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 comparatively little advance was made until Schlosser and Ellis, in a paper on Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the Compound Ascidians into notice. It was not, however, until the commencement of the nineteenth century, as a result of the careful anatomical investigations of Cuvier[[66]] upon the Simple Ascidians, and of Savigny[[67]] upon the Compound Ascidians, that the relationship between these two groups of Tunicata was conclusively demonstrated. Up to 1816, the date of publication of Savigny's great work, the few Compound Ascidians previously known had been generally regarded as Alcyonaria or as Sponges; and although many new Simple Ascidians had been described by O. F. Müller[[68]] and others, their internal structure had not been investigated. Lamarck[[69]] in 1816, chiefly as the result of the anatomical discoveries of Savigny and Cuvier, instituted the class Tunicata, which he placed between the Radiata 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 Péron in 1804, and Salpa described by Forskål in 1775.
Chamisso, in 1819, 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[[70]] 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.
In 1845, Carl Schmidt[[71]] first announced the presence in the test of some Ascidians of "tunicine," a substance very similar to cellulose; and in the following year Löwig and Kölliker[[72]] confirmed the discovery, and made some additional observations upon this substance and upon the structure of the test in general. Huxley,[[73]] in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology, and affinities of the pelagic Tunicates, Pyrosoma, Salpa, Doliolum and Appendicularia. These important forms were also investigated about the same time by Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart.
The most important epoch in the history of the Tunicata is the date of the publication of Kowalevsky's celebrated memoir[[74]] upon the development of a Simple Ascidian. The tailed larva had been previously discovered and investigated by several naturalists, notably by H. Milne-Edwards,[[75]] P. J. van Beneden, and Krohn; but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not been understood. It was reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the Vertebrate embryo. He showed that the relations between the nervous system, the notochord, and the alimentary canal are practically the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration. 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 Chordata, which includes the Tunicata (or Urochordata), Balanoglossus and its allies (Hemichordata), Amphioxus (Cephalochordata), and the Vertebrata (or Craniata). Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by Kupffer,[[76]] Giard, and others.
In 1872 Fol[[77]] added largely to the knowledge of the Appendiculariidae, and Giard[[78]] 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 the species described by von Drasche,[[79]] from the Adriatic, and those discovered by the "Challenger" expedition.[[80]] The structure and the systematic arrangement of the Simple Ascidians have been discussed of recent years mainly by Alder[[81]] and Hancock, Heller,[[82]] Lacaze-Duthiers,[[83]] Traustedt,[[84]] Roule, Hartmeyer, Sluiter[[85]] and Herdman.[[86]] In 1874 Ussoff investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle. In an important paper published in 1880, Julin[[87]] drew attention to the similarity in structure and relations between this gland and the "hypophysis cerebri" of the Vertebrate brain, and insisted upon their homology. Metcalf has recently added further to our knowledge on this and related matters.
The Thaliacea or pelagic Tunicata have of late years been the subject of several very important memoirs. The researches of Todaro, Brooks,[[88]] Salensky,[[89]] Seeliger,[[90]] Korotneff,[[91]] and others have elucidated the embryology, the gemmation and the life-history of the Salpidae; and Grobben, Barrois,[[92]] and more especially Uljanin,[[93]] have elaborately worked out the structure and the details of the complicated life-history of the Doliolidae. Finally we owe to the labours of Metschnikoff, Kowalevsky, Giard, Hjort, Seeliger, Ritter, Van Beneden and Julin, much detailed information as to development and life-history, the process of gemmation and the formation of colonies, which has added greatly to our knowledge of the position and affinities of the Tunicata and of their natural classification.
Structure of a Typical Ascidian.
If a typical "Simple Ascidian," such as the common British Ascidia mentula (Fig. 15), or Ascidia virginea, be examined alive and expanded in sea-water it will be seen to bear on the upper surface two short projections, each terminated by a wide tubular opening, through which the animal, when touched, can emit jets of water with considerable force—thus accounting for the popular name "sea-squirts." The rest of the body is covered by the dull grey tough cuticular outer "test" or "tunic" (hence Tunicata) by means of which the animal is attached to a rock or other foreign body. One of the tubular openings, the mouth or "branchial aperture," is terminal, and indicates the morphological anterior end; it is surrounded by eight lobes. The other opening, the cloaca or "atrial aperture," is on the dorsal edge, from one-third to one-half way down the body, and is bounded by six lobes only; consequently the two apertures, and so the ends of the body, can be distinguished externally by the number of lobes—an important matter. The area of attachment is usually the posterior part of the left side; in Fig. 15 the animal is seen from the right hand side.
If a little carmine-powder, or some other insoluble particles be scattered in the water in which the Ascidian is living, the particles will be seen to converge to the branchial aperture and be sucked in by the inhalent current entering the body. After a short interval a certain proportion of the particles will be shot out from the atrial aperture with the exhalent current.
These particles have passed through the pharyngeal portion of the alimentary canal and the cloacal passages, with the water used in respiration, but a considerable amount of such particles taken in with the water do not reappear, as they are retained by the nutritive organs and pass along the remainder of the alimentary canal with the food. The current of water passing in at the branchial and out at the atrial aperture is of primary importance in the life of the Ascidian. Besides serving for respiratory purposes it conveys all the food into the body and removes waste matters both intestinal and renal, and also expels the reproductive products from the body.