Natural History of Britisli 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 ee 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. <e 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 gernmule 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 Phil. Journ. xvii. 412. 
f Grant, in the Lancet, 1834, Vol. i. p. 229. 
NO. III. 11 
