Natural History of British Zoophytes. 245 



" from their moving, irritable, and free condition of animalcules, 

 to that of fixed and almost inert zoophytes, exhibits a new meta- 

 morphosis 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 immoveably to one spot, the reproductive germs would 

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

 mechanism as we have described, been carried to a distance, and 

 spread over the bosom of the deep. 



The evolution of the gemmules, subsequent to their fixation, has 

 been minutely traced by Professor Grant and Mr Dalyell. When 

 the bud falls from the crested head of Tubularia indivisa, slight pro- 

 minences, enlarged at the tips, pullulate from the under surface, 

 and the " nascent animal" elevating itself on these rudiments of the 

 tentacula, as on so many feet, enjoys the faculty of locomotion. " Ap- 

 parently selecting a site, it reverses itself to the natural position 

 with the tentacula upwards, and is then rooted permanently by a 

 prominence, which is the incipient stalk, originating from the under 

 part of the head. Gradual elongation of the stalk, afterwards con- 

 tinues to raise the head, and the formation of the zoophyte is per- 

 fected."* — So the worm-like embryo of the vesiculiferous Hydroida, 

 a few days after its exclusion from the vesicle, becomes stationary 

 and contracts into a circular or spherical spot which always retains 

 its original colour. It is transparent and soft, but in a short time 

 some opaque fleshy spots are visible within it, and are separated by a 

 thin homogeneous transparent substance, which is to form the future 

 polypidom. " As yet it is exceedingly minute, soft, and gelatinous ; 

 but in the progress of its growth, the soft, thin, homogeneous sub- 

 stance of the exterior becomes more dense, embracing the first form- 

 ed parts of the fleshy substance, indeed all parts, and the whole 

 jelly, with its thin covering, and continues to advance and to radi- 

 ate. Then we observe a stem beginning to rise from the centre of 

 these radii of roots, which are, in fact, the first formed parts that 

 the little round gemmule shoots out. So that the gemmule is be- 

 come, not a polypus but a root. It begins then to rise from the cen- 

 tre of the roots, and at length to divide ; so it will at length form 

 on its branches a cell, at the bottom of which cell will gradually be 

 developed a polypus."t In the Flustra and other ascidian zoophytes 

 the process is very similar, but in these, instead of the rootlets and 

 little embryo stalk, a cell is the part first formed, in which a po- 



* Dalyell, in Edin. New Pliil. Journ. xvii. 412. 

 f Grant, in the Lancet, 1834, Vol. i. p. 229. ' 

 NO. III. E, 



