382 TuNiCAi'A. MOLLUSCA. Ascidians. 
morphoses. The larvjB are like tadpoles in shape, are 
they ai’e sombre in their colours, but in the sunny 
solitary and free, swimming rapidly by the aid of the 
regions of the south their hues assume the brightest 
undulatory movements of a long tail. Tliey attach 
dyes, and vie with those of the corallines and actiniie 
themselves occasional!}'; they then change their form. 
that people the bed of the ocean. It was in the 
lose their tail, and after a few days’ free and locomotive 
Ascidian Tunicaries that MM. Audouin and Milne 
life, finally fix themselves. In addition to reproduction 
Edwards first discovered the metamorphoses of the 
by ova the compound Timicaries have been ascertained 
Mollusca, and their discoveries have since been ex- 
to possess a second mode, viz. — by gemmation. This 
tended by the laborious researches of Sars and Loven. 
process is superinduced upon the young Ascidian after 
It was among these singular beings that Van Hasselt 
its development from the ovum. “ The individuals 
discovered “ a heart of such extraordinary character. 
formed by the gemmation of the primary bud of the 
changing incessantly its auricle to ventricle, and its 
young Ascidian are, instead of being detached, retained; 
ventricle to auricle — its arteries to veins, and its veins 
the process of gemmation being regulated so as to pro- 
to arteries.” Among the Salpian Tunicaries it was. 
duce the characteristic pattern in which the different 
moreover, that Chamisso made the no less extraordinary 
individuals are grouped in the mature compound ani- 
discovery that a ‘ Salpa mother is not like its daughter 
mal.” — {Owen.) The greater number of the Tunicaries 
or its own mother, but resembles its sister, its grand- 
are fixed in their adult state; but there are others that 
daughter, and its grandmother.’ The Pyrpsomes afford 
are always free, and only found in the open ocean. 
a spectacle to the naturalist of unexampled beauty 
They thus form two natural groups. Of the first or 
when, seen by myriads from the vessel in the night. 
fixed group, some of the individuals become entirely 
they gleam with phosphorescent radiance, forming vast 
distinct, in others they remain connected by a vascular 
shoals of mimic pillars of fire, illuminating all around 
canal, and in a third set all the individuals become 
with a green unearthly glare. The most curious fea- 
blended into a common mass. These groups form the 
ture in the history of these soft-shelled mollusks 
Solitary, the Social, and the Compound ascidians of 
is the fact that many among them form communities 
authors. The animals belonging to the class Tunicata 
of beings, like the corals, a commonwealth of beings 
are all marine, and are numerous in all seas, being 
bound together by common and vital ties Each star is 
found from low water to a considerable depth. “ Ad- 
hering to rocks and sea-weeds,” says Mr. Adams, 
“ these strange, bag-shaped, leathery bodies may be 
a family, each group of stars a community — individuals 
are linked together in systems, systems combined into 
masses.” 
seen along the strand at low water, ejecting when 
The Class Tunicata has been divided into two orders 
touched the sea-water to some distance ; and on the 
or divisions by Dr. Fleming, according as the inner or 
ocean their lengthened sinuous chains, or pellucid 
muscular tunic or sac is more or less detached, or is 
phosphorescent tubes, cannot fail to arrest the eye of the 
adherent to the external tunic. Those individuals 
voyager. The compound forms exhibit, in the varied 
which have this inner sac more or less detached from 
arrangement of the individuals composing the general 
the external sac are termed Dicliitonida ; and those 
mass, a number of elegant stems and flowers of curious 
which have it adhering throughout to the external tunic 
and complicated design. Towards the northern shores 
are termed Monochitonida. 
Order— DICHITONIDA. 
Inner sac more or less detached from the external 
containing few or many irregularly- shaped leathery 
tunic ; united to it at the two orifices. Branchiae large, 
bodies, fixed to sea-weed, rock, or shell by one extremity 
equal, spread on the inner surface of this sac. Bran- 
or by one side, free at the other, and presenting two 
dual orifice with an inner membranaceous ring and 
more or less prominent orifices, from which on the 
circle of tentacles. 
slightest pressure the sea-water is ejected with great 
This order contains four families, three of which 
force. On the sea-shore, when the tide is out, we find 
are fixed, and the fourth floating. 
Family I. — ASCIDIID.® {Simple Ascidians). 
similar bodies attached to the under surface of rougli 
stones. They are variously, often splendidly coloured, 
but otherwise they are unattractive or even repulsive 
in aspect. These creatures are Ascidiat, properly so 
This family has been thus characterized: — Animal 
called. Numbers of them are found clustering among 
simple, fixed, solitary or gregarious; not united into 
tangles, like branches of some strange semitransparent 
groups by a common integument ; oviparous ; not 
fruit. They are very apathetic and inactive, living 
gemmiparous ; sexes united ; branchial sac simple 
upon microscopic creatures drawn in with currents of 
or disposed in (eight to eighteen) deep and regular 
water by means of their ciliated respiratory oigans. 
folds. 
The leathery case is often incrusted with stones and 
The species of Ascidians are very abundant in the 
shells, decorated with parasitical though ornamental 
northern seas, on the British coasts, the northern shores 
plumes of corallines, and not seldom perforated by 
of Europe, and in the Mediterranean. “ Barely is the 
bivalves which lodge themselves snugly in the tough 
dredge drawn up from any sea-bed at all prolific in 
but smooth skin.” Some of the Ascidians appear to 
submarine creatures,” says Professor Forbes, “without 
have been well known to Aristotle ; and his description 
