THE DESCENT OB ORiaiN OF MAN 213 



earlier geological periods, and were constructed on what is 

 called a generalized type, tliat is, they presented diversified 

 affinities with other groups of organisms. The Lepidosiren 

 is also so closely allied to amphibians and fishes, that natu- 

 ralists 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 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 diversified 

 class of fishes, namely, the lancelet or amphioxus, is so dif- 

 ferent 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. Goodsir perceived that the 

 lancelet presented some affinities with the Ascidians, which 

 are invertebrate, hermaphrodite, marine creatures perma- 

 nently attached to a support. They hardly appear like ani- 

 mals, and consist of a simple, tough, leathery sack, with 

 two small projecting orifices. They belong to the Mollus- 

 coida of Huxley — a lower division of the great kingdom 

 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" has lately 

 observed that the larvae of Ascidians are related to the 

 Vertebrata in their manner of development, in the relative 



^' 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 slietched 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. 



" "M^moires de I'Acad. des Sciences de St. P^tersbourg," tom. x., No, 

 16, 1866. 



