SUBDIVISION I. TETHVDES.
In this subdivision the inner tunic is detached from the external coveting, except at the two orifices. The branchix are large, unequal, and cover the walls of the great cavity. The opening to this cavity is surrounded on the inside with a mernbranaccous, denticulated ring, or with a circle of filaments.
The animals of this Order are all fixed to other bo dies, without the power of displacing themselves. The branchial and anal openings are neither opposite, nor do they communicate directly with each other. The bran cilia] cavity is open at its upper extremity only, and the aperture is fringed with tentacula ; the gills of both sides are continuous. The genera are distributed into two sections.
This contains the animals formerly included in the genus Ascidia of Linmzus. They are distinguished by the separate or independent existence of the different individuals. In some species many individuals adhere to the rock in clusters, and appear to constitute one ani mal ; but these can be easily separated, as they possess no common covering or organical connection. The ge nera of this section arc four in number, and are distri buted into two families. In the first family the orifices are furnished with four rays, and the genera are Boltenia and Cynthia, the former represented by the Ascidia pe dunculata, having the body supported on a stern, while in the latter the body is sessile, as in C. conchilega, a na tive of the British seas.
In the second family, the anal and branchial openings are either destitute of rays, or they exceed four in num ber. The genera are two. In the P/1allusia the body is sessile. The branchial orifice usually possesses eight rays, and the anal one six. The P. mentula is very common on rocks and old oysters in the British seas. The P. intestinalis and nrunum are likewise natives. In the genus Clavelina the body is pedunculated, and the apertures arc destitute of rays. The C. Iepadiformis has been added to the British Fauna by the industry of Dr. Leach. The genus Mammaria of Muller will probably constitute a third family in this section, when its struc ture has been determined. The body is smooth, having the base fixed to sea-weeds, and the summit perforat ed by a single aperture. One species, the AI. mam milla, inhabits the British seas, having been observed at Leith by Professor Jameson, and at Belfast by Mr. Tem pleton. The genus Bipapillaria of Lamarck is involved in equal obscurity.
In this section are included animals which were for merly placed among the zoophytes, and chiefly in the genus ?lIcyonium. They are compound animals, many individuals being united under a common covering. They admit of division into three families.
In the first family the branchial and anal orifices have six rays each. The genera of this family arc three in number. The Diazona is distinguished by the body be ing sessile, orbicular, and consisting of n single system or group of individuals. The D. violates or is the only known species. In the Distoma the body is ses sile and polymorphous, and the individuals are arranged in systems or groups. The D. variolosus, first observed
by Gartner, is a native of our seas, and is the Alcyonium ascidioides of Pallas. In the Sigillina, the body is conical and consisting of a single group of indivi duals The S. australis of Savigny is the only known species.
In the second family the branchial orifice c.nly has six regular rays. The genus Synoicum has the body cylindrical, vertical, and pedunculated with a single sys tem. The S. turgens of Phipps, a native of the Arctic seas, is the only species which has been described. In the month of August 1817, we observed a Synoicum, adhering to a rock in the sea at the Isle of May. It dif fered from the turgens, as described by Savigny, chiefly in the smoothness of the skin. In the Sydneum the body is inversely conical, vertical, pedunculated, with a single system. The S. turbinatum, observed by Dr. Leach as a native, is the only known species of the genus. In the genus illnidium, the body is variously shaped, ses sile, consisting of many systems, which are destitute of a central cavity. The ..4.ficus, or Alcyonium pulmona ria of Ellis's zoophytes, belongs to this genus. In the Polyclinum the body is likewise sessile, but the indivi duals are in systems with central cavities. The Didem num is sessile, spongy, and incrusts other bodies. The anal opening is obscure.
In the third family, both the branchial and anal orifi ces are destitute of rays. The body invests foci and rocks, and consists of several systems. In the genus F.uccclium, the systems are destitute of central cavities. In the genus Botryllus, on the other hand, there is a central cavity to each system. The last genus contains the following British species : B. Leachii, Borlassii, Scholosseri, and Conglomeratus.
The animals which remain to be considered, as con stituting this order, arc free, and float about in the wa ter. Many individuals arc united, forming a conical bag. The anal and branchial openings are diametrically oppo site to each other. The branchial cavity is open at both ends, the anterior aperture is destitute of tentacula, but furnished with a denticulatcd ring. The branchix are disjoined. There is only one genus in this order known, which has been termed Pyrosoma. It contains the fol lowing species, verticillata, paniculata, and allanticum.
This subdivision embraces animals whose inner tunic adheres to the external one at all points ; whose bran chial orifice is furnished with a valve ; and whose bran chix consist of two united narrow leaves.
The animals here referred to were first formed into a separate genus by Brown, in his History of Jamaica, under the title Thalia, a name now usurped by Flora. Forskael afterwards instituted his genus Salpa, and Sir Joseph Banks the Dagysa, for the reception of similar animals. The name or Forskael has been preferred by naturalists. Few of the species have been examined in a recent state. They appear to be numerous, and to ex hibit a considerable variety of character ; perhaps suffi cient to warrant their distribution into several distinct genera. They appear to inhabit chiefly the seas of warm climates.