244 Natural History of British Zoophytes. 
sides which originate from the centre of the stem. In Flustrae, cal- 
careous Cellariae, and some others, the ova are formed in the cells, 
but exterior to the bodies of the polypi, which disappear before the ova 
arrive at maturity. In the Lobularise, Gorgoniae, Spongiae, Clione, 
&c. the ova are formed and matured in the common fleshy substance 
of the body before they advance to be discharged through the poly- 
pi, or the fecal orifices."* 
The gemmules exhibit considerable variety in colour, they are 
milk-white, yellow, red, pink or green, but sometimes the colour is 
not fully developed until near maturity. In all the ascidian polypes 
they are globular or have a tendency to that form, and appear to pre- 
serve it until after their discharge from the body ; but in the Hy- 
droida this is not the case, for although spherical in their earlier 
stages, " their shape alters on approaching maturity ; it elongates, 
becomes elliptical, next prismatic, and at length each corpusculum 
issues as a perfect animal from the orifice of the vesicle," and exhi- 
bits in figure and in motion much resemblance to the little leech- 
like Planariae. (Fig. 5.t) The gemmules are all 
clothed with vibratile cilia similar to those of 
the tentacula of the parents. Sometime pre- 
vious to their discharge from the polypidom 
they are observed to be in a constant rota- 
tory motion, by which their birth seems to be 
facilitated ; and now at liberty in the water they move and swim 
about as if they were guided by volition and sense, whirling at the 
same time on their own axis.J 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 beautiful 
forms particular to each species, as the Supreme Being has ordered 
and determined. The transformation of the ova, says Dr Grant, 
* Edin. Journ. of Science, No. 14. 
f The figures represent the gemmules of Plumularia pluma. For the draw- 
ing I am indebted to my friend Dr Coldstream of Leith. 
\ In reference to those of Flustra carbasea and the observation appears to 
be very generally applicable Dr Grant says " they are very irritable, and are 
frequently observed to contract the circular margin of their broad extremity, and 
to stop suddenly in their course when swimming ; they swim with a gentle glid- 
ing motion, often appear stationary, revolving rapidly round their long axis, with 
their broad end uppermost, and they bound straight forward, or in circles, with- 
out any other apparent object, than to keep themselves afloat till they find them 
selves in a favourable situation for fixing and assuming the perfect state." Edin. 
New Phil. Journ. iii. 117. 
