Tunicata

Definitions

  • WordNet 3.6
    • n Tunicata tunicates
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Webster's Revised Unabridged Dictionary
    • pr. n. pl Tunicata (Zoöl) A grand division of the animal kingdom, intermediate, in some respects, between the invertebrates and vertebrates, and in modern classifications considered a subphylum of the vertebrates; called also urochordata. They were formerly classed with acephalous mollusks. The body is usually covered with a firm external tunic, consisting in part of cellulose, and having two openings, one for the entrance and one for the exit of water. The pharynx is usually dilated in the form of a sac, pierced by several series of ciliated slits, and serves as a gill.☞ Most of the species when mature are firmly attached to foreign substances, but have free-swimming larvæ which are furnished with an elongated tail and somewhat resemble a tadpole. In this state the larva has a urochord and certain other structures resembling some embryonic vertebrates. See Ascidian Doliolum Salpa Urochord, and Illust. of Social ascidian, under Social.
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Century Dictionary and Cyclopedia
    • tunicata A class, superclass, or phylum of animals intermediate between and connecting the invertebrates with the true vertebrates, now made a prime division of chordate animals (see Chordata); the ascidians, tunicaries, or sea-squirts. The evidence of vertebrate affinity or character is chiefly in the larval state, when there is a sort of notochord, the urochord (see Urochorda), which in one group persists in the adult (see Appendiculariidæ). The tunicates are so called from the thick, tough, leathery integument or tunic, the name having been given by Lamarck in 1816 to the forms then known, and the class having been placed in his system between the worms and the radiates. The tunicates had before been regarded as polyps or even as sponges; with Cuvier they formed a division (Nuda) of mollusks; afterward and for many years they were considered as molluscoids, and associated with or approximated to the brachiopods and polyzoans. The discovery of the urochord by Kowalevsky in 1866 gave the first evidence of their proper position among chordate animals, and consequently of their vertebrate affinity. They were thereupon regarded as the “ancestors” of the vertebrates, of which, however, they appear rather to represent a degenerate or retrograde side-shoot. The developmental history is intricate and perplexing. Alternation of generation has been determined for the whole group, and some members of it occur under two distinct forms. One of the most remarkable peculiarities of Tunicata is the presence in the integument of tunicin, a kind of animal cellulose—cellulose having been supposed to be peculiar to plants. Tunicates are very dissimilar to one another in outward appearance, though they conform to a type of structure most parts of which can be clearly homologized with those of vertebrates. An ordinary simple ascidian resembles a leathern bottle fixed at the base, and provided with two openings, through one of which water is indrawn, and through the other of which it can be expelled with some force when the animal contracts, whence the name sea-squirt; other fanciful names are sea-pear, sea-peach, sea-pork, and sea-potato. Other tunicates, also fixed, are social, aggregate, or colonial; some are free-swimming, or fixed and free at different stages of their development, and of the free forms some are simple and others are linked in chains. The salps and pyrosomes are phosphorescent. All tunicates are marine; most live on the shore or surface, but some at great depths. Their classification has been almost as changeable as their location in the system. The arrangement of H. Milne Edwards (1826, and long current with little modification) has been entirely remodeled. According to the latest views, Tunicata rank as a class divided into three orders: Larvalia, tailed when adult, represented by the family Appendiculariidæ (see cut under Appendicularia); Thaliacea, free-swimming, simple or compound, without a tail in the adult, and either cyclomyarian (Doliolidæ) or hemimyarian (Salpidæ and Octacnemidæ) (see cuts under Doliolidæ and Salpa); and Ascidiacea, of which there are three groups or suborders— Salpiformes, resembling salps in being freeswimming, colonial, and luminous, with one family, Pyrosomatidæ; Compositæ, fixed, reproducing by gemmation and so forming compound organisms, with seven families, of which Botryllidæ is the best-known, a member of it having been described in 1756; and Simplices, fixed (exceptionally free) and solitary (rarely social—that is, imperfectly composite), with four families, Molgulidæ, Cynthiidæ, Ascidiidæ, and Clavellinidæ. The last named are the social ascidians; the second and third families are each divided into subfamilies ranked as families by some writers, and are also the largest families, represented by the numerous genera and species which come most frequently under observation, and to which the common name ascidian is specially pertinent. (See cuts under Ascidia and gastrulalion.) A former broader arrangement, which ignored the peculiarities of the Larvalia, was into two orders, by means of which the salps and the doliolids on the one hand were contrasted with all other tunicates on the other; and each of these orders had a number of different names. Also called Ascidioida.
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Chambers's Twentieth Century Dictionary
    • n Tunicata a class of remarkable animals, many of which are popularly known as Ascidians or sea-squirts—now regarded as occupying a lowly place among vertebrate or chordate animals
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Etymology

Webster's Revised Unabridged Dictionary
NL. See Tunicate
Chambers's Twentieth Century Dictionary
Fr. tunique—L. tunica, an under-garment of both sexes.

Usage

In literature:

These are marine mollusks, without shells, belonging to the family Tunicata.
"The People's Common Sense Medical Adviser in Plain English" by R. V. Pierce
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