ACEPHALA NUDA. 533 



tnbcrance, but the cloak is prolonged into certain points. And of tiiese somehave a single point at each extremity, 

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

 are simi)ly ova! or cylindrical. 



The Ascidi* (Jscidia, 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 tlie 

 water of respiration, and the other is the yent. Their Ijranchiae form a large sac, at the bottom of 

 which the mouth is situated, and near the month is the mass of viscera. The envelope is much wider 

 than the cloak properly so called. Tliis is fibrous and vascular; and we perceive on it one of tlie 

 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 e.xhiljit 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 remarkahle for the long pedicle which supports them. M. Savigny, from his own researches 

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

 coriaceous, branchial sac plaited longitudinally.' PAaWiuia differs from the preceding in the brancliial sac not being 

 plaited ; their envelope is gelatinous. Clav€f/hia,—ih^ branchial sac without plaits, not reaching the bottom of tlie 

 envelope, the body pedunculate, the envelope gelatinous. liollenia,—t\ie 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 great number 

 of species. Mr. Macleay has more recently proposed two genera, the Ci/sliiif^ia and Dendi-odoa, on distinctions of 

 the same nature. 



THE SECOND FAMILY OF THE ACEPHALA NUDA,— 

 Thl: 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, iinlependently of their peculiar organization, is opposed to this idea, is that, 

 according to the observations of M.M. 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 lil'e. Tlieir 

 branchiic form, as in the .\sci(lia, a large sac, wliicli tlie food must traverse before it can reach the 

 uiouth : 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 Biphora;, an opening at each end. Such are 



The Botryllus, Gccrtn., — 

 That has an oval form, adherent to various foreign bodies, and united by tens or twelves, like the rays 

 of a star. The Inaiicliial 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. ^A■bel] an o) ifice is irritated one animal contracts only, but if 

 the irritation is ap|ilieil to tlie centre, they all contract. These minute creatures attach themselves to 

 Ascidiffi, sea-weeds, \c. In some species three or four starred clusters appear to he 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, wdiich 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 the 



cavity of the tube. We might thus compare a Pyrosoma to a great number of tiie 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. atlanticiim), 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 cartila- 



• To M. Savign)' we are indebted for our kiiowledcre of Oie singular i known the iieculicir strueture of the BofrjUus and of the Py 

 orgBiiiiatioii of this inmiiy, which was formerly confoanded with the Seethe admirable wupk o/ S«vig„y oij liiverlebr-/j:d ArWals, |ia 

 Zonphytes. At the same time, MM. Desmnreat and Lesueur made | 





