HYDRA. 105 



simple active atom, — the planula had been discharged from the unwieldy 

 Medusa, it was converted to a stationary hydra ! 



This new animal was provided with a complement of eight arms, yet 

 80 immature as to be of unequal dimensions. Different groups, under 

 metamorphosis, shewed the utmost irregularity in respect to evolution, to 

 their shape, and proportions, figs. 9, 10 : nor was it until thirteen days later, 

 or three Aveeks after their birth, that any appeared with eight regular 

 tentacula. 



Thus was a most perplexing problem solved — the Hydra tuba proved 

 to have sprung of a Medum. 



The planulae of the arborescent zoophytes already described, and 

 many whereof we have yet to treat, are generated in external pods 

 or vesicles, from whence they issue when mature into active life ; next 

 becoming quiescent, and metamorphosing to the zoophyte . Here the host 

 of planulffi is contained in bunches of capsules reposited in the ova- 

 rian sacs or cavities above specified as belonging to the under surface of 

 the Medusa. The different capsules vary in size and in form — Plate XVI. 

 fig. 1. 



There is no doubt of the planulae being originally ova ; and that their 

 evolution advances as in the vesicles of the arborescent zoophytes, or as 

 the rudimentary embryo retained in the capsule of the Tubularia. 



I have seen a stream of cremacious-like matter absolutely flow from 

 the ovarian sacs of the Medusa, proving exclusively incalculable legions 

 of planulae. 



Much irregularity prevails in everything connected with the history 

 of these animals. I cannot specify the longest period of activity by the 

 planula. I have observed some in motion during 18 days, and some, from 

 the Mcdma capillata, during 19 days. The latter always advanced with 

 the smaller extremity first, during progression, whether from natural 

 habits or supervening incident. 



The metamorphosis of that species of the Chrysaora, the more par- 

 ticular subject of these observations, commences very speedily, often within 

 forty-eight hours, or even sooner, if the planula be kept still, which is es- 

 sential. But there is no uniformity either in progress or in perfection. 



VOL. I. O 



