Appendix to Cionistes reticularis. 



279 



the polypary ; in other species of Atractylis they arise from 

 the reticulated base of the zoophyte. In certain species of 

 the genera Sertularia and Campanidaria^ marsupial forms 

 occur which bear no homological relation to each other. 



The gradual transition in the Hydroidae from the simple 

 generative sac to the perfect Medusa is exceedingly interest- 

 ing. I attempt to indicate it in the following sketch : — 



Generative elements (spermatozoa or. 

 ova) contained in a simple generative 

 sac or dilatation of the ectoderm; 'pla- 

 centa formed of endodermal floor of 

 sac, 



Placenta protruding into generative sac, 

 and forming " spadix" (Allman) sur- 

 rounded by generative elements, . 



branched and permeating them, 



— or folding round single ovum, 



Placenta adherent to summit of genera- 

 tive sac ; summit of sac furnished 

 with cluster of large thread- cells ; sac 

 the equivalent of the peduncle of 

 Coryne gravata, . 



Summit of sperm-sac furnished with a ) 

 row of tentacles indicating the pre- ( 

 sence of a non-differentiated subum- f 

 brella, ) 



Generative sac transformed into a free\ 

 walking medusoid ; peduncle furnished I 

 with branched tentacles, as in Bon- \ 

 gainvillea ; subumbrella not differen- [ 

 tiated, its presence indicated by eye- I 

 specks and otoliths ; umbrella absent, / 



Ovisac fixed, enclosed in a differentiated } 

 subumbrella with lateral and circular > 

 canals and tentacles, . . ) 



Hydra. 



Coryne. 

 Hydr actinia. 



Campanularia 



lacaerta 



(male). 

 Eudendrium 



rameum (fe- 

 male). 

 Eudendrium 



arbusculum 



(male). 

 E. confertum 



(female).* 



Laomedea 

 Loveni 

 male). 



EleutJieria, 

 medusoid of 

 Clavatella 

 (Hincks). 



Simple gene- 

 ' rative sac. 



/ 



1 Generative sac 

 become a pe- 

 duncle ("ma- 

 nubrium," 

 Allman). 



Subumbrella 

 present, but 

 not differ- 

 entiated. 



/ 



Laomedea ") Subumbrella 

 Loveni (fe- > differen- 

 male). ) tiated. 



* In the ovisac of E. confertum (see figure in margin), 

 which I have had an opportunity of examining through 

 the kindness of Mr Alder, the endoderm and eGtoderm 

 at first adhere together at the summit of the sac, and 

 at this point a few large thread-cells occur in the ecto- 

 derm. A similar occurrence of adhesion and thread-cells 

 is found in the sperm-sac of E. arbusculum, and also in 

 the false and mouthless peduncle of the medusoid of 

 Coryne gravata. 



