Tunicata occur either fixed or free, solitary, aggregated or in colonies (see Fig. 27, p. 64). The fixed forms, found on the sea-bottom, are usually termed "Ascidians," those that are solitary or merely aggregated being "Simple Ascidians" or Monascidiae, and those that are organically united into a colony being "Compound Ascidians" or Synascidiae. The colonies have been produced by budding, a process which is very general in the group, and the members of the colony are conveniently known as "Ascidiozooids." Some exhibit {36}alternation of generations, and all pass through remarkable changes in their life-history, nearly all of them undergoing a retrogressive metamorphosis. OUTLINE OF HISTORY. 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 {37}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.