HYDRA. 107 



"V\1ien portions of the circular stratum were presented to the micro- 

 scope, I could not discover any difference between the planulse there and 

 those of the preceding Medusa. They were white, solid, elliptical, and all 

 in lively motion. Their extreme minuteness rendered the determination 

 of their exact form very diflScult by the naked eye. 



Many of the planulne were now set apart in different vessels, to re- 

 main for a certain time undisturbed. 



The progress of metamorphosis, on the whole, narrowly resembled 

 that of others, but perhaps with less irregularity, from the quiescence pre- 

 served ; and some interesting illustrative facts were disclosed. 



One of the vessels, a narrow cylinder, six inches deep, being examined 

 in ten days, I found the water covered by a thick scum like cambric paper, 

 and so tenacious that portions had to be cut out by scissors for the micro- 

 scope. 



Such portions proved wholly composed of metamorphosing planula*, 

 which were generally rounder, not so quadrangular, or rather so obtusely 

 cushion-shaped as the former ; nevertheless exhibiting considerable variety 

 of figure. Four originating tentacula had become evident in a few, and 

 in one of this description the distended orifice of the mouth exposed the 

 internal cavity below. — Plate XVI. fig. 15. No planulse now visible 

 were unchanged. 



All these subjects seemed imbedded in the scum, or they were per- 

 haps adhering to the under surface of it, for in further progress many 

 might be seen hanging down, as if suspended from that surface. 



The quantity of scum thus covering the surface of the water in the 

 vessels containing planukie is very great. An observer might readily con- 

 clude that its presence and profusion indicated decay. But it is not so. I 

 venture to conjecture that it may be more probably considered a conco- 

 mitant on metamorphosis. 



At first the planulaj crawl at the bottom of the vessel : very few under 

 ordinary circumstances rise on the sides, nor are they to be often seen 

 swmming. But after some time, a scum is formed on the surface of the 

 most limpid element, even in tall vessels, amidst which they are motion- 

 less, in thousands. I know not how they rise, whether by some super- 



