THP] ELASMOBRANCH FISHES 



109 



Tlie eoracomandibularis {c.md.) in the shark is seen ui)()n removing the 

 skin and the first and second ventral constrictors. In the rays it lies directly 

 under the skin. In 8cynin\(s {c.md., fig. 110a) it is an unpaired muscle which 

 originates directly from the coracoid cartilage and is inserted near the man- 

 dibular symphysis. That the muscle is of a paired nature, however, is shown 

 by its being innervated by paired nerves. In Raja its paired condition is indi- 

 cated anteriorly, and in Aetolxitis the two muscles are entirely separate. 



The coracohyoideus muscle {chij., fig. 110) is seen lying at the sides of the 

 eoracomandibularis {Heterodontus,&g. 110b) . It arises from the fascia around 

 the arcus communes and is inserted on the 

 basihyoid and lower part of the hyoid 

 arch. In Scymnus (fig. 110a) it passes 

 more directly forward from the most an- 

 terior arcual segment than it does in Hep- 

 tanchus. In Raja (see fig. 107b) the coraco- 

 hyoideus is small and lies more or less over 

 the eoracomandibularis. It is inserted on 

 the ventral part of the basihyal segment. 



The coracobranchiales (c.hr.) are the 

 deepest of the hypobranchial muscles and 

 support the floor of the pericardial cavity. 

 They consist of five separate parts in pen- 

 tanchid sharks, six in Hexanchns, and 

 seven in Heptanchiis. In the rays they 

 arise in a common mass. The first coraco- 

 branchialis (c.hr.) is large in Scymnus; 

 and the remaining muscles, except the last 

 which is tlie largest, are smaller. They are 

 normally inserted on the ceratobranchial 

 segment of the branchial arches. 



Muscles op the Fins 



Fig. 111. Development of the muscles 

 to the pectoral fin, Acanthias. (From 

 Erik Miiller.) 



1-33, muscle buds; I-XII, nerves. 



In Elasmobranchs the muscles of the 

 paired fins arise in the embryo as buds 



from the ventral downgrowth of the myotome. Varying numbers of these buds 

 take part in the formation of the fins in different species of Elasmobranchs. 

 After the dorsal somite is separated off from the lateral plates its myotome 

 grows rapidly both dorsally and ventrally. As the ventral downgrowth 

 passes the fin area it gives off the muscle buds as lateral growths. The muscle 

 bud separates into an anterior and a posterior primary bud, each of which in 

 turn divides into an upper and a lower secondary bud so that from one myo- 

 tome four buds arise. The two of these l)uds which are dorsal in position go to 

 make up dorsal fin muscles, while the two ventral in position compose the 

 ventral musculature of the fin (fig. 111). 



