ii OUTLINE OF HISTORY 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 1 and Audouin made a series of observa- 

 tions on living Compound Ascidians, and amongst other discoveries 

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

 ment into the young Ascidian. 



In 1845, Carl Schmidt 2 first announced the presence in 

 the test of some Ascidians of " tunicine," a substance very similar 

 to cellulose ; and in the following year Lowig and Kolliker 3 

 confirmed the discovery, and made some additional observations 

 upon this substance and upon the structure of the test in general. 

 Huxley, 4 in an important series of papers published in the 

 Transactions of the Eoyal and Linnean Societies of London from 

 1851 onwards, discussed the structure, embryology, and affinities 

 of the pelagic Tunicates, Pyrosoma, Salpa, Doliolum and Appendi- 

 cularia. These important forms were also investigated about the 

 same time by Gegenbaur, Vogt, H. Mliller, Krohn, and Leuckart. 



The most important epoch in the history of the Tunicata is 

 the date of the publication of Kowalevsky's celebrated memoir 5 

 upon the development of a Simple Ascidian. The tailed larva had 

 been previously discovered and investigated by several naturalists, 

 notably by H. Milne -Ed wards, 6 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 develop- 

 ment 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- 



1 Mem. Instit. Paris, xviii. 1842. 



2 Zur vergl. Physiol. Wirbellos. Thiere, Brunswick. 



3 Comptes Eendus, Paris, xxii ; and Ann. Sci. Nat. ser. 3 (Zool.) v. 



4 Phil. Trans. 1851 ; Trans. Linn. Soc. xxiii. 1860. 



6 Mem. Acad. St. Peterslourg (7), x. 1866. 6 Mem. Instit. Paris, xviii. 1842. 



