British Protozoa and Zoophytes. 125 



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. Prof. Allman's term 

 ' blastostyle,' applied to the reproductive polyp, is apt to mis- 

 lead, as it indicates that the alimentary and reproductive polyps 

 are not homologous parts. Still more decidedly does that ac- 

 complished naturalist confuse the homology of these parts 

 by applying the same term to the branched pedicle of the ag- 

 gregated generative sacs of Tubularia indivisa, which is merely 

 formed of the conjoined and elongated pedicles of the individual 

 sacs. 



It is impossible to construct any classification of the Hydroid 

 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, medusoids spring from the latter part 

 of the zoophyte. In Atractylis 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 the polypary ; in other species of Atractylis 

 they arise from the reticulated base of the zoophyte. In 

 certain species of the genera Sertularia and Campanularia, mar- 

 supial forms occur which bear no homological relation to each 

 other. 



The gradual transition in the Hydroidse from the simple 

 generative sac to the perfect Medusa is exceedingly interesting. 

 I attempt to indicate it in the following sketch : — 



Generative elements (spermatozoa or ova)") 



contained in a simple generative sac or I „ , 



dilatation of the ectoderm; placenta M r ' 



formed of endodermal floor of sac J 



Placenta protruding into generative sac, ] p 



and forming 'spadix' (Allman) sur- \ j? r H ne ' ,■ • 



rounded by generative elements J ti V aractima - 



{Campanularia 

 lacerata 

 (male). 

 f Eudendrium 

 rameum (fe- 

 male). 



Simple gene- 

 rative sac. 



