34 



PROF. AGASSIZ S 



[PLATE XXIX A FRESH WATER POLYPUS, WITH 

 A SIMPLE CAVITY AND A MOVEABLE STEM.] 



vibratory cicil'a, by which they first moved, are 

 finally cast. There is a depression forming upon 

 the summits, and then two little horn-like appen- 

 dages grow out. [Fig. C.] They grow larger. [Fig. 

 D.] The tentacles grow longer, the depressions 

 still deeper, and then there is finally a central cav- 

 ity with four distinct tentacles. [Fig. E.] Then 

 there will be a little Hydra like animal, with eight 

 tentacles, a cential cavity, and a peduncle by 

 which it is attached. [Fig. F ] 



This is the first development of the germ of the 

 common medusse, the jelly fishes of this shore, 

 which are known in Boston harbor under the 

 name of sun-fishes. When it is grown somewhat 

 larger, a contraction takes place under the rows of 

 those tentacles, which have become more numer- 

 ous. In this stage of growth buds may also be 

 found. (Fig. H) New individuals may thus 

 arise from buds on the sides of this simple stem, 

 and these new individuals may grow to a consid- 

 siderable size with the parent stalk before they 

 separate. But at last they will separate, and grow 

 by themselves and form new sticks. So that we 

 have here two modes of reproduction among me- 

 dusae; in the first place, from eggs, which grow 

 into polyp-like animals, (Plate XIX, fig. A F) and 

 secondly, by buds which will produce new individ- 

 uals, (fig. H.) The bad being separated from the 

 main body, will even form new colonies, and so 

 on, (Fig. H.) At first these buds differ somewhat 

 from the parent stock, but soon assume the same 

 character, differing slightly when they are finally 

 freed. 



There are animals in which the successive buds 

 differ much more. There are in this (Plate XXVIII) 

 Campanularia, as it is called, buds which give rise 

 to animals with large tentacles, and there are oth 

 ers with shorter tentacles, and there are even 

 others of a differently pe; so that the various buds 

 which grow from one stock may differ widely 

 and yet be buds of one and the same stock. - 

 Here, in the young Medusas (Plate XIX) we see 

 that only one kind of buds arise but there has 

 been still another mode of reproduction and multi 

 plication observed in the same animal (Plate XIX, 

 fig. I). The stem, on growing longer and higher, 

 (Fig. G ) will begin to divide by transverse contrac- 

 tions into articulations. There are at first, simple 

 folds noticed in the skin, scarcely deepened to any 

 extent, but gradually growing deeper and deeper, 

 so that at last it seems as if a pile of discs were 

 heaped upon each other, (Plate XIX, fig. I,) the 

 lower part of which is a simple stem, as in Fig. G, 

 and the upper part, still consisting of a row of ap- 

 pendages as they have grown upon the summit of 

 this little Polyp and Serrate (Figs H and G). Next, 

 the edges of the discs begin to be fringed, (Fig. I,) 

 the cut growing deeper and deeper, these serra- 

 tures assume a regular form, and the contraction 

 growing successively deeper and deeper, those ser- 

 rated discs, almost separated from each other,form 

 a pile of loose discs simply connected by a central 

 axis. And as soon as the Polyp has divided into 

 this series of discs, the upper tentacles, that is to 

 say, the tentacles of the primivite Polyp, with the 

 upper disc, die away. What formed first the prin- 

 cipal part of the growing animal, dies away, ex- 

 cept the basal attachment,which remains; and next, 

 in the remaining pile, the uppermost disc frees it- 

 self from the pile and begins to swim. But the 

 moment it is free it assumes an inverted position, 

 (Fig. K) ; those fringes which were upwards, now 

 are turned downwards. The inner surface, which 

 was first upward, is now downward also. In this 

 way, a series of these serrated discs (Fig. L) are 

 successively freed from a primitively undivided 

 stem, by gradual transverse articulations, to form 

 as many independent individuals (Fig. T), which 

 after all can be traced to one single egg. 



There are finally quite a number of individuals 

 formed, which have arisen simply by transverse 

 division, and by the successive modifications 

 which each of these discs has undergone. And, 

 after freeing themselves, the Ephyrss, as they are 

 called, (Fig. J M) will undergo such changes as to 

 assume those structural peculiarities which char- 

 acterise the perfect Medusae. The tube will be- 

 come hollow. The cavity will enlarge, and that 

 will have its tubes, branching into the disc by va- 

 rious canals, (Fig. M.) Those canals will circulate 

 fluid around the disc, and finally the complicated 

 structure of Medusae (Plate XIX. Fig. M.) is pro- 

 duced by the addition of fringes on the edge ; and 

 the growth of processes on the side of the stomach 

 which give rise to the egg, the eggs always hang- 



