THE ELASMOBRANCH FISHES 



155 



PRODUCTION OF RESPIRATORY CURRENT 



For tlie sharks and rays in general the respiratory current is produced by 

 the interaction of the complicated series of buccal and ])haryngeal muscles 

 which insure that when the current enters the mouth the external clefts close 

 and when the clefts oi)en. the mouth closes. In general the action is as follows : 

 By the contraction of the ventral, longitudinal, or hyj^obranchial musculature 



Fig. 148. A. Part of a gill pocket of Cetorhinus. (From Pavesi.) B. Diagram of a section 

 parallel to the gill rakers. 



&./•., branchial ray; fl., filaments; g.r., gill rakers; g.p., gill pocket; ia., internal branchial 

 aperture. 



the floor of the mouth and pharynx is lowered, thus enlarging the Ijuccal and 

 pharyngeal rooms, at the same time that the mouth is opened. Into this cavity 

 the water rushes. The adductors then act, closing the mouth and at the same 

 time flexing the epi- and ceratobranchial segments of the arches, thereby 

 spreading apart the cartilaginous branchial rays and causing the pockets to 

 enlarge. The water now enters the pockets and is then forced out through 

 the external clefts by the contraction of the constrictor and interbranchial 

 muscles. By this action of the muscles a rhythm is produced which under 

 conditions of rest is about thirty-five respirations a minute (Heterodontus 

 francisci) . 



