Chap. VI. 



Affinities and Genealogy. 



159 



Prof. Owen, 22 the Ichthyosaurians — great sea-lizards furnished 

 with paddles— present many affinities with fishes, or rather, 

 according to Huxley, with amphibians ; a class which, including 

 in its highest division frogs and toads, is plainly allied to the 

 Ganoid fishes. These latter fishes swarmed during the earlier 

 geological periods, and were constructed on what is called a 

 generalised type, that is, they presented diversified affinities with 

 other groups of organisms. The Lepidosiren is also so closely 

 allied to amphibians and fishes, that naturalists long disputed in 

 which of these two classes to rank it ; it, and also some few 

 Ganoid fishes, have been preserved from utter extinction by 

 inhabiting rivers, which are harbours 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 diversified 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, &c. ; so that it was classed by 

 the older naturalists amongst the worms. Many years ago Prof. 

 Goodsir 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 

 Molluscoida of Huxley — a lower division of the great kingdom 

 of the Mollusca ; but they have recently been placed by some 

 naturalists amongst the Vermes or worms. Their larvae some- 

 what resemble tadpoles in shape, 23 and have the power of 

 swimming freely about. M. Kovalevsky 2i 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 

 dorsahs of vertebrate animals; and in this lie has been since 



23 At the Falkland Islands I had 

 the satisfaction of seeing, in April 

 1833, and therefore some years be- 

 fore any other naturalist, the loco- 

 motive larva) of a compound Asci- 

 iia^, closely allied to Synoicum, 

 but apparently generieally 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 bv m* 



under a simple microscope, plainly 

 divided by transverse opaque parti- 

 tions, which I presume represent 

 the great cells figured by Kovalev- 

 sky. At an early stage of develop- 

 ment the tail was closely coiled 

 round the head of the larva. 



24 ' Memories de l'Acad. dee 

 Sciences de St. Pjtersliourg,' torn x. 

 No. 15, I860. 



