^6 ASCIDIANS 



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 tlie name of Tethyum. 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 Eoyal 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 ^ 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 0. 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 Tuxicata, 

 which he placed between the Eadiata 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 Peron in 

 1804, and Salpa described by Forskai 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 



' Mim. Mus. Paris, ii. 1815. ^ Mini. s. I. Jnim. s. Vert. Pt. ii. Paris, 1816. 



' Zoologia Danica, iv. 1806. 



■* Hist. Nat. d. Anim. sans Vert. Paris, 1815-1822, t iii. 



