ACErilALA NUDA. 383 



tuberance, but the cloak is prolonged into certain points. And of tnese some have a single point at each extresnity, 

 others have two, three, or even more at the oral extremity ; some have one only at that end ; and the greater number 

 are simply oval or cylindrical. 



The Ascidi.e {Ascidia, Linn.), Thefyon of the Ancients. 

 The cloak and its cartilaginous envelope, which is frequently very thick, resemble sacs everywhere 

 closed, except at two orifices, w-hicli correspond to the tubes of many Bivalves, one of which admits the 

 water of respiration, and the other is the vent. Their branchiae 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 properly 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 distance. They abound in every sea, and some of them are eaten. 



Some species are remarkable for the long pedicle which supports them. M. Savigny, from bis own researches 

 and mine, has attempted to subdivide the Ascidife into several subgenera: such are Cynthia, — body sessile, envelope 

 coriaceous, branchial sac plaited longitudinally. Phalbma 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, — the body pedunculate, and the envelope coria- 

 ceous. He also takes into consideration the number and form of the tentacula which encircle the inside of tlie 

 branchial orifice, but their characters, in part anatomical, cannot yet be applied with certainty to a great 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 mass, so that they 

 seem to communicate organically with each other, and in this respect to connect the MoUusca with the 

 Zoophytes ; but what, iiulependently of tlieir 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 

 branchia: form, as in the Ascidia, a large sac, which the food must traverse before it can reach the 

 mouth : fheir 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 Botrvllus, Goertn., — 

 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 tJie centre of a star. When an orifice is irritated one animal contracts only, but if 

 the irritation is applied to tlie centre, they all contract. Tliese minute creatures attach themselves to 

 Ascidia3, sea-weeds, &c. In some species three or four starred clusters appear to be piled upon one 

 another. 



The Pvrosom.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 branchial orifices are pierced near these points, and the vents open into tlie 

 cavity of the tube. We might thus compare a Pyrosoma to a great number of the stars of a Botrvllus 

 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 tjiey have been hitherto con- 

 founded with the Alcyonia. The mass of the viscera of each individual is more or less prolonged in the cartila- 



• To M. Savigny wc are iiidibted 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 Sec the admirable work of Savigny on Invertelirated Animals, part ii. 

 Zoophytes. At the same lime, MM. Desmarest and I.esucur made | 



