392 L. DONCASTEli. 



no support to the view that tliey are modified nepliridia, for 

 they are almost certainly formed chiefly if not wholly from 

 the actual cells of the ovary, while the external aperture is, 

 like the vas deferens, ectodermal. Although, therefore, the 

 Annelid theory appears at first sight very probable, yet a 

 further examination of the facts on which it is based shows 

 that it is open to grave doubt, and that the resemblance may 

 be very probably due to convergence of type. 



Of the remaining chief views of the affinities of the 

 Cha^toguatha, those, namely, which place them with the 

 Nematoda or near the Euteropneusta, the latter is open to 

 the objections which affect the Annelid theory ; for if the 

 segments of Sagitta are not homologous, but have arisen in 

 different ways and at different times, no ground remains for 

 a theory of their relationship which depends on the view that 

 the three segments correspond with the three divisions of the 

 body-cavity in Balanoglossus aud its allies. The only view, 

 therefore, that remains to be discussed is that of affinity with 

 the Nematodes. 



The Chsetognatha have been associated with the Nematodu 

 chiefly in consequence of the very similar arrangement of the 

 muscles in four longitudinal groups, and this comparison 

 would be much strengthened if the muscles were shown to be 

 certainly parts of the cells which line the body-cavity. 



The following comparisons are more doubtful, but it may 

 be noticed that the vas deferens of Sagitta bears considerable 

 similarity to the excretory duct of a Nematode, for each is 

 ectodermal aud lies in the lateral area between the dorsal aud 

 ventral muscles, aud in rare cases in Nematodes (e. g. 

 Lecanocephalus) the duct is said to open into the body- 

 cavity. The oviduct of Sagitta may be compared with that 

 of a Nematode in being continuous with the ovary and 

 formed from the same rudiment. The development of 

 Nenuitodcs is unfortunately imperfectly known, but it is of 

 interest that the genital organs arise, as in Sagitta, by the 

 vei-y early separatiou of two large cells, which hiter multiply 

 and form the sexual glands aud their ducts, and in the latter 



