TUNICATA. (Tuniciers, Fr.; Earkt Ilinscheter and Sce-Scheiden, Germ.) —The Tunica ta are molluscous animals, having no calcified shell, but a more or less coriaceous envelope or tunic, whence their name. This external coat or test is either bag-shaped and provided with two apertures, or is tube-shaped and open at the ends. They have no distinct head, and no organs serving as arms or feet ; they are provided with a muscular and a nervous system ; and with well-defined or gans of respiration, digestion, circulation, and generation.
They are exclusively marine, and are widely spread from the arctic to the tropical seas. Sessile or foot-stalked on the rock, or in crusting seaweed and other bodies, their external form is seldom of graceful contour ; yet the arrangement of the individuals in the compound masses often exhibits curious and elegant designs. The floating forms, however, with their lengthened, sinuous chain, or tapering tubes, pellucid, rainbow-tinted, or, by night, brightly phosphorescent, are sur passed by no terrestrial object. The other, fixed, forms, not altogether destitute of ele gance of colour in the northern seas, become in warmer climates more and more rich in variegated hues, and in the tropics are amongst the most resplendent living gems of ocean's parterres.
The earliest notice of the tunicate animals is made by Aristotle, who gives a very cor rect account of the anatomical and zoological characters of a simple Ascidian, which he calls rrf0a0v.* They, however, attracted no further notice until comparatively late times. Ron delet t gives indifferent figures and descriptions. Gesner and Aldrovandus, uniting Rondelet's Tethya with those given by Belon which are Alcyonia, were among the first who gave rise to the confusion that long existed in the history of these animals. Linnms in the 4th edition of his System Naturte placed a Tethyuin in his system, under the appellation of Tethys; he pointed out at the same time that the animal of bivalve Molluscs was a Tethys, showing that he was aware of the analogy of the Bivalves with the Tunicates. In other respects, however, he added to the confusion. After that Bohatsch § and Plancusll figured and described some species. Baster 11 describes a species, and gives it the name of Aseidiunt (from arson, a skin bottle), at the same time adding a very just remark on the analogy of its internal struc ture with that of the oyster. This analogy
was noticed also by Pallas, who proposed the union of the 7'ethyunz and Aseidium**, which Linnaeus carried out in the 12th edit. of his System Naturw, uniting the species, noticed by Bohatsch and Kcemg, under the name of Ascidia, and confining the name Tethys to the inhabitants of the Bivalves. Subsequently, O. F. Miiller O. Fabricius t, DicquemareI, Pallas§, and others, described and figured several Aseidice, which Brugiere and Gmelin If collected together in their re spective works, without adding much to the knowledge of the group. Cuvier's first ob servations on the simple Ascidians were begun in 1797, and his memoir on their anatomy was published in 1815." About the same time were published the researches of Schalktt on the anatomy, and of Carus It on the anatomy and development of the Ascidians. The memoirs of Peron and Lesueur §§ on the Pyrosonia, Desmarest and Lesueur NI on the Botryllus, and particularly the elaborate work of Savigny IT on the simple and compound Ascidians, enabled naturalists to make rapid advances in the knowledge of this family. With respect to the Salpians, Brown***, For skahlftt, and Tilesius III were the first to figure and describe any forms of this group. Considerable confusion with regard to these forms existed in the classifications of Linnaeus, Pallas, Brugiere, and Bose, and indeed in the earlier writings of Lamarck and Cuvier, until the latter had an opportunity of work ing out the anatomical characters of these animals.
The earlier works on the various branches of this subject have been succeeded by the publication of the researches of many dis tinguished naturalists. We are chiefly in debted to the labours of Cuvier, Savigny, Carus, MacLeay, and Van Beneden, for in formation on the structure and development of the simple Ascidians ; to the researches of Cuvier, Kuhl and Van Hasselt, Chamisso, Eschricht, and Krohn, for the history of the Salpians ; and to Lesueur, Desmarest, Sa vigny, Lister, Sars, Milne-Edwards, Audouin, and Forbes, for our knowledge of the com pound ascidian forms.