20 ANTHOZOA HYDROIDA. 



resemblance of the common Hydra, which settles, roots itself, 

 and glides insensibly into the resemblance of its parent 

 species. 



There is still another mode of propagation amongst these 

 zoophytes, viz. by ciliated ovules. These are produced by 

 all those genera which have external ovarian vesicles, and by 

 some naked species. They proceed as usual from the common 

 pulp or fleshy axis, and are matured in the vesicles, which 

 fall off on their discharge. But the maturation seems to be 

 in two degrees. First, the ovules, after they have become 

 distinct and separate from the central pulp in the vesicle, and 

 after they have obtained a self-rotatory motion, are observed 

 to lose their sphericity on approaching maturity ; their shape 

 alters, " it elongates, becomes elliptical, next prismatic, and at 

 length each corpusculum issues as a perfect animal from the 

 orifice of the vesicle," and exhibits in figure and in motion 

 much resemblance to the little leech-like Planarise ; hence 

 this ovule was called a planule by Sir J. G. Dalyell. Se- 

 condly, the ovules are discharged in their oviform condition 

 and clothed with vibratile cilia. So soon as at liberty, these 

 move and swim in the water as if they were guided by voli- 

 tion and sense, whirling at the same time on their axis, and 

 stopping occasionally as if in search of a situation on which to 

 fix/''" This freedom to move whither they list may continue 

 for several hours, or even for two or three days, before a 

 proper site for their permanent stay and future growth is 

 found, when they begin to shoot up rapidly into those beauti- 

 ful forms particular to each species, as the Supreme Being has 

 ordered and determined. The transformation of the ova, 

 says Dr. Grant, "from their moving, irritable, and free con- 

 dition of animalcules to that of fixed and almost inert zoo- 

 phytes, exhibits a new metamorphosis in the animal kingdom, 

 not less remarkable than that of many reptiles from their first 

 aquatic condition, or that of insects from their larva state." 

 One purpose of this mobility in the ova is obvious: it is a 

 means ordained for their diffusion, for, the parents being fixed 

 immovably to one spot, the reproductive germs would have 

 dropt and sprung up at their roots, had they not, by some 



* See Couch in Ann. N. Hist. xv. p. 162 ; Corn. Fauna, iii. p. 5. 



