BY N. A. COBB. 181 



blance existing between the adults of the two genera x y u r i s 

 and Rhabditis* have been often remarked. So long as the 

 adults only are considered the resemblances are open to either of 

 two interpretations. 



1. The resemblances may be due to a close genetic relationship. 



2. They may be due to the reaction of the environment, which 

 is to some exteat similar for the two genera. 



In the latter case a close genetic relationship is supposed not 

 to exist, and no verj'- striking resemblance would be expected to 

 obtain at any period of life on which the selective influence of the 

 environment had not been exerted. The embryonic life is probably 

 that on which the environment would have least influence. 

 Consequently if the embryos of Rhabditis and O x y u r i s 

 resemble each other, or if the similarity between the average 

 mature form of either of these genera and the embryos of the 

 other be greater than that between the adults of the two genera, 

 we should be justified in preferring the first interpretation. In 

 either case we naturally appeal to the ontogenetic evidence before 

 us. If the first interpretation be the true one, a study of the 

 development would betray the relationship, which should be one 

 of the following : — 



1. The one genus a direct or iudirect derivative from the 

 other. 



2. Both genera separate derivatives from an older form. 



In the latter case the successive developmental phenomena of 

 the two forms should diverge. In the former case, the natural 

 supposition being that O x y u r i s is the younger group, the 

 larvai of O x y u r i s should have greater aflinity with Rhab- 

 ditis than have its adults. 



Now this seems to be the case. Our figure repi'esents an 

 oesophagus for instance which is much more Rhabditis-like 

 than the cesophagus of the adult O x y u r i s. The median 

 swelling containing the large cells must. I think, be interpreted 

 as a bulb, and it is at this point that a bulb or something like 



* In the discussion which follows I have in mind also the allied genera 

 Diplogaster, Anguillula, Cephalobus, &c. 



