78 MORPHOLOGY. 



tubes, whose Ccavity is continuous with the original cavity of the bud, and which are lined by a 

 continuation of the endodcrm of the bud. There is no difficulty in recognising in these tubes the 

 radiating canals of the future medusa, and in the web of ectoderm which unites them the 

 umbrella. 



From the central point of the area included between the bases of the fom- canals another 

 hernial process (C d and D d) has already begun to make its appearance, composed of ectoderm 

 and endoderm, and containing a prolongation of the original bud cavity. It advances as a thick 

 process in the axis of the cup, and is at once recognisable as the future manubrium. 



The four peripheral processes continue to elongate, and are soon seen to be dilated into 

 bulb-like expansions at their extremities (E, F). The bulbs increase in size, and come in 

 contact by their sides ; while one of them, enlarging much more rapidly than the three others, 

 gives a marked preponderance to its side of the bud, and makes the distal end of the bud appear 

 as if obliquely truncated. It then begins to extend itself beyond this distal end into a thick, 

 hollow tentacle. 



In the mean time the four bulbs which had come in contact have coalesced, and their 

 cavities now communicate with oue another (G) ; but, by the gradual enlargement of the 

 distal end of the bud, the bulbous ends of the radiating canals are again drawn away from one 

 another. The communication, however, between their cavities is not thereby interrupted, but 

 continues to be maintained by a tubular elongation of their original points of union ; and in this 

 tube we now recognise the circular canal of the medusa. 



The cavity of the umbrella is still closed by the more external of the two laminas into which 

 the ectoderm had originally split at the distal end of the bud. In the final stage this lamina is 

 either perforated in the centre, in order to form the velum, or, what I now believe to be more 

 probable, it entirely disappears, and the velum is formed by a centripetal extension of the 

 ectoderm on a plane with the bulbous extremities of the radiating canals, at the time when 

 these bulbs are withdrawn from contact with one another, in order to form the circular 

 canal. 



The manubrium, previously imperforate, has now acquired a mouth at its extremity. The 

 solitary tentacle, too, has now become elongated, and presents its characteristic moniliform struc- 

 ture, the umbrella rapidly contracts and expands with vigorous systole and diastole, and the 

 medusa at last hangs on its stalk, a true Steenstriij)ia, ready to break away from the restraint of 

 its fostering hydranth, and enter upon an independent existence (H, and PI. XIX, fig. 5). 



From the above account of the development of the medusa bud, it will be seen that here 

 also I am not entirely in accordance with the views expressed by Agassiz on this subject. The 

 distinguished American naturalist gives a very detailed account of the process as he has inter- 

 preted it in the development of the medusa-bud springing from his Syncoryne mirabilis, and in 

 which he describes this development as starting with the separation of the endoderm from the 

 ectoderm in the primordial tubercle, and the inversion of the endoderm into itself, so as to form 

 the cup of the future umbrella. " In doubhng on itself the retreating fold does not press 

 closely on all points upon the stationary one, but leaves four equidistant spaces into which the 

 chymiferous fluid penetrates."^ These four spaces are the foundation of the four radiating canals, 

 which would thus originate in an entirely different way, and have a significance entirely different 



' ' Natural History of the United States,' vol. iv, p. 192, &c. 



