OF THE ACALEPH^ OF NORTH AMERICA. 225 



The existence of this type on the American shores was first mentioned by Dr. Gould, 

 in his Report on the Invertebrate Animals of Massachusetts, in which Oceania tubulosa 

 of Sars is enumerated among the animals occurring along these shores. They were among 

 the earlier Medusne I saw on this continent, Dr. Gould himself having kindly showed 

 me the localities where he procured them. Since that time, early in the spring of lui.7, 

 I have also obtained, by dredging, specimens of the Coryna of Boston Bay, from which 

 the perfect Medusa is derived. 



We have here a double question to settle, — first, whether the European species 

 is identical with that of North America, and, next, under what generic name this species 

 should finally stand in our system. This is a matter of no slight importance, now that 

 the zoological nomenclature can no longer be viewed in the same light as formerly. 

 The knowledge of alternations of generations of animals of most heterogeneous forms, be- 

 longing to the same specific types, calls for a precise understanding about their systematic 

 names. As the question has not yet been discussed, 1 shall, I trust, be allowed to say a 

 few words upon it, although it is not my intention to describe on this occasion what I 

 know of the metamorphoses of this animal, but only to allude to its structure, reserving 

 my account of the embryology of Medusae for another paper. 



Reviewing all the facts which have been brought to light on these points, and availing 

 myself of my own investigations, I have shown, in my Lectures on Comparative Em- 

 bryology,* that Hydroid Polypi are not simply a lower form of stemmed animals, pro- 

 ducing at a given period more highly organized Medusae, but that they are themselves, 

 by their structure, real Medusa?, differing from the free forms with which they alternate 

 mainly in their being attached by a stem, and much less in their real structure than was 

 perhaps supposed at the time when the so-called Hydroid Polypi were still considered 

 as members of the class of Polypi. If I am right in this respect, — if Sarsia and other 

 naked-eyed Medusae can no longer be considered as the productions of animals belonging 

 to another class, — if we have really to remove the so-called Hydroid Polypi to the 

 Medusae proper, and simply to view them as alternate generations of these Medusae, or as 

 different modes of existence of animals of one and the same species, — then there can be 

 no doubt of the propriety of describing all these phases of their development and alternate 

 generation under one and the same generic name ; by which method we shall at once 



* Twelve Lectures on Comparative Embryology, delivered before the Lowell Institute, December and Jan- 

 uary, 1848-49. Boston, 1849. 8vo. — See also a paper read before the American Association for the Ad- 

 vancement of Science, held at Cambridge in August, 1849, entitled, On the Plan of Structure and Homologies 

 ef Radiated Animals, with reference to the systematic position of the so-called Hydroid Polypi. 



