142 
Z. HYDROIDA. 
Plumularia. 
ii. 259. Templeton in Mag. Nat. Hist. ix. 466. Risso, L’Europ. merid. 
y. 313 La P. enfaux, Blainv. Actinolog. 477. 
Hab. On shells and rocks near low water-mark and in deep water. j 
A common and very elegant species, generally from 4 to 6 inches in 
height, rising in wide spiral turns, and sending out from its filiform 
percurrent stem, at regulated intervals, alternate spreading plumous 
branches which are placed one above the other on the outer side. 
Pinnae alternate, bifarious. In young specimens the branches are 
two-ranked and alternate, and I have seen this character remain in one 
specimen of considerable size. There are no cells on the spiral 
stem, but they occur on the branches as well as on the pinnae, and 
are arranged in two rows pointing alternately to opposite sides. There 
is a fine figure of the coralline in the centre of the curious frontis- 
piece to Ellis’s Essay ; and the magnified figure in tab. 38 is a more 
correct representation of the cells than that given in tab. 7, which 
has been drawn from a dried specimen. The ovarian vesicles are of 
uncertain occurrence, and I have seldom seen them ; they are scattered 
irregularly on the branches, stalked, ovate or pear-shaped, with a short 
tubulous aperture, and occasionally wrinkled longitudinally when dry 
“ This species is very common in the deeper parts of the Frith of 
Forth ; its vesicles are very numerous, and its ova are in full matu- 
rity at the beginning of May. The ova are large, of a light-brown 
colour, semi-opaque, nearly spherical, composed of minute transpa- 
rent granules, ciliated on the surface and distinctly irritable. There 
are only two ova in each vesicle ; so that they do not require any 
external capsules, like those of the Campanularia, to allow them suf- 
ficient space to come to maturity. On placing an entire vesicle* 
with its two ova, under the microscope, we perceive through the 
transparent sides, the cilise vibrating on the surface of the contained 
ova, and the currents produced in the fluid within by their motion. 
When we open the vesicles with two needles, in a drop of sea- water, 
the ova glide to and fro through the water, at first slowly, but after- 
wards more quickly, and their cilise propel them with the same part 
always forward. They are highly irritable, and frequently contract 
their bodies so as to exhibit those singular changes of form spoken off 
by Cavolini. These contractions are particularly observed when they 
come in contact with a hair, a filament of conferva, a grain of sand, or 
any minute object ; and they are likewise frequent and remarkable at 
the time when the ovum is busied in attaching its body permanent- 
ly to the surface of the glass. After they have fixed, they become 
flat and circular, and the more opake parts of the ova assume a radiat- 
ed appearance ; so that they now appear, even to the naked eye, like 
