234 Proceedings of the Royal Physical Society. 



through the apertures of the segmental organs of the earth- 

 worm. 



M. Perrier * has described a Nematoid, Dionyx Lacazii, 

 which he found encysted in the transverse muscles of Ponto- 

 drilus, the male agreeing with the free-living forms by the 

 possession of two intromittent spicules and an accessory 

 piece. This affinity is to be explained, according to M. Perrier, 

 by supposing that this species, like Ascaris nigrovenosa, is a 

 free-living form which passes into a parasitic stage. 



A third way of explaining the affinities shown by certain 

 parasitic genera to the free-living forms, is to be found in the 

 similarity of conditions of life ; in D. Pleurochoetce, for instance, 

 living in the perivisceral cavity of Pleurochoeta, which is 

 freely open to the exterior by a series of large dorsal pores, 

 as well as by the apertures of the .generative organs, the 

 conditions of life must be very much like those enjoyed by 

 many free-living forms. 



The following is a brief resume of the characters in which 

 Dicelis Pleurochoetoe approaches the free-living forms : — The 

 presence of a long caudal appendage in both male and 

 female, the relative thickness of the cuticle, the cellular 

 structure of the oesophagus, and the presence of three spicules 

 in the male. In the structure of its generative organs, how- 

 ever, it conforms to the type usually found in the parasitic 

 forms, but the female agrees with certain free Nematoids in 

 producing but few eggs. Dr Cobbold, in a paper read before 

 the Linnsean Society, on November 17, 1881, described a 

 parasite of the ostrich {Strongylus Douglasii) which has this 

 character, and it is worth while remarking that the two 

 genera Strongylus and Dicelis are very closely allied. 



Explanation of Plate. 



Fig. 1. Posterior end of body of female, D. PlcurochcetK, x 200. 

 Fig. 2. Optical longitudinal section of cliitinous cuticle in male ; upper 

 surface of body, x 540. 



Fig. 3. Do. ; lower surface, x 540. 



Fig. 4. Head from ventral side showing the single " mouth papilla," x 540. 



Fig. 5. Portion of tail of female, x 540. 



Arch, de Zool. Exp., t. ix., 1881. 



