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or Urochorda Tunicata

tail, qv and larva

TUNICATA, or UROCHORDA, a class of animals of the phylum Chordata (q.v.), commonly represented by the Ascidians (q.v.) formerly much misunderstood. Since Kowa lewsky's description in 1866 of the development of an ascidian, it has been clear that these animals, together with all the other tunicates, must be associated in the zoological system with Amphioxus and the true vertebrates. Most tunicates pass through a free-swimming, tadpole-like, larval stage (and a few remain permanently in this condition) which has in the tail or swimming organ a notochord and a tubular central nervous system, both of which develop in essentially the same manner as in other vertebrates. The fore part of the alimentary canal is perforated by pores or stigmata opening to the exterior and serves as a respiratory organ. After a short free-swim ming period, the larva attaches itself in typical cases and the tail is gradually absorbed with its contained organs, so that both notochord and tubular nerve cord disappear, a remnant of the latter in the body becoming the gang lion of the adult. The Tunicata comprise three

orders: The L,arvacea, including forins of small size and simple structure, which retain the lar val tail throughout life. Appendicularia is an example found abundantly among the surface fauna of our coasts. The Thaliacea, including almost exclusively free swimming pelagic forms which have no tail in the adult and seldom a tailed larva. They form compound colonies and exhibit an alternation of generations. Salpa (q.v.) is an example. The Ascidiacea, includ ing usually fixed, simple or compound forms, with usually well-marked larva, but no tail in the adult state. Molgula cynthia, Boltenia and Ainarceciunt are common genera on our coasts. See AscinIAN. Consult Herdman, 'Challenger Reports,' Vols. VI and XIV, Lon don (1882 and 1886) Verrill, (Invertebrates of Vineyard Sound' (Washington 1874).