TU NI C A T A 



{By W. A. Herd/man, D.Sc, Professor of Natural History, University College, Liverpool.) 



Simple 

 Ascid- 

 ians. 



Com- 

 pound 

 Ascid- 



Cuvier 

 and 



Lamarck. 



Cham- 

 isso and 

 alterna- 

 tion of 

 genQia- 

 tions. 

 Circula- 

 tion. 



Milne- 

 Edwards. 



Carl 

 Schmidt. 



THIS group of animals was formerly regarded as con- 

 stituting along with, tlie Polyzoa and the Brachio- 

 poda the invertebrate class Molluscoidea. It is now known 

 to be a degenerate branch of the Ghordata, and to be more 

 nearly related to the Vertehrata than to any group of the 

 Invertehrata. 



HiSTOEY.l 



More than two thousand years ago Aristotle gave a 

 short account of a Simple Ascidian under the name of 

 Tethyum. He described the appearance and some of the 

 more important points in the anatomy of the animal. 

 From that time onwards to little more than a century ago, 

 although various forms of Ascidians had been briefly de- 

 scribed by writers on marine zoology, comparatively little 

 advance was made upon the knowledge of Aristotle. 

 Schlosser and Ellis, in a paper containing a description of 

 Botryllus, published in the Philosophical Transactions of 

 the Eoyal Society for 1756, first brought the Compound 

 Ascidians into notice ; but it was not until the commence- 

 ment of the 19th century, as a result of the careful ana- 

 tomical investigations of Cuvier (i) upon the Simple 

 Ascidians and of Savigny (2) upon the Compound, that 

 • the close relationship between these two groups of the 

 Tnnieata was conclusively demonstrated. Up to 1816, 

 the date of publication of Savigny's great work {2), the 

 few Compound Ascidians then known had been generally 

 regarded as Alcyonaria or as Sponges; and, although 

 many new Simple Ascidians had been described by O. F. 

 Miiller {4) and otiiers, their internal structure had not 

 been investigated. Lamarck {3) in 1816, chiefly as the 

 result of the anatomical discoveries of Savigny and Cuvier, 

 instituted the class Tunicata, which he placed between the 

 Badiata and the Vermes in his system of classification. 

 The Tunicata included at that time, besides the Simple 

 and the Compound Ascidians, the pelagic forms Pyrosoma, 

 which had been first made known by P6ron in 1804, and 

 Salpa, described by Forsk^l in 1775. 



Chamisso in 1820 made the important discovery that 

 Salpa in its life-history passes through the series of changes 

 which were afterwards more fully described by Steenstrup 

 in 1842 as "alternation of generations"; and a few years 

 later Kuhl and Van Hasselt's investigations upon the same 

 animal resulted in the discovery of the alternation in the 

 directions in which the wave of contraction passes along 

 the heart and in which the blood circulates through the 

 body. It has since been found that this observation holds 

 good for all groups of the Tunicata. In 1826 H. Milne- 

 Edwards and Audouin made a series of observations on 

 living Compound Ascidians, and amongst other discoveries 

 they found the free-swimming tailed larva, and traced its 

 development into the young Ascidian. Milne -Edwards 

 (5) also founded the group of "Social" Ascidians, now 

 known as the Clavelinidse, and gave a classification of the 

 Compound Ascidians which was universally accepted for 

 many years. From the year 1826 onwards a number of 

 new and remarkable forms were discovered, as, for instance, 

 some of the Boltminse (Macleay), CAeZyosoma (Broderip and 

 Sowerby and afterwards Eschricht), Oikopleura (Mertens), 

 Perophora (Lister), Pelonaia (Forbesand Goodsir), Chondro- 

 stachys and Diplosoma (Denis Macdonald), Dmzona (Forbes 

 and Goodsir), and Bhodosoma (Ehrenberg, and afterwards 

 Lacaze-Duthiers). 



In 1845 Carl Schmidt {6) first announced the presence 



1 Only the more important works can he mentioned here. For a 

 more detailed account of the history of the group and a full biblio- 

 graphy see {17) ill -the list of works at the end of this article. 



in the test of some Ascidians of "tunicine," a substance 

 very similar to cellulose, and in the following year Lowig 

 and Kolliker (7) confirmed the discovery and made some 

 additional observations upon this substance and upon the 

 structure of the test in general. Huxley (<?), in an im- Huxley, 

 portant series of papers published in the Transactions of 

 the Eoyal and Linnean Societies of London from 1851 on- 

 wards, discussed the structure, embryology, and affinities 

 of the pelagic Tunicates Pyrosoma, Salpa, Doliolum, and 

 Appendioularia. These important forms were also investi- 

 gated about the same time by Gegenbaur, Vogt, H. Miiller, 

 Krohn, and Leuckart. The most important epoch in the 

 history of the Tunicata is the date of the publication of 

 Kowalevsky's celebrated memoir upon the development of Kowa- 

 a Simple Ascidian (p). The tailed larva had been previously levsky: 

 discovered and investigated by several naturalists — notably ?'*'^^ 

 H. Milne-Edwards (5), J. P. van Beneden (lo), and KrOhn ■'*"*• 

 (11) ; but its minute structure had not been suflSciently 

 examined, and the meaning of what was known of it had 

 not been understood. It was reserved for Kowalevsky in Relation- 

 1866 to demonstrate the striking similarity in structure ship to 

 and in development between the larval Ascidian and the yerte- 

 vertebrate embryo. He showed that the relations between 

 the nervous system, the notochord, and the alimentary 

 canal are much the same in the two forms, and have been 

 brought about by a very similar course of embryonic de- 

 velopment. This discovery clearly indicated that the 

 Tunicata are closely allied to Amphioxus and the Yerte- 

 brata, and that the tailed larva represents the, primitive 

 or ancestral form from which the adult Ascidian has been 

 evolved by degeneration, and this led naturally to the view 

 usually accepted at the present day, that the group is a 

 degenerate side-branch from the lower end of the phylum 

 CAorc^ato, which includes the Tunicata (Urochorda), Amphi- 

 oxus (Cephalochorda), and the Vertehrata. Kowalevsky's 

 great discovery has since been confirmed and extended to 

 all other groups of the Tunicata by Kupffer {is), Giard Kupffer, 

 {ij and is), and others. Important observations upon Giard, &c. 

 the process o^ gemmation and the formation of colonies in Gemma- 

 various forms of Compound Ascidians have been made by tion. 

 Krohn, Metschnikofi', Kowalevsky, Ganin, Giard, Delia 

 Valle, and others, and have gradually led to the establish- 

 ment of the general principle, that all the more important 

 layers of the bud are derived more or less directly from 

 the corresponding regions in the body of the parent. 



In 1872 Pol {14) added largely to the knowledge of thepol, &c. 

 Appendiculariidx, and Giard (75) to that of the Compound 

 Ascidians. The latter author described a number of new 

 forms and remodelled the classification of the group. The 

 most important additions which have been made to the 

 Compound Ascidians since Giard's work have been those 

 described by Von Drasche {16) from the Adriatic and 

 those discovered by the "Challenger" expedition {17). 

 The structure and the systematic arrangement of the Simple 

 Ascidians have been mainly discussed of recent years by 

 Alder and Hancock {iS), Heller {19), Lacaze-Duthiers 

 {20), Traustedt {21), and Herdman {17, 22). In 1874 

 tjssoff (.?j) investigated the minute structure of the nervous Sub- 

 system and of the underlying gland, which was first dis- neural 

 covered by Hancock, and showed that the gland has aS'^"*! 

 duct which communicates with the front of the branchial ^Qj.g^j 

 sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle, 

 tubercle. In an important paper published in 1880 Julin 

 {24) drew attention to the similarity in structure and rela- 

 tions between this gland and the hypophysis cerebri of the 

 vertebrate brain, and insisted upon their homology. He 

 suggests that they perform a renal function. The Thaliacea 



2A 



