278 



Proceedings of the Boyal Physical Society, 



Generative sacs attached to reproductive polyp without ~) Eudendrium 

 mouth or tentacles ; summit of polyp surmounted by a > confertum 

 cluster of large thread-cells, as in . . ) (Alder). 



Generative sacs or medusoids attached to reproductive } Cionistes, 

 polyp without mouth, tentacles, or cluster of thread- > Sertularia, 

 cells, as in ) Campanul 



Reproductive polyp divided longitudinally into several' 

 portions, each surmounted by its cluster of large thread- 

 cells ; sperm-sacs formed, as in Hydra, by simple dila- 

 tation of the ectoderm ; each division of polyp trans- 

 formed into a " moniliform" sperm-sac, as in . 



Generative sacs or medusoids attached to the polypary, 

 as in . . ... 



aria. 

 Eudendrium 

 arbusculum 

 (T. S. W.), 

 E. capillare ? 

 (Alder). 

 / Atractylis 

 (T. S. W.), 

 Hydractinia 

 (Alder and 

 T. S. W.), 

 Cordylophora 

 (Aliman). 



It will thus be seen that there is a very gradual transition 

 from the alimentary polyp to the reproductive polyp, and 

 from the latter to the simple generative sac. Professor All- 

 man's term " blastostyle " applied to the reproductive polyp, 

 is apt to mislead, as it indicates that the alimentary and re- 

 productive polyps are not homologous parts. Still more 

 decidedly does that accomplished naturalist confuse the 

 homology of these parts by applying the same term to the 

 branched pedicle of the aggregated generative sacs of Tubu- 

 laria indivisa, which is merely formed of the conjoined and 

 elongated pedicles of the individual sacs. 



It is impossible to construct any classification of the Hy- 

 droid Zoophytes on the form or position of their generative 

 sacs or medusoids, as these vary not only in different species 

 of the same genus, but also in males and females of the 

 same species. Thus, in Eudendrium rameum the sperm-sacs 

 are moniliform, the egg-sacs single ; the former are attached 

 to the alimentary polyp, the latter to the polyp and also to 

 the polypary. In Hydractinia, although the generative sacs 

 generally spring from the reproductive polyps, they are also 

 found attached to the polypary ; and in a most interesting 

 species of this genus lately discovered by Mr Alder, medu- 

 soids spring from the latter part of the zoophyte. In Atrac- 

 tylis ramosa, T. S. W. {Eudendrium ramosum, Van. Ben.), the 

 medusoids, the males and females of which differ in shape, 

 spring from the polyps, from club-shaped bodies, and from 



