30 HISTORY OF THE HUMAN BODY 



the lower representatives of this group, but becomes more or 

 less bony in the higher. The gill-slits no longer open directly 

 and separately to the outside, as in their selachian ancestors, 

 but are grouped together and covered by a gill-flap or oper- 

 culum. 



The two remaining groups of fishes, teleosts and dipnoans, 

 represent independent lateral branches that have specialized 

 in accordance with certain definite lines and are consequently 

 not in the direct line of man's ancestry. Such groups often 

 form collateral testimony of considerable morphological value 

 and are thus not without importance even in the present line 

 of speculation. The teleosts have an almost completely ossi- 

 fied skeleton and are the descendants of the bony ganoids, with 

 which they are so closely connected through intermediate 

 forms that the separation between them is mainly an artificial 

 one.* They are essentially a modern group and constitute 

 the great majority of the fishes in the world to-day, thus 

 taking the place of the ganoids of earlier times. The dipnoi 

 are represented by but three forms, one found in Africa, one 

 in Australia and one in South America. They are fresh-water 

 fishes and are remarkable for their power of sustaining long 

 periods of drought by digging into the mud, and breathing 

 air through a modified air-bladder. They were thus for- 

 merly considered the link between fishes and amphibians, but 

 later researches into their structure do not confirm this view. 



As a matter of fact the amphibians seem to have come from 

 the ganoids, although by means of forms now lost, and to have 

 developed first into the Stegocephali, a group wholly extinct 

 but well represented by fossil remains occurring in and about 

 the coal deposits. These had many of the characteristics of 

 our modern amphibians, but possessed scales arranged 

 in definite rows, organs which are entirely lacking in all 

 living representatives of this Class, with the exception of the 



* Although the employment of the two terms " ganoid " and " teleost " 

 is a convenient one in comparative anatomy, modern ichthyologists tend 

 strongly to the rejection of both terms and the fusion of the two groups 

 into a single one, the Teleostomi. Cf. Appendix. 



