Chap. VI.] AFFINITIES AND GENEALOGY. 197 



dite, marine creatures permanently attached to a support. 

 They hardly appear like animals, and consist of a simple, 

 tough, leathery sack, with two small projecting orifices. 

 They belcmg to the Molluscoida of Huxley — a lower di- 

 vision of the great kingdom of the Mollusca; but they 

 have recently been placed by some naturalists among the 

 Vermes or worms. Their larvaB somewhat resemble tad- 

 poles in shape, 21 and have the power of swimming freely 

 about. Some observations lately made by M. Kowa- 

 levsky, 22 since confirmed by Prof. Kuppfer, will form a 

 discovery of extraordinary interest, if still further ex- 

 tended, as I hear from M. Kowalevsky in Naples he has 

 now effected. The discovery is that the larvss of As- 

 cidians are related to the Yertebrata, in their manner of 

 development, in the relative position of the nervous sys- 

 tem, and in possessing a structure closely like the chorda 

 dorsalls of vertebrate animals. It thus appears, if we 

 may rely on embryology, which has always proved the 

 safest guide in classification, that we have at last gained 

 a clew to the source whence the Vertebrata have been de- 

 rived. We should thus be justified in believing that at 

 an extremely remote period a group of animals existed, 

 resembling in many respects the larva? of our present As- 

 cidians, which diverged into two great branches — the one 

 retrograding in development and producing the present 



21 I had the satisfaction of seeing, at the Falkland Islands, in April, 

 1833, and therefore some years before any other naturalist, the locomo- 

 tive larvae of a compound Ascidian, closely allied to, but apparently gen- 

 erically distinct from, Synoicum. The tail was about five times as long 

 as the oblong head, and terminated in a very fine filament. It was 

 plainly divided, as sketched by me under a simple microscope, by trans- 

 verse opaque partitions, which I presume represent the great cells figured 

 by Kowalevsky. At an early stage of development the tail was closely 

 coiled round the head of the larva. 



22 ' Memoircs de l'Acad. des Sciences de St. Fetersbourg,' torn. x. No. 

 15, 1806. 



