactly as in some species of Opegaster and Ope- 

 coelus. 



The presence or absence of an anus is sometimes difficult to determine. 

 In fact, it can be said for D. vitellosum that an anus is almost present. 

 Study of the toto-mount could not determine this character with certain- 

 ty. It was thought probable that an inconspicuous anus was present. The 

 posterior end of the woiTn was cut into serial cross sections 7.5 [x thick. 

 These sections show that the posterior part of the united ceca approaches 

 very close to the ventral surface of the body and comes in direct contact 

 with the cuticula. The cuticula even seems to be thin at this spot, but no 

 connection to the outside could be seen. It was finally decided that an anus 

 was lacking. If an anus should be present in this species, it would become 

 a member of the genus Opegaster and very similar to O. parapristipo- 

 matis. 



After studying examples of all the following genera the writer is con- 

 vinced that Opecoelus, Opegaster, Dactylostomum, Coitocaecum, and 

 Opecoelina are rather closely related genera and should be classified in 

 the same subfamily, Opecoelinae. 



OPECOELUS and OPEGASTER 



The genera Opecoelus Ozaki, 1925 and Opegaster Ozaki, 1928 are 

 very similar. In Opegaster the vitellaria, or at least "rudimentary vitel- 

 laria," extend anterior to the acetabulum, while in Opecoelus the vitel- 

 laria are postacetabular. In Opegaster, the seminal vesicle does not extend 

 appreciably beyond the acetabulum as it does in Opecoelus. The acetabu- 

 lum is often stalked in Opecoelus, but not in Opegaster. The presence or 

 absence of papillae on the acetabulum has not been considered a generic 

 character, and both conditions are to be found in different species of each 

 genus. 



Actually some species seem close to the border line between the 2 

 genera. Thus, Opegaster tamori has vitellaria barely anterior to the ace- 

 tabulum and a seminal vesicle reaching halfway to the ovary. Further- 

 more, the "rudimentary vitellaria" might not be considered by some as 

 vitellaria, in which case some species exhibit the vitellarian condition of 

 Opecoelus and the cirrus-sac extent of Opegaster. 



