368 
MOLLUSCA. 
Mollusca. et sur les Ascidies simples, inserted in his Mémoires sur les 
Sy Animaux sans Vertébres. 8vo, Paris, 1816. 
The covering of the animals of this group consists of an 
external and internal sac or tunic, which are either entirely 
united or unconnected, except at the apertures. The sur- 
face is smooth in some, and rough in others, and in a few 
species defended by an artificial covering of agglutinated 
shells and sand. The sacs are furnished with muscular 
bands, and are capable of contraction. Some of the species, 
by means of contractile movements, float about in the water 5 
others, receiving that element into the branchial cavity, and 
ejecting it forcibly at the opposite one, push themselves for- 
ward. Many, however, are fixed during life to seaweeds 
and stones. 
The apertures of the tunic are two in number, unless in 
the doubtful genus Mammaria. The one, frequently the 
largest, is destined for receiving the water into the cavity 
to supply the mouth and gills. This is termed the dranch- 
ial cavity. The other is destined for the exit of the water, 
the eggs, and the faeces, and termed the anal opening. 
These apertures are sometimes placed near each other, at 
other times at opposite extremities of the body, and vari- 
ously provided with tentacula or valves. 
The mouth is simple, destitute of spiral arms, and open- 
ing in the anterior of the cavity of the body between the 
branchiz, as in the other Acephala. It possesses neither 
jaws nor tentacula. The alimentary canal is very simple, 
and can scarcely be distinguished into gullet, stomach, and 
intestine. The food is soft, and such as the bounty of the 
waves bestows. The liver adheres to the stomach, and in 
many species is divided into distinct lobes. 
The circulating system appears to be reduced to asingle 
systemic ventricle. The gills cover the walls of the cavity, 
in the form of ridges, more or less complicated, and seldom 
symmetrical. 
The reproductive organs consist of an overium, either 
simple or complicated, with some additional glands, the uses 
of which have not beenascertained. The species are con- 
sidered as hermaphrodite, and independent of reciprocal im- 
pregnation. They appear not only to be oviparous, but to be 
gemmiparous and compound, many individuals being organi- 
cally connected, and capable of simultaneous movements. 
They are all inhabitants of the sea. 
Ist Subdivision.—Dichitonida. 
Interior tunic detached from the external one, and united 
only at the two orifices. 
The branchiee are large, equal, and spread on the central 
walls of the inner sac. The branchial orifice has an inner 
membranaceous denticulated ring, or a circle of tentacula. 
ist Tribe. 
Body permanently fixed to other bodies. 
In this tribe the branchial and anal orifices are not op- 
posite each other, and do not communicate through the 
branchial cavity. This cavity at its opening is furnished 
with tentacular filaments. The branchiz are conjoined an- 
teriorly. 
A. Simple. 
This division includes the genus Ascidia of Linnzus. 
The individuals are independent of each other, and although 
they frequently adhere together in clusters, they are desti- 
tute of a common covering, or organical connexion. 
1. Apertures furnished with four rays. 
The animals of this group have the external tunic cori- 
aceous, dry, opaque, rough, folded, and frequently covered 
with extraneous bodies, or inclosing such. The branchial 
orifice has four rays, the anal one the same, or divided trans- 
versely. The branchie are divided longitudinally into per- 
sistent regular deep folds. 
a. Body pedunculated. 
The peduncle, in this division, may be said to have its 
rise in the summit of the body, which it serves to suspend. 
Sorme. 
The abdomen is lateral. The meshes of the branchiz Mollusca. 
are destitute of papillz. 
Genus Bourenta.—The tentacular filaments of the 
branchial circle are compound. ‘There is no liver, and the 
ovarium is compound. Only one species is known, B. fus?- 
Savigny, Mem. tab. i. f. 1., and tab. y. f. 5. It is 
the Vorticella Boltent of Lin. and the Ascidia Clavata of 
Shaw. 7 
6. Body sessile. 
M. Savigny describes this group as a genus, which he 
terms Cynthia, which he divides into four sub-genera. 
(A.) Tentacular filaments of the branchial orifice com- 
pound. The folds of the branchize more than eight in num- 
ber. The liver distinct, and surrounding the stomach. 
Ovarium divided, with one division at least on each side the 
body. The intestine destitute of a rib. 
Genus Cynruta.—Meshes of the branchiz unchanged 
by the folds. C. Momus. Sav. tab. i. f. 2. 
Genus Casrra.—Meshes of the branchie interrupted 
by the folds. C. Diona of Sav. tab. vii. f. 1. The Ascidia 
quadridentata of Forskael. 
(B.) Tentacular filaments of the branchial orifice simple. 
The folds of the branchize eight in number, four on each 
side, and the meshes uninterrupted. Intestine strengthen- 
ed by a cylindrical rib from the pylorus to the anus. Liver 
absent or indistinct. 
Genus Sryeta.—Ovarium divided, one division at least 
on each side. .S. Canopus. Sav. tab. viii. f. 1. 
Genus Panpocra.—Ovarium single, and situate in the 
fold of the intestine. The Ascidia conchilega, a native 
species is the type. 
2. Apertures with indistinct rays, or more than four.’ 
The external tunic is here soft, easily cut, and translu- 
cent. The rays (when existing) of the branchial orifice 
amount to eight or nine; and those of the anal to six at 
least. The branchize are destitute of longitudinal folds. 
The tentacular filaments of the branchial circle are simple. 
Liver indistinct. Ovarium single. 
a. Body pedunculated. 
The stalk is here placed at the base, and serves to sup- 
port the body, being of an opposite character from that of 
the Boltenia. 
Genus Ciuavetrna.—Branchial and anal orifices with- 
out rays. Angles of the branchial meshes simple. Intes- 
tine destitute of a rib. he Ascidia clavata of Pallas, and 
the A. lenadiformis of Miller belong to this genus; the 
latter of these is now recorded as a British species. 
As connected with this group the small Ascidia, figured 
and described by Mr. Lister in the Philosophical Transac- 
tions for 1834, deserves particular notice, as the following 
important observations which he has recorded, sufficiently 
indicate. They will indeed be perused with peculiar inter- 
est by all who are practically engaged in the study of mol- 
luscus animals. 
This compound animal occurs in groups that consist of 
several individuals; each having its own heart, respiration, 
and system of nutrition, but fixed on apeduncle that branches 
from a common creeping stem, and all being connected by 
a circulation that extends throughout. Their parts of such 
transparency that their interior is easily seen. Their ex- 
ternal shape is that of a pouch, compressed at the sides, and 
fixed at the hind part of its base upon the peduncle. 
Its two openings are in the form of very short tubes; that 
of the mouth at the top of the pouch, and that of the funnel 
in front. The longest diameter, from the peduncle to the 
space between the openings, is about 085 inch. : 
The outer covering is a tough coat, acontinuation of the 
peduncle, more pliable near the openings; lined interiorly 
with a soft substance or mantle, in which a ramifying cireu- 
lation is very distinct. A great part of the interior is occu- 
pied by the branchial sac, which is subcylindrical, flattened 
So 
