ACEPHALA NUDA. 
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tuberance, but the cloak is prolonged into certain points. And of these some have a single point at each extremity, 
others have two, three, or even more at the oi-al extremity ; some have one only at that end ; and the greater number 
are simply oval or cylindrical. 
The Ascidi.e {Ascidia^ Linn.), Thetyon of the Ancients. 
The cloak and its cartilaginous envelope, which is frequently very thick, resemble sacs everywhere 
closed, except at two orifices, which correspond to the tubes of many Bivalves, one of which admits the 
water of respiration, and the other is the vent. Their branchise form a large sac, at the bottom of 
which the mouth is situated, and near the mouth is the mass of viscera. The envelope is much wider 
than the cloak propei'ly so called. This is fibrous and vascular ; and we perceive on it one of the 
ganglions between the two tubes. These animals attach themselves to rocks and other bodies, and are 
deprived of all power of locomotion ; the chief sign of vitality which they exhibit consists in the ab- 
sorption and evacuation of water through one of their orifices : when alarmed, they eject it to a con- 
siderable distanee. They abound in every sea, and some of them ai'e eaten. 
Some species are remarkable for the long pedicle which supports them. M. Savigny, from his own researches 
and mine, has attempted to subdivide the Ascidiae into several subgenera: such are Cynthia, — body sessile, envelope 
coriaceous, branchial sac plaited longitudinally. Phallusia differs from the preceding in the branchial sac not being 
plaited ; their envelope is gelatinous. Clavellina, — the branchial sac without plaits, not reaching the bottom of the 
envelope, the body pedunculate, the envelope gelatinous. Boltenia—i\\& body pedunculate, and the envelope coria- 
ceous. He also takes into consideration the number and form of the tentacula which encircle the inside of the 
branchial orifice, but their characters, in part anatomical, cannot yet be applied with certainty to a gi eat number 
of species. Mr. Macleay has more recently proposed two genera, the Cystingia and Dendrodoa, on distinctions of 
the same nature. 
THE SECOND FAMILY OF THE ACEPHALA NUDA,— 
The Aggregata, — 
Comprises animals more or less analogous to the Ascidia, but united in a common m.ass, so that they 
seem to communicate organically with each other, and in this respect to connect the Mollusca with the 
Zoophytes ; but what, independently of their peculiar organization, is opposed to this idea, is that, 
according to the observations of MM. Audouin and Milne Edwards, the individuals at their birth live 
and swim about separately, and only become united at a certain subsequent period of their life. Their 
branchiae form, as in the Ascidia, a large sac, which the food must traverse before it can reach the 
mouth : their principal ganglion is likewise between the mouth and the anus, and the disposition of the 
viscera and of the ovary is very nearly similar.* 
Nevertheless some have, like the Biphorae, an opening at each end. Such are 
The Botryllus, Gcertn., — 
That has an oval form, adherent to various foreign bodies, and united by tens or twelves, like the rays 
of a star. The branchial orifices are at the outer end of the rays, and the vents open in a common 
cavity, which is in the centre of a star. Wlien an orifice is irritated one animal contracts only, but if 
the irritation is applied to the centre, they all contract. These minute creatures attach themselves to 
Ascidige, sea-weeds, &c. In some species three or four starred clusters appear to be piled upon one 
another. 
The Pyrosom.e, Peron. — « 
Are united in great numbers, so as to form a large hollow cylinder, open at one end, and closed at 
the other, which swims in the ocean by the alternate contraction and dilatation of the individual animals 
which compose it. These terminate in points on the exterior, so that the whole surface of the cylinder 
is bristled with them ; the branehial orifices are pierced near these points, and the vents open into the 
cavity of the tube. We might thus compare a Pyrosoma to a great number of the stars of a Botryllus 
that had been strung in a line together, but the whole mass remaining moveable. 
The Mediterranean and Atlantic produce some large species, the animals of which are arranged with but little 
regularity. They sparkle during the night with all the brilliancy of phosphorus. A small species is also known 
(P. atlanticum), in which the animals are arranged in very regular rings. 
The remaining species of this family have, like the typical Ascidia, the vent and the branchial aperture near 
each other, on the same extremity of the body. All that are known are fixed, and they have been hitherto con- 
founded with the Alcyonia. The mass of the viscera of each individual is more or less prolonged in the cai'tila- 
* To M. Savigny we are indebted for our knowledge of tlie singular i known the peculiar structure of the Botryllus and of the Pyrosoma. 
organization of this family, which was formerly confounded with the See the admirable work of Savigny on Invertehrated Animals, part ii. 
Zoophytes. At the same time, MM. Desmarest and Lesueur made ] 
