FREE-SWIMMTNG SPOROSACS OF THE HYDROID GENUS DICORYNE. 259 



The fully developed blastostyle lias a narrow base of attachment to the stolon or to 

 a hydranth stem, a wide middle region on which the sporosacs are borne, and a 

 narrower, dome-shaped distal region bearing plentiful nematocysts. The blastostyle, 

 which has neither mouth nor tentacles, is enveloped at its base by a simple 

 hydrotheca similar to that of a hydranth. 



A full-grown blastostyle bears usually from eighteen to twenty-four sporosacs, but 

 occasionally more are present ; the largest number observed was thirty-four. All the 

 sporosacs on a blastostyle are of the same sex, and throughout any one colony the sex 

 of the blastostyles is uniform. 



The free-swimming sporosac is provided with a single tentacle * and is ciliated 

 all over. The female sporosac is sub-spherical and bears in every case only a single 

 oocyte, at one side of which a well-developed spadix is present. The male sporosac 

 is usually rather smaller, and more oval in form, and the mass of spermatozoa is 

 arranged round an axial spadix. 



Further details of the various portions of the colony may now be given. 



Stolon (PI. VI, fig. 1 ; PI. VIII, figs. 14, 15). 



The external surface of the stolon or hydrorhiza is moderately smooth, i.e. without 

 corrugations, and there are no internal chitinous projections of the perisarc such as 

 are found supporting the walls in several gymnoblastic and calyptoblastic hydroids, 

 e.g. Podocoryne anechinata, Ritchie ; Sertularia heterodonta, Ritchie ; Plumularia 

 lagenifera, var. septifera, Torrey. Branches are produced from the stolon strands 

 (fig. l), and for a time their ends are free and rounded, but later they anastomose 

 with neighbouring strands of the stolon, and thus the meshwork becomes complex. 

 The perisarc of the stolon is about 6 (jl thick and of a brown colour. The histology 

 of the ccenosarc does not present any unusual feature. 



Regenerative Capacity of the Stolon. — A point of considerable biological interest 

 may be noted here. It has already been remarked that on all the colonised shells, 

 except one, almost all the larger hydranths are disintegrated and their stems empty 

 of ccenosarc. One colony was evidently dead when preserved, but even in the least 

 promising of the others a few very small hydranths, with only six tentacles (fig. 1, H i), 

 can be seen growing out directly from the stolon, thus giving evidence that, in spite 

 of. the destruction of the more visible portions of the colony, a residue of living cells 

 still remains within the stolon ready to start into being a new series of individuals. 

 Probably these young hydranths had developed during the few days the colonies had 

 been living under favourable conditions in an aquarium in the Zoological Station. 

 Microscopic examination of the stolons from these colonies shows that although 

 many parts are empty, other portions contain a plentiful amount of normal ccenosarc. 

 This living ccenosarc had been protected and shut off from the influence of deleterious 



* One abnormal sporosac with a bifid tentacle and two others each with two tentacles have been observed 

 (see p. 277). 



