ACALEPHiE. " 187 



those of Dalyell (1836), established a series of metamorphoses carried 

 on from the Hydra gelatinosa or Hydra tuba, to the medusa state. 

 Our next question, at that date, would have been; — What does this 

 Hydra tuba come from ? For this part of the history of the Medusa, 

 we are indebted to Prof. Siebold (1838), who traces the development 

 of the ova of the Medusa ( Cyancea aurita) to the Hydra tuba, as has 

 been already detailed ; during which, as in Campanularia, a ciliated 

 monadiform planula precedes the polype stage ; and thus the two 

 detached series of links are united into one harmonious metagenetic 

 cycle. 



Confirmation by different observers is not wanting. The accurate 

 Sars, pursuing his researches, followed out, independently, the whole 

 series of changes of the ova of the Medusa, from the infusorial to the 

 polype-type, thence to the scyphistoma and strobila ; finally to the 

 budding forth of the flattened segments, and the splitting up into a 

 pile of Medusa3. The details, with figures, were published in 1841.* 

 But Dalyell has shown us that the polype larva may have propagated 

 hundreds of hydrse, before it issues into the pile of Medusae. The 

 sea which washes our coasts is sometimes covered by millions of 

 these little Medusae. Dalyell narrates, in his beautiful work, that 

 one summer having selected a specimen of a Medusa of the genus 

 Chrysaora, he kept it alive in a capacious jar of sea-water for a few 

 days. On removing it from the jar in which it had been placed, he 

 found that a quantity of brownish matter, like dust, remained at the 

 bottom. Subjected to the microscope this proved to be a host of 

 animated creatures in quick and varied motion ; each was of an oval 

 form, ciliated, actively moving, like the planulae. Some of these, 

 after a period of ten or twelve days, become developed into stationary 

 hydr^e.f 



Such is the nature of the full and satisfactory evidence on which 

 the main facts of the generation of the Medusae are established. In 

 comparing different stages of the very interesting development of 

 the Cyancea aurita to Infusories and Polypes, it must be under- 

 stood that such comparisons are warranted only by a similarity of 

 outward form and of the instruments of locomotion and prehension. 

 The essential internal organisation of the persistent lower forms of 

 the Zoophyta is wanting in the transitory states of the higher ones. 

 A progress through the inferior groups is sketched out, but no 

 actual transmutation of species is effected. The young Medusa, 

 before it attains its destined condition of maturity, successively re- 



* CLIL t CIX. vol. i. r- 103. 



