Temperature and Veriebm 17 



In considering the increase in number and corresponding 

 reduction in size of the vertebras of northern fishes, it is often 

 very difficult to distinguish between primitive simplicity, such 

 as the salmon and herring show, and the lack of complexity 

 which may be due to " Panmixia " or the cessation of selec- 

 tion — examples of which may be found in the LiparididcB and 

 perhaps in the cod and arctic blennies. 



We have also, in connection with the process of ichthyiza- 

 tion, something of what Professor Dana calls " Cephalization.'' 

 Features of this are (i) the attachment of the shoulder-girdle 

 to the skull, which occurs in most recent fishes, but which is 

 carried to co-ossification in the case of some of the most spe- 

 cialized, {BalistidcB, Tetrodojitidce , etc.). (2) The attachment 

 of the pelvis to the shoulder-girdle or to the head, shown in 

 the spiny-rayed fishes and their allies, and (3) the modification 

 and specialization of various bones of the jaws and gill arches, 

 which is in the most specialized forms often accompanied by 

 co-ossification or by reduction in number of the bones con- 

 cerned. Connected with these changes is the gradual reduc- 

 tion or loss of the air-bladder, which is a degenerate lung, 

 doubtless used for air-breathing by the ganoid ancestors of 

 the modern fishes. In the spiny-rayed fishes it is a closed 

 sac, often so small as to be functionless and very often it is 

 wholly absent. 



NUMBERS OF VERTEBRA. 



We may now consider in detail the numbers of the vertebrae 

 in the different groups of fishes : 



Lancelets. — In the different species of Branchiostoma or 

 lancelet, a group which stands at the bottom of the vertebrate 

 series, probably diverging from the fish-stock before the 

 formation of a brain or organs of special sense, the number 

 of segments is large, from 50 to 80. 



Lampreys. — In the lampreys and hag-fishes, low and to 

 some extent primitive types, which show no trace of limbs 

 or jaws, the vertebrae are cartilaginous and numerous, being 

 little specialized. The number in species examined is more 

 than a hundred, the range being perhaps from 100 to 150. 



