114 THE EVOLUTION OF MAN. 



Fig. 191. — Embryo of a Shark (Scymnus lichia), seen from ventral side : 

 V, pectoral fins (in front of these five pairs of gill-openings) ; h, ventral fins ; 

 a, anal opening ; s, tail fin ; k, external gill-tufts ; d, yelk-sac (the greater 

 part of this has been removed) ; g, eye ; n, nose ; m, mouth fissure. 



Fig. 192. — Developed Man-shark (Carcharias melanopterus), seen from 

 the left side : )-j first, r^ second dorsal fin; s, tail fin; a, anal fin; v, pectoral 

 fins ; h, ventral fins. 



developed in another direction from the Primitive Fishes ; 

 this line leads upward through the Dipneusta group to the 

 important class of Amphibia. 



This significant relationship between the three groups 

 of Fishes has been placed beyond all doubt by the re- 

 searches of Gegenbaur on the subject. The lucid discussion 

 on the " systematic position of the Selachii " which that 

 author inserted in the introduction to his classic study of 

 the "head skeleton of the Selachii," must be regarded as 

 definitely proving this important relation.^*^ In Primitive 

 Fishes (Selachii), however, the scales (skin appendages) 

 and the teeth (jaw appendages) are identical in formation 

 and structure, while in the other two groups of Fishes 

 (Mucous and Osseous Fishes) these organs have already 

 become distinct and differentiated. Moreover, in Primitive 

 Fishes, the cartilaginous skeleton (the vertebral column 

 and the skull, as well as the members) is of the simplest 

 and most primitive nature, of which the bony skeletons 

 of Mucous and Osseous Fishes must be regarded as a 

 modification. It is true that in certain respects (in the 

 structure of the heart and of the intestinal canal) Mucous 

 Fishes fully coincide with Primitive Fishes, and differ from 

 Osseous Fishes. But a comparative review of all the 

 anatomical relations plainly shows that the Mucous Fishes 

 constitute a connecting group between Primitive and 



