26 LABORATORY DIRECTIONS 



to each other and to the thyroid, produce the changes in the 

 tension on the vocal cords. The vocal cords are elastic tissue 

 covered over with a mucous membrane. The slit-like opening 

 between the vocal cords is the glottis. The trachea, extending 

 below the larynx, is supported by cartilaginous rings, which are 

 open on the dorsal side. The cartilages of the larynx are 

 derived from modified visceral arches. 



appendicular Skeleton 



Typically, vertebrates have two pairs of limbs, pectoral and 

 pelvic, each with its girdle of bones connecting it with the body. 

 As a matter of adaptation these limbs undergo great modifica- 

 tion in various forms, and one, or even both pairs, may be 

 entirely absent. From the amphibians to the mammals the 

 limbs of all forms are easily recognized as homologous. But 

 the limbs (fins) of fishes are of an entirely different structure, 

 and although probably homologous, are not easily compared 

 with those of higher vertebrates. 



Above the fishes the parts of the pectoral girdle and forelimb 

 may be homologized with the parts of the pelvic girdle and hind 

 limb. Each part of one is represented by a similar part in the 

 other. 



Girdles and Limbs of the Shark 



The pectoral girdle Is the U-shaped arch of cartilage which 

 supports the pectoral fins. It is made up of right and left 

 halves united by a ligament in the mid-ventral line. The 

 articular {glenoid) surface for articulation of the fin is well up 

 on the side. The part of the girdle dorsal to this is known as the 

 scapular region, the most dorsal part of which forms a distinct 

 cartilage, the suprascapular. The part of the girdle ventral 

 and medial to the glenoid surface is the coracoid region. 



The pectoral fin is attached to the pectoral girdle at the 

 glenoid surface. It consists of: (a) three hasalia, large cartilages 

 articulating with the glenoid surface; (b) many radialia or fin 

 rays extending beyond the basalia; and (c) actinotrichia, the 

 very numerous horny threads extending beyond the radialia and 

 supporting the greater portion of the fin. 



