FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 267 



Blacksod Bay, Plymouth:, lent by Dr E. J. Allen, F.R.S., from the collections in the 

 Laboratory of the Marine Biological Association, Plymouth ; and (c) North Sea, 

 lat. 57° 43' N., long. 0° 38' E., in a collection received for identification by one of us 

 from the North Sea Investigation Committee of the Scottish Fishery Board. 



It is a matter for regret that the specimens from each of these localities bear 

 male sporosacs only. As in the Neapolitan examples these are clustered upon blasto- 

 styles borne upon the hydrorhiza or upon the hydrocaulus, but in D. conferta they 

 are relatively more frequent on the latter. 



The spermatogonia are first to be distinguished in the ectoderm of the blastostyle 

 on account of the deeper stain absorbed by their nuclei. They do not migrate : their 

 situation already indicates the position of the future sporosacs. In all the blasto- 

 styles of D. conferta which we have sectioned the mesogloea is of unusual thickness, 

 averaging 6 ft and reaching even 13 /a, and this induces a slight modification in the 

 early development of the gonophore. For whereas in the Neapolitan examples of 

 D. conybearei the mesogloea, even at the earliest stage, was bent outwards into the 

 sporosac-mound, in D. conferta the abrupt edges of mesogloea at the junction between 

 blastostyle and gonophore show that a disc of the mesogloea has been almost com- 

 pletely resorbed, the thin film remaining being then pushed outwards by the growing 

 endoderm taking part in the formation of the sporosac. 



The early development of the sporosac follows the course described for the 

 Neapolitan examples (see pp. 265, 266). There is a large endodermal spadix which 

 either has a narrow central lumen or is solid, a thin film of mesogloea, and a thick 

 cap of ectoderm consisting of a mass of spermatogonia with more peripherally placed 

 covering cells. The sporosac is invested with a cuticle considerably thicker and 

 usually developing at an earlier stage than that present in the Neapolitan ones, 

 and foreign particles are adherent to it. 



The subsequent development is marked by a constriction in the proximal portion 

 of the sporosac. This proceeds to such an extent that finally a neck is formed 

 containing only a narrow cylinder of endoderm invested by a thin film of mesogloea 

 which is separated by a considerable interval from the cuticle still strengthening the 

 connection between the newly developed sporosac-stalk (see p. 263) and the sporosac 

 proper (PI. VIII, fig. 16). The blastostyle type of ectoderm is retained in the 

 sporosac-stalk, but in the sporosac a thin ectoderm of almost uniform cells, with 

 relatively large nuclei, is formed by rapid division. With the appearance of two 

 tentacles, the origin of which dates from the- beginning of the constriction at the 

 sporosac-neck, the sporosac assumes bilateral in place of radial symmetry. The 

 tentacles originate as a couple of buds which arise opposite each other from the wall 

 of the sporosac immediately distal to the narrow neck. Pushing distalwards they 

 are retained alongside the sporosac by the cuticle. With the maturing of the 

 reproductive products the cells of the spadix undergo regenerative changes, and in 

 most cases the spadix exhibits a distinct central cavity. The cuticle enveloping the 



