36 ASCIDIANS CHAP. 
alternation of generations, and all pass through remarkable 
changes in their lfe-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 Zethyum. He 
described the appearance and some of the more important points 
in the anatomy of the animal. From that time onwards com-: 
paratively 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 investiga- 
tions of Cuvier ! upon the Simple Ascidians, and of Savigny 7 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. Miiller® and 
others, their internal structure had not been investigated. 
Lamarck* 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 Forskal 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 
1 Mém. Mus. Paris, ii. 1815. 2 Mém. s. l. Anim. s. Vert. Pt. ii. Paris, 1816. 
3 Zoologia Danica, iv. 1806. 
4 Hist. Nat. d. Anim. sans Vert. Paris, 1815-1822, t. iii. 
