PROF, AGASSIZ S 



another class, that of Medusae; ihe next among ra>- 

 diata, whose embryology we have to investigate. 

 But it is out of the question to understand the 

 changes which Medusae undergo,without knowing 

 their structure, and this structure is not only very 

 complicated, but it has been little studied and is 

 still obscure. I stand, therefore, with a very diffi 

 cuit task before me, and I ask your indulgence 

 upon this point. 



Let me begin by pointing out a few diagrams, 

 and saying a few words upon the figures before 

 you. 



(PLATE XIX YOUNG MEDUSAE.) 



Here (Plate XIX, fig's A B G) are outlines of a 

 family which has been described by Sars, the dis 

 tinguished Norwegian naturalist, as a peculiar 

 polypus, under the name of Scyphistoma. Here 

 (Fig. I) are other figures, which have been also 

 described by Sars as polypes, under the name of 

 Strobila. Here (Fig. J) is another free animal, de- 

 scribed under the rame of Ephyra. And here 

 (Fig. M) is another, found on the shores of the At- 

 lantic, both in Europe and in the United States, in 

 the temperate zone, which belongs to the genus 

 medusas. As to the class to which these various 

 animals belong, I may mention that the two ge- 

 nera, Strobila and Scyphistoma, were referred to 

 polypi, and the other two (Fig. J M) to jelly-fish- 

 es, or Medusoe. Now, gentlemen, it has been as- 

 certained within a few years, both by Sars and 

 Yon Siebold, that all these figures are the various 



stages of growth of one and the same animal. 

 We have here (Plate XIX) the metamorphoses of 

 one and the same animal changes which take 

 place in the growth of an egg. This (A) is an 

 egg,as it is laid by a Medusae. Here (Plate XX, A) 

 (PLATE XX. POLYPI CORYN^E, SYNCORYN^E, 



PODOCORYNuE ) 



we have a still more extraordinary structure 

 (syncoryna). You see these stems terminated by a 

 rosy colored head, from which tentacles, half a 

 dozen or more, arise, and out of these various bo- 

 dies, a little tubercle here, a more prominent one 

 there, and another bell-shaped here, with tentacles 

 around its opening. Here is another form (Fig, 

 B) called Podocoryna, by Sars, from which va- 

 rious kinds of buds arise, which do not resemble 

 the primitive stem; also much larger buds, which 

 differ still more, and which are at a certain time 

 freed and grow into other animals. Indeed, stems 

 of polypi, from which arise buds of medusae or 

 jelly-fishes, budding from polypi-like stems, be- 

 coming free and growing into a regular, simple, 

 isolated jelly-fish, like this (Plate XIX, N) ; this is 

 the case here, (Plate XX) a bud which grows into 

 a jelly-fish. It is, however, out of the question, 

 that in its different stages of growth, an animal 

 could belong to various classes, or that an animal 

 of one class could give rise by budding to animals 

 of another class. Therefore, it is perfectly obvious, 

 from the nature of these well authenticated facts, 

 that there has been a want of understanding of 

 these phenomena when they were first described ; 

 and it was not until a few years ago, when Steer- 

 strupp found out the key to this astonishing com- 

 plication, by ascertaining that there is an alter- 

 nation in the mode of reproduction of many ani- 

 mals, which takes place in different ways in the 

 animal kingdom. In some, there are eggs laid, 

 which eggs give rise to animals different from 

 their parents, and these in their turn give rise to 

 eggs, from which arise animals similar to their 

 grandparents and different from their parents. 

 In other cases, animals lay eggs which go to form 

 individuals different from themselves, and these 

 individuals, by buddinfir,or transverse division, pro- 

 duce forms which are freed, grow, and then re- 

 semble the parent, by a complicated process of 

 metamorphoses. 



However, though Steerstrupp, for the first time, 

 brought out these conclusions distinctly, he 



