250 CONTRIBUTIONS TO THE NATURAL HISTORY 



HIPPOCRENE 



I HAVE selected the genus Hippocrene for a special consideration of the naked-eved 

 Medusae, for the very obvious reason, that it can most easily be managed under the 

 microscope when alive, owing to its smaller size, and also, in some degree, to its peculiar 

 form, which makes it easier to keep it in a given position. This genus was established 

 nearly simultaneously, and quite independently, by two different authors. Mertens, 

 during his voyage round the world, saw a species to which he gave the name of Hip- 

 pocrene, which was described and figured in the Transactions of the Imperial Academy 

 of Sciences of St. Petersburg, for the year 1835. Lesson, on the other hand, described 

 it in the Annates des Sciences Naturelles, under the name Bougainvillia.* 



All we know at present of the structure of this remarkable genus is to be gathered 

 solely from the sources just mentioned, and from Lesson's work on Acalephae, forming one 

 of the continuations of Buffon, publisiied by Roret. Professor Forbes has added most 

 valuable information upon the British species. There is, however, one point in the 

 history of this animal as yet entirely unsettled. The other naked-eyed Medusae seem 

 to be all derived from hydroid Polypi, but the generation of Hippocrene has not been 

 sufficiently traced to have led to the knowledge of its alternate generation ; and though 

 I have seen its eggs laid, they are so minute as to escape the attention of the naked eye, 

 when dropped into water. The suggestion I have made,t that Hippocrene might be 

 the free generation of Tubularia, rests simply upon certain analogies between the germs 

 developed in the ovarian bunches of Tubularia and the full-grown Hippocrene, and by no 

 means upon a direct investigation of its metamorphoses. It remains, therefore, to be seen, 

 whether Hippocrene is really the alternate state of existence of Tubularia, or whether it 

 originates from some other hydroid Polyp. 



That Hippocrene should be one stage of generation of some hydroid Polyp is made 

 more probable, since the true relations of this little Medusa with Sarsia and other 

 naked-eyed Medusae have been fully ascertained, and I expect that it will be found to be 



•Although Professor Edward Forbes, in his Monograph of the Naked-Eyed Mediis(P,has preserved the 

 generic name of Lesson in preference to that of Mertens, upon due consideration I am inclined to go back to 

 the name of Mertens ; for, though Forbes mentions the year 1829 as the date of Lesson's genus, I cannot find 

 any definite indication of its establishment prior to the year 1836, in the Annales des Sciences Naturelles. 

 It is true Lesson was the first to describe the species upon which this genus rests ; but he established it under 

 the name of Cyanea Bougainvillei in Duperrey's Voyage round the World. 



t Lectures on Comparative Embryology. 



