II OUTLINE OF HISTORY a7 
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! 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” 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 ° 
confirmed the discovery, and made some additional observations 
upon this substance and upon the structure of the test in general. 
Huxley,* 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 Appendi- 
cularia. These important forms were also investigated about the 
same time by Gegenbaur, Vogt, H. Miiller, Krohn, and Leuckart. 
The most important epoch in the history of the Tunicata is 
the date of the publication of Kowalevsky’s celebrated memoir ” 
upon the development of a Simple Ascidian. The tailed larva had 
been previously discovered and investigated by several naturalists, 
notably by H. Milne-Edwards,° 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 Mém. Instit. Paris, xviii. 1842. 
2 Zur vergl. Physiol. Wirbellos. Thiere, Brunswick. 
3 Comptes Rendus, Paris, xxii; and Ann. Sci. Nat. ser. 3 (Zool.) v. 
4 Phil. Trans. 1851; Trans. Linn. Soc. xxiii. 1860. 
5 Mém. Acad. St. Pétersbourg (7), x. 1866. 8 Mém. Instit. Paris, xviii. 1842. 
