AFFINITIES AND GENEALOGY. 181 



been preserved from utter extinction by inhabiting rivers, 

 which are harbors of refuge, and are related to the great 

 waters of the ocean in the same way that islands are to 

 continents. 



Lastly, one single member of the immense and diversi- 

 fied class of fishes, namely, the lancelet or amphioxus, is so 

 different from all other fishes, that Hackel maintains that 

 it ought to form a distinct class in the vertebrate kingdom. 

 This fish is remarkable for its negative characters; it can 

 hardly be said to possess a brain, vertebral column, or heart, 

 etc. ; so that it was classed by the older naturalists among 

 the worms. Many years ago Prof. Groodsir perceived that 

 the lancelet presented some affinities with the Ascidians, 

 which are invertebrate, hermaphrodite, 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 belong to the 

 Mulluscoida of Huxley a lower division of the great king- 

 dom of the Mollusca; but. they have recently been placed 

 by some naturalists among the Vermes or worms. Their 

 larvae somewhat resemble tadpoles in shape,* and have the 

 power of swimming freely about. M. Kovalevsky f 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 ani- 

 mals ; 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 fur- 

 ther, 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 classi- 



* At the Falkland Islands I ^ad the satisfaction of seeing, in April, 

 1833, and therefore some years nefore any other naturalist, the loco- 

 motive 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 fila- 

 ment, 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 develop- 

 ment the tail was closely coiled round the head of the larva. 



f " Memoires cle 1'Acad. des Sciences de St. Petersbourg," torn, x, 

 No. 15, 1866, 



