TITE TUNICATA. 321 
strument, it alone feels the injury, and retires, without any 
others being conscious of the danger, or of the hurt inflicted on 
their mate. Tbe polyzoa propagate by gemmation and by ova 
or eggs, which, germinating on the inner surface, escape at a later 
period into the visceral cavity, and are finally discharged into 
the wide sea, so to fulfil their mission in creation, and people the 
shores of every clime with myriads of busy workers in horn and 
in lime, which, with subtle chemistry, they draw from a fluid 
quarry and build up in textures of admirable beauty and 
heaven-ordered designs.” 
Each polyzoon begins with a single ovum. The original or 
seminal cell of a flustra or lepralia has no sooner fixed itself 
upon some stone, shell, or alga, than new buds begin to shoot 
forth, which in their turn produce others from their unattached 
margins, so as rapidly to augment the number of cells to a very 
large amount. Thus a common specimen of Flustra carbasea 
presents more than 18,000 individual polyzoa, and as each of 
these has about twenty-two tentacula, which are again furnished 
with about a hundred cilie a piece, the entire polyzoary pre- 
sents no less than 396,000 tentacula and 39,600,000 ciliz. The 
Rey. Dayid Landsborough calculated that a specimen of Flustra 
membranacea five feet in length by eight inches in breadth had 
been the work and the habitation of above two millions of in- 
mates, so that this single colony on a submarine island was about 
equal in number to the population of Scotland. As the tentacula 
are numerous in this species, four thousand millions of ciliz 
must have provided for its wants, about four times the number 
of the inhabitants of this globe! 
The Tunicata are so called because their soft parts are not 
enclosed in a calcified shell such as invests the majority of their 
class, but in a more or less coriaceous envelope or tunic which 
is either bag-shaped, and provided with two apertures, or tube- 
shaped, and open at the ends. They present a strong resem- 
blance to the Polyzoa, not merely in their general plan of 
conformation, but also in their tendency to produce composite 
structures by gemmation; they may, however, be at once dis- 
tinguished from them by the absence of the ciliated tentacula 
which form so conspicuous a feature in the external aspect of a 
flustra or a retepore. Their branchiwe, which have generally 
the form of ridges (e), occupy a large sac, forming, as it were, 
