﻿Miss Agnes Crane — On Certain Living and Fossil Fishes. 209 



III. — On Certain Geneka of Living Fishes and their Fossil 



Affinities.^ 

 By Miss Agnes Crane. 



ON first thonglits, it may seem that the lowest group of verte- 

 brates, of all the divisions comprised in the animal kingdom, 

 might be most easily described, and its zoological limits defined ; 

 but, on examination, the fishes prove to be most cm'ioiisly linked to 

 the invertebrata below and the amphibian reptiles above. In fact, 

 it is not easy to draw the lines positively between them, and to say 

 where the true vertebrates begin, or where the piscine characters 

 are merged in the reptilian. It is proposed to refer to some of 

 the most aberrant forms of living fish and their fossil affinities ; 

 then, briefly passing in review the distribution of the various 

 families in geological time, to see how far descent with modification 

 is traceable in this class of vertebrates. 



The Lowest Vertebrate. — It is well known that the lowest ver- 

 tebrate form is the anomalous lancelet (^Amphioxus lanceoJatiis), which 

 is found burrowing in sand banks on our southern shores and in the 

 Mediterranean. The position which this singular species should occupy 

 in the animal kingdom has long been a subject of debate among natu- 

 ralists. Some, like Agassiz, separate it entirely from all other fishes, 

 while Haeckel proposes to place it in a distinct division of the Verte- 

 brata, and Professor Semper removes it from the vertebrates altogether. 

 But Professors Owen^ and Huxley,^ considering it to possess the 

 rudiments of a skull and brain, with the elements of a vertebral 

 column, retain it among the fishes, and it forms the first or lowest 

 order of their respective systematic arrangements. In Amphioxus, 

 which ranges from one and a half to two inches in length, the 

 vertebral column is notochordal throughout life, that is to say, com- 

 posed of a membraneous rod inclosed in cartilage; and as there is no 

 enlargement of the skull for the reception of the brain, the animal 

 tapers nearly equally at either end. The skin is scaleless, lubricous, 

 and so transparent that the internal structure is visible, and the eyes 

 are not more fully developed than in the common leech. The mouth 

 is vertical, jawless, and suctorial, and is furnished with vibratile 

 cilia. The lancelet possesses neither heart nor swimming bladder, 

 and is without ribs and even rudimentary limbs. In all other fishes 

 respiration is effected by means of water passing through the mouth 

 and escaping by the gills, or their equivalents ; in this species 

 it traverses the whole interior of the animal and escapes by a special 

 pore on the under surface of the body. Professor Goodsir^ long ago 

 called attention to this peculiar mode of respiration, and noticed the 



1 Eead before the Brighton Nat. Hist. Soc. Feb. 8th, 1877, and the Geologists' 

 Association April 6th, 1877. 



The writer is much indebted to Dr. Giinther, F.R.S., and Mr. "W. Davies. F.G.S., 

 of the British Museum, for information kindly imparted, and facilities and assistance 

 afforded in the examination of specimens. 



2 Anatomy of Vertebrates, vol. i. 



3 Preliminary Note on the Structure of the Skull and Brain in Amphioxus lanceo- 

 latus, Proceedings of the Royal Society, 1874, No. 157, December. 



* Transactions of the Eoyal Society of Edinburgh, toI. xv.pl. 11, p. 259. 



DECADE II. — VOL. IV. — NO. V. l4 



