14 THE ELASMOBRANCH FISHES 



We have thus far spoken only of the fins wliich are propelling. In a type like 

 Myliohatis (fig. 8), direction of the course is effected through the paired fins. 

 In TJrolophus (fig. 9) the horizontal course is also directed by the pectorals, 

 but the vertical direction is controlled in large part by the caudal which is 

 used effectively as a rudder. 



In the sharks other fins are of service in directing the course. The use of the 

 directive fins may made out by a series of experiments. If a rubber band is 

 put over the pectorals of a young shark with the caudal fin free, there residts 

 a downward swimming of the fish, the pectorals functioning as organs for 

 directing the horizontal course in the water. In function the pelvics are acces- 



—'Ai--' ;.j_ui£i-i. 



Fig. 24. Embryo of ScyUium canicula showing early epidermal fin-folds. (From Mayer.) 



al., anal fin; df., dorsal fin-fold; dl., second dorsal fin; pL, pelvic fin; pt., pectoral fin; 

 V.I., ventral fin-fold. 



sory to the pectorals. Both the dorsals and the anal may be bound down with- 

 out interfering seriously with propulsion. These fins are of service, however, 

 in keeping the fish in a vertical plane. 



FOEM OF FIN IN ITS BEGINNING 



It is evident that the form of the fin in present-day Elasmobranchs differs 

 from that of the ancestral type. And the question arises, What was the form of 

 the ancestral fin ? To this inquiry many answers have been given, few of which 

 have gained a hearing. We may mention briefly two of the best known theories 

 for the origin of the paired fins. One is the lateral fin-fold theory of Balfour 

 and Thacher; the other is the gill-arch theory of Gegenbaur. 



According to the fin-fold theory the ancestors of the present-day fishes are 

 supposed to have possessed a median dorsal fold {df., fig. 24) which was con- 

 tinued over the tail as the dorsal lobe of the caudal fin and then forward to the 

 anal region as the ventral lobe, {v.l.) ; anterior to the anal region the ventral 

 lobe separated into right and left lateral folds not greatly unlike the meta- 

 pleural folds of Amphioxus. In certain regions of the dorsal and the lateral 

 folds greater development ensued than at the interspaces. These parts of the 

 fin-fold hence increased in size and became the present unpaired and paired 

 fins, while the intermediate parts finally dropped out. 



The gill-arch theor}^ of Gegenbaur holds that the framework of the girdles 

 for the paired fins and of the fins themselves have been derived from the gill 

 arches (g.a., fig. 25) and their attached branchial rays (h.r.). The arch itself 

 represents the girdle and the formation of the skeleton of the fin proper was 



