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Show 430 MOLLUSCA. wards, at first live and swim separately, only becoming united at a certain subsequent period, a fact which is in direct opposition to this opinion. Their branchire, as in the Ascidire, form a la1•ge sac, traversed by the aliment before it arrives at the mouth; their principal ganglion is also situated between the mouth and the arms; a nearly similar disposition obtains with respect to the viscera and ovary( 1). Notwithstanding this, some of them, like the Biphora, have an opening at each extremity. Such is the BoTRYLLus, Grert., Of an oval form, fixed on various bodies, and united by tens 01• twelves, like the rays of a star. The branchial ot·ifices are at the external extremities of these rays, and the anus terminates in a common cavity, which is in the centre of the star. If an orifice be il'l'i.tated, but a single animal contracts; if the centre he touched they all contract. These very small animals attach themselves to cet·tain Ascidire, Fuci, &c.(2) In some particular species, three or four stars appeared to be piled one on the other(3). PYROSOMA, Peron. The Pyrosomre unite in great numbers, forming 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. The latter tet·minate in a point on the exterior, so that the whole external surface of the tube is brig,. tied with them; the bt·anchial orifices are pierced neat· these points, and the anus debouches in the internal cavity of the cylinder. A Pyrosoma may thus be compared to a great number of stars of Bo· trylli strung together, the whole of which is movable( 4). (1) It is toM. de Savigny that we arc indebted for our recent knowledge ofthe singular organization of the whole of this family, formerly confounded with the Zoophytes properly so called. At the same time, Messrs Desmarets and Lesueur made known the particular structure of the Bob·ylli and Pyrosomm. See the ex· cellent work of M. S:.wigny in his Mem. sur les anim. sans verteb., part II, fasc. I. (2) See Desmarets and Lesueur, Bullet. des Sc. May 1815;--Botryllus stellatus, Grertner, or .!l.lcyunium Scltlosseri, Gm., Pall., Spic. Zool., X, iv, 1-5. (3) Botryllus conglomeratus, Grert., or .!l.lcyonium conglomeratum, Gm.; Pall., Spic. Zool. X, iv, 6. (4) See Desmarets and Lesueur, Joe. ci.t. ACEPH:A.LA NUDA. 431 The Mediterranean and the Ocean produce large species the . ' animals of which are arranged with but little regularity. They exhibit a phospho~esc~nt appearance during the night( 1 ). A smaller species Is also known(2) where the animals are arranged in very regular rings. The remainder of these aggregated Mollusca, like the ordinary Ascidire, have the anus and branchial orifice approximated to the same extremity. The species known are all fixed, and till now they have been confounded with the Alcyonia. The visceral bundle of each individual is more or less extended into the common cartilaginous or gelatinous mass, more or less narrowed or dilated in certain }lOints ; but each orifice always forms a little six-rayed star on the· surface. We unite them all under the name of POLYCLINUM(3). Some of them are extended over bodies like fleshy crests( 4). Others project in a conical or globular mass( 5), Or expand into a disk comparable to that of a flower or of an Actinia(6), or are elongated into cylindrical branches supported by slender pedicles, &c.(7), or form parallel cylinders(8). Recent observations even seem to show that the EscHAR.£, hitherto placed among the PoLYPI, belong to this family of the Mollusca(9). (1) Several of the Polyclina and .!l.plidia of Savigny. (2) Pyrosoma atlanticum, Peron., Ann. du Mus., IV, lxxii;-Pyrosoma gigas, Desmar., and Lesueur, Bullet. des Sc. June 1815, pl. v, f. 2. (3} The Pyrosome elegant, Lesueur, Bullet. des Sc., June 1815, pl. v, f. 2. (4) It is f1·om the number of strangulations, that is to say, the greater or less separation of the branchia:, stomach and ovary, that M. de Savigny has formed his PoLYctxNuM, ArLIDIUllt, DIDEMlltullr, Euc.IELIUM, DuzoN.A., SIGlLLIN.A., &c. which, in our opinion, need not be retained. Here, also, should come the .!l.lcyonium ficus, Gm.; the Distomus variolosus, Grertn., or .!l.lcyonium ascidioi'de3, Gm., Pall., Spic. Zoo!., X, iv, 7. (5) The Eucmlium, Savig.; the Distomi are arranged in the same manner. (6) The genus Diazona, Sav., consisting of a large and beautiful purple species discovered near Ivice by M. Delaroche. (7) The genus Sigillina, Sav., whose cylindrical branches are frequently a foot long, and the animals, slender as threads, but three or four inches. (8) The genus Synocium, Lam. (9) Messrs Audouin and Milne Edwards on the one hand, and M. de Blainville on the other, have lately verified this fact, which the observations of Spallanzani previously seemed to announce. |