20 



PROF. AGASSIZ S 



Now I have been able to trace the eggs of an 

 Oohinra which lived on this shore, and they, as well 

 as the voung star-fishes were free animals ; and 

 also were observed during winter 



The nuestion is now, whether there are not 

 among Fchinoderms, as among other low animals, 

 though the fact has not been traced by direct ob- 

 servations phenomena similar to what has been 

 observed among Jelly-fishes, where alternate gen- 

 erations take place, where animals of a peculiar 

 character are produced in one generation, from 

 which spring animals of another character, and 

 generation after generation alternately, the primi- 

 tive types are reproduced. 



That these must be some phenomenon of that 

 kind I can scarcely doubt, when I see other ani- 

 mals indicating a similar change, which has been 

 also observed by Johannes Muller, during summer. 



Here is a frame similar to that of the little ophi- 

 ura, containing within also a more opaque body, 

 with an opening below considered as a mouth, and 

 a connecting tube. 



The external frame,also formed of a solid,gelati- 

 nous mass, in the interior of which there are calca- 

 rious nets,and on the edges of which there are again 

 vibratory cilia all around these stems (Plate XI, fig. 

 A). And there are several groups of these vibra- 

 tory cilia in the form of crescent-shaped epaulettes 

 on the four corners of the animal. Here is a figure 

 of the same, (Plate XI, fig. B,) seen from above. 

 We have in this being, the same arching appenda- 

 ges which were noticed in Plate XII , but instead 

 of giving rise in their connection to a projecting 

 centre, they form a more rounded vault, from 

 which the elongated sticks hang down, diverging 

 somewhat. From the four corners, however, hang 

 down the four longest of these arms, and over the 

 arms of the corners are the fringed epaulettes; and 

 from one side two equally developed, between 

 which the mouth opens, an oesophagus and stom- 

 ach occurring in the centre, as in the young Ophiu- 

 ra. After the interior mass has undergone some 

 changes, you see, however, a very curious differ- 

 ence, which distinguishes at once this animal from 

 the other; a disc, which is observed upon that spher- 

 ical mass, namely, (Plate Xf, fig. B) the mouth as 

 it is called by Muller still hanging underneath, as 

 seen in another figure at the same stage of growth, 

 but viewed in profile (Plate XI, fig. C), where the 

 spherical mass with its lower tube is placed verti- 

 cally, and where that new disc formed upon iCs 

 surface is placed obliquely on one side of the up- 

 per portion. This disc has at this period five 

 somewhat prominent tubercles upon its surface, as 

 is seen here (Plate XI, figs. C and B), which will 

 become more developed (Plate XI, figure D) 

 The disc will grow larger over what was formerly 

 the main mass, the appendages will be somewhat 

 reduced in their lerrgth, and also in the develop- 

 ment of their vibrating cilia. And from that time 

 in addition to this, the five tubercles will be elong- 

 ated into five tubes or suckers, (Plate XI. fig. E.) 



[PLATE XI YOUNG SEA URCHINS j 



and there will be spines coming out between them, 

 the oesophagus and mouth being reduced and finally 

 disappearing. At this period,the young animal con- 

 sists, therefore, of a circular disc upon a spheroidal 

 body,with elongated suckers coming out of its edge, 

 with spines between them ; but the suckers, instead 

 of being in pairs, as they are in the Ophiura, are 

 only five in number (Plate XI. fig. F.) ; the main 

 mass of the primitive sphere forms still a spherical 

 body under the shield, the shield itself bending over 

 the spherical mass (Plate XI. fig. F). This disap- 

 pears, however, more and more, and at last there 

 is a little flattened, sea urchin-like animal pro- 

 duced, with at first five suckers, next with ten, 

 (Plate XI. fig. G.) with large spines alternating 

 with them, the greater portion of the spherical 

 body remaining, nevertheless, soft, as there are not 



