amongst the Vermes or worms. Their larvae somewhat resemble tadpoles in
shape (23. At the Falkland Islands I had the satisfaction of seeing, in
April, 1833, and therefore some years before any other naturalist, the
locomotive larvae of a compound Ascidian, closely allied to Synoicum,
but apparently generically distinct from it. The tail was about five
times as long as the oblong head, and terminated in a very fine
filament. It was, as sketched by me under a simple microscope, plainly
divided by transverse opaque partitions, which I presume represent the
great cells figured by Kovalevsky. At an early stage of development the
tail was closely coiled round the head of the larva.), and have the
power of swimming freely about. Mr. Kovalevsky (24. Memoires de
l Acad. des Sciences de St. Petersbourg, tom. x. No. 15, 1866.) has
lately observed that the larvae of Ascidians are related to the
Vertebrata, in their manner of development, in the relative position of
the nervous system, and in possessing a structure closely like the
chorda dorsalis of vertebrate animals; and in this he has been since
confirmed by Prof. Kupffer. M. Kovalevsky writes to me from Naples,
that he has now carried these observations yet further, and should his
results be well established, the whole will form a discovery of the
very greatest value. Thus, if we may rely on embryology, ever the
safest guide in classification, it seems that we have at last gained a
clue to the source whence the Vertebrata were derived. (25. But I am
bound to add that some competent judges dispute this conclusion; for
instance, M. Giard, in a series of papers in the Archives de Zoologie
Experimentale, for 1872. Nevertheless, this naturalist remarks, p.
281, L organisation de la larve ascidienne en dehors de toute
hypothese et de toute theorie, nous montre comment la nature peut
produire la disposition fondamentale du type vertebre (l existence
d une corde dorsale) chez un invertebre par la seule condition vitale
de l adaptation, et cette simple possibilite du passage supprime
l abime entre les deux sous-regnes, encore bien qu en ignore par ou le
passage s est fait en realite. ) We should then be justified in
believing that at an extremely remote period a group of animals
existed, resembling in many respects the larvae of our present
Ascidians, which diverged into two great branches the one retrograding
in development and producing the present class of Ascidians, the other
rising to the crown and summit of the animal kingdom by giving birth to
the Vertebrata.
We have thus far endeavoured rudely to trace the genealogy of the
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