110 



THE ELASMOBRANCH FISHES 



It is evident that the number of segments tliat take part in the formation 

 of buds for the pectoral fin is fewer in the sharks than in the rays. This fact is 

 clear when we consider two types like Mustelus and Torpedo, in the former of 

 which the fin is relatively narrow and in the latter is of great extent. Accord- 

 ing to Maurer (1912), in the embryo of Mustelus only ten segments contribute 

 to the formation of the musculature of the pectoral fin ; while in Torpedo there 

 are twenty-six such segments. 



The further course of the development of these buds in two forms like the 



above has been studied in great 

 detail because of the bearing which 

 such development has on the lateral 

 fin-fold theory. That in a type like 

 Mustelus segments (myotomes) an- 

 terior to the pectoral fin and between 

 the pectoral and pelvic fins form 

 buds which atrophy without enter- 

 ing the fin is taken by those wdio 

 accept the lateral fin-fold theory to 

 mean that the fin previously had a 

 much greater anteroposterior extent 

 than at present; and it is hence in 

 agreement with what would be ex- 

 pected from that theory. 



Muscles of the unpaired fins are 

 formed in essential respects like 

 those of paired fins. As the myotome 

 grows dorsally to the middorsal line 

 it gives off buds in the regions of the 

 dorsal fins and the dorsal lobe of the 

 caudal fin. Each bud for the un- 

 paired fins divides into an anterior 

 and a posterior bud but no further 

 division takes place since the buds from opposite sides unite to form the muscu- 

 lature of the unpaired fins. Ventral buds arise from the tip of the tail forward 

 to the anal region. Tlie more posterior of these supply the ventral lobe of the 

 caudal fin, while those in the region of the anal fin, in forms in which an anal 

 fin develops, provide musculature for that fin. 



Fig. 112. Adult pectoral muscles, Sqtialus 

 sucJdii. (Evelyn Forsythe, orig.) 



cL, gill cleft; d.b., dorsal bundle; d.r.m., 

 dorsal radial muscle of pectoral fin; l.h., lat- 

 eral bundle ; U., lateral line ; ms., myoseptum ; 

 tr., trapezius muscle ; v.h., ventral median 

 muscle. 



MUSCLES OF THE CLASPERS 



The muscles which control the claspers are usually more complex than those 

 described for Heptanchus. In Chlamydoselachus, however, few points of modi- 

 fication are shown. The principal change is noted in the area of the adductors. 

 While the adductor in the notidanids is a long muscle, in Chlamydoselachus 

 (ad., fig. 113) it is relatively broad and fan-shaped. Here, too, the external 



