There are very few fossil organisms that can be interpreted as ascidians: ascidians have no hard skeleton elements or calcareous shells, making it unlikely that their soft bodies can be fossilized. The single known fossil species (from the Pliocene), Cystodytes incrassatus, belongs to the widely distributed recent genus Cystodytes, characterized by the presence of relatively large (up to 0.04 in, or 1 mm, in diameter) spicules of discoid form, allowing easy identification. Another, much more ancient (about 300 million years old) form, Jaekelocarpus okla-homensis, has been interpreted as a tunicate, but recent ascidian taxonomists do not support this view. Thus, in the absence of paleontological data, phylogenetic relationships of ascidi-ans can be understood mainly on the basis of the morphological characteristics of the recent forms.
The nature and relationships of ascidians were not understood for a long time, although these common marine animals were known even to Aristotle, more than 2,300 years ago. Earlier authors, including Carl Linnaeus, placed colonial ascidians in Zoophytes, a compound group that contained many different unrelated taxa, while the solitary ascidians were regarded as members of the phylum Mollusca. This view was based mostly on a wrong opinion, that the tunic, or the test, covering the body of adult ascidians is a modification of the calcareous shell of mollusks; furthermore, solitary ascidi-
ans and bivalve mollusks both are filter feeders and have two siphons, intruding and extruding. In 1816 J. C. Savigny first recognized the common nature of the solitary and colonial ascidians, as well as some pelagic forms, and in the same year J. B. Lamarck created the group Tunicata. He believed that it was a distinct class between Alcyonaria and Vermes, although other authors still treated this group as a class Mol-luscoides of Mollusca. From 1867 to 1872 the Russian embryologist A. Kowalevsky published several works on larval development and morphological characteristics of ascidi-ans and was the first to recognize the close relationship between ascidians and chordates. Kowalevsky showed that the development of ascidians was similar to that of lancelets, a small group of primitive chordate animals now placed in the subphylum Cephaolchrodata of the phylum Chordata.
There are three important features linking ascidians and other tunicates with the chordate animals. First there is the presence of a notochord, a rod of specialized cells in the tail of ascidian larvae. The notochord disappears during the metamorphosis from free-swimming larva to sessile adult ascidian. In lancelets it is well developed in adults. In adult vertebrates the chorda dorsalis is wholly retained only in some fishes or is represented by remnants between the vertebrae. Second, a dorsal hollow nerve chord also is present only in ascidian larvae and disappears in adults. It lies dorsal to the notochord,
as in chordates, whereas it is always ventral in other phyla of invertebrate animals. Third, pharynx perforations, or gill slits, known in Ascidiacea as "stigmata," are present in all chordates, at least during embryonic development, but such structures are never present in invertebrate animals.
Class Ascidiacea belongs to the subphylum Tunicata and is divided into two orders: Enterogona (with two suborders, Aplousobranchia and Phlebobranchia) and Pleurogona (with one suborder, Stolidobranchia). The definition of the orders is based entirely on embryonic characters and reflects the origin of the atrial cavity—from a single or paired dorsal invaginations. For practical purposes, taxonomists use mostly suborders, saying that the ascidians are divided into three main groups, Aplousobranchia, Phlebobranchia, and Stolido-branchia. This subdivision is based mostly on the morphological features of adults, in particular, the structure of the branchial sac, the position of the gut, and some other features. This subdivision does not reflect colonial organization: Stolidobranchia and Phlebobranchia contain mostly solitary species but also include colonial species, and most Aplouso-branchia are colonial, but some species are solitary.
There are 24 families, and, at present, about 185 (from about 240 described) genera are treated as valid. The number of valid species is more difficult to count; the estimated number is 2,500-2,800. The interesting peculiarity of the ascidian classification is the relatively small number of genera in relation to the number of species. More than half of all known species belong to 10 very large genera listed here in decreasing order of number of species: Aplidium, Didemnum, Molgula, Polycarpa, Ascidia, Styela, Eudistoma, Pyura, Cnemidocarpa, and Synoicum. Most genera are well separated and can be identified easily, but species identification is often much more difficult. In general, the current ascidian classification seems to be close to the ideal, or "natural," system; that is, it appears to reflect the real phylogenetic relationships between taxa.
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