FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 277 



borne sporosacs can be regarded as degenerate hydranths. Although there are 

 exceptions to this postulate, the general criticism advanced by Weismann seems 

 to be valid. , 



Further, the suggestion that the sporosacs of Dicoryne are degenerate hydranths 

 which have become secondarily the bearers of sexual products seems to be at variance 

 with the actual order of development observed in D. conybearei. For there, in 

 the case of both male and female sporosacs, the first indication was the differentiation 

 of the germ cell or cells, the stimulus of the presence of which apparently brought 

 about the origin and growth of the sporosacs. On' the hydranth-hypothesis we should 

 have expected that the hydranth-bud or hydranth-sporosac would have first been 

 indicated, and that secondarily the germ cells would have been differentiated therein 

 or migrated thereto. Kuhn (1910, p. 103) favours Goette's view, but the theory 

 that the sporosac of Dicoryne is a modified hydranth is unsatisfactory in a general 

 way, because in asserting it a far-reaching assumption of homology is made which, 

 owing to the simplicity of the structures concerned, seems incapable of being either 

 proved or definitely disproved. 



A close examination of the anatomy, and especially of the development of the 

 sporosacs of two species of Dicoryne, has left us with the impression that too great 

 efforts have been made to bring these essentially simple structures into line with 

 theories necessitating detailed and not infrequently obscure homologies. It seems 

 to us that the sporosac of Dicoryne is essentially an outgrowth of almost the simplest 

 possible form, produced apparently by the stimulus due to the presence of the germ 

 cells. The ectoderm in which these lie grows 'pari passu with them and forms the 

 envelope for them, the subjacent endoderm — which soon grows out as a "spadix" — 

 is applied to the germ cells and carries the supply of nutrient material necessary for 

 their growth. This bud-like outgrowth, like other simple buds in the Hydrozoa, 

 produces one or two tentacles, similar in structure to those of a polyp, and 

 eventually becomes free. The most striking difference between such a bud and 

 any other known in the Hydrozoa is its ciliation, which is probably an adaptive 

 development. The free-swimming sporosacs of Dicoryne are the only ciliated re- 

 productive members known in the Hydrozoa. 



Abnormal Sporosacs of Dicoryne conybearei. 



Amongst the many free sporosacs examined three only show any departure from 

 the normal, and in each case the variation relates to the tentacle. Both sexes are 

 represented in this short abnormal series, which exhibits two types of variation. 



In one of the male sporosacs, otherwise normal, the tentacle is bifurcated near the 

 tip (text-fig. 3, A), both branches possessing ectoderm and endoderm cells of the 

 usual type and arrangement. 



