HYDRA. 109 



The natural position of the hydra in maturity, being pendent, may 

 account for its tendency to rise upwards in taller vessels, "with the com- 

 mencement or the progress of the change. 



As the advances of metamoi-phosis evidently depend on circumstances, 

 they must be attended with great irregularity. Many planulae decay with- 

 out any sensible altei'ation : in others the change advances tardily and with 

 difficulty : the difference of a fortnight's progress, or much more, will be 

 seen, in the state of two portions of the same brood at a given time ; and 

 in many, though great, it is never completed. But sometimes its progress 

 is distinct during the course of a long observation. I speak of the pro- 

 geny of JNIedusaj in general. 



A nascent hydra may acquire eight tentacula within twelve days of 

 its birth from the Medusa. But amidst above a thousand specimens of 

 the same brood, I could discover only one with twelve tentacula in forty- 

 five days. — PI. XVI. fig. 14. Many others had then eight, which were 

 long and deeply muricate. But some had only four, quite as long, and 

 as deeply muricate. None ever gained more than twelve : nor of at least 

 two thousand hydrse, originating at first in a vessel, did any survive, in 

 seventy days from their production. 



Unless in very favourable positions, it is by no means easy to follow 

 the history of such minute specks as those of which we treat. 



The most convenient method of observation is when they are affixed 

 to watch-glasses. The progeny of the Medusae, while yet planulse, should 

 be transferred to a flat vessel, some inches in diameter, filled to any height 

 ■vvith water. One or more watch-glasses may be then inverted above the 

 planulse, and the whole allowed to remain perfectly still for several days. 

 Meantime the planulae rising from the bottom, in the course of metamor- 

 phosis, are intercepted from the surface of the water by the watch-glass, 

 to which they will adhere. I have sometimes found them do so in two 

 days. After remaining there two or three days longer, the watch-glasses 

 should be removed to another vessel of water, and kept in an inclined po- 

 sition, the concavity where the planulas adhere being downwards. Many 

 young hydrae will be next seen, pendent by the prolonged apex, in the 

 course of eight days. It is preferable to have only a few in adhesion, in- 



