184 HYDROID^. Part IV. 



those among the Hydroids proper, from which no free Meduste arise, are not to be 

 associated with the Polyps ; their special structure and mode of reproduction showing 

 them to be genuine Acalephs, as I trust to be able presently to prove, upon a broad 

 basis of carefully considered facts. It may, however, facilitate the perusal of the next 

 few chapters, if I add, that among the Hydroids, as I limit this order here, two 

 more or less distinct forms occur, one of which leans towards the Polyps by their 

 general ap2)earance, though in their structure they agree with the brood of the 

 Discophoraj, and not with that of Polyps ; the other, resembling in every respect 

 genuine Medu.s£e. Between these extreme forms, there exists every possible gra- 

 dation, from Hydroids assuming medusoid characters, to true MedusDe, with some 

 of the characteristic structural features of the Discophoras either abortive or entirely 

 wanting. Again, among these Acalephs we find a great variety of combinations 

 of individuals : some forming comjDound communities, either attached to the ground 

 or entirely free, in Avhich the hydroid elements are predominant, and the medu- 

 soid elements assume the appearance of simple reproductive organs; others, forming 

 similar compound communities, in which the hydroid and medusoid elements are 

 more equally combined; others forming free compound communities, in Avhich the 

 medusoid elements are predominant, and the hydroid elements more or less subordi- 

 nate ; and, finally, others still in which the hydroid elements appear only in the 

 young brood. 



As the mode of development of the Medusa long known under the name of 

 Sarsia, and its genetic relations to the Hydroid described under the names of Coryne 

 and Syncoryna, afford the best opportunity of proving that free Meduste may be 

 produced by Hydroids, I shall make a beginning with this type, and first refer to 

 the publications in which the information already' on hand, resi^ecting its history, 

 may be found. In this type the hydroid and medusoid forms of the animal 

 appear separately, in alternate generations; the hydroids forming communities or 

 colonies which are attached to the gi'ound, while the medusae, budding from their 

 branches, become free, and are found, at certain periods of the year, floating in 

 the water as independent Acalephs, with distinct sexual organs, the males and 

 females being developed upon different hydroid communities. 



REFERENCES TO THE PAPERS IN WHICH THE INFORMATION NOW ON HAND UPON THE 

 GENUS CORYNE OF GAERTNER MAY BE FOUND. 



Coryne, Gaertner, in Pallas, Spic. Zool., fasc. 10, 1774, p. 40, Tab. IV. Fig. 8, A, a. 

 " Johnston, Brit. Zooph., 2d ed., 1847, p. 36, et Fig. 

 " Gosse, Devonshire coast, 1853, p. 189, PI. IX., &c. 

 " Alder, Catal. Zooph., 1857, p. 12, PI. VII. 



