RESP1RA TOR Y SYS TEM. 523 



brane which lines the body cavity ; the rest of the gut lies 

 freely. Rectum, ureters, and genital ducts all communicate 

 with the exterior through the common terminal chamber 

 or cloaca. An abdominal pore opens on each side of the 

 cloacal aperture, and puts the body cavity in direct com- 

 munication with the exterior. Excepting mouth cavity and 

 cloaca, the gut is lined by endoderm. 



Respiratory system.- -The first apparent gill-clefts the 

 spiracles open dorsally behind the eyes. Each contains 

 a rudimentary gill on the anterior wall, supported by a 



FIG. 251. Upper part of the dorsal aorta in the skate. 



After Monro. 



i/.ei., Dorsal aorta; c., coeliac artery; ;//., superior mesenteric ; 

 s.cl. , subclavian ; *./'., efferent branchial vessels, three formed 

 from the union of nine ; v., vertebral ; c., carotid. 



spiracular cartilage. Through the spiracles water may enter 

 or leave the mouth. 



There are other five pairs of gill-clefts, separated by par- 

 titions, and with ventral apertures. The first is bounded 

 anteriorly by the hyoid arch, posteriorly by the first branchial 

 arch. The hyoid arch bears branchial filaments on its 

 posterior surface ; the first four branchial arches bear gill 

 filaments on both surfaces ; the fifth branchial arch bears 

 none. Each set of branchial filaments is called a half gill, 

 and as the first four branchial arches bear a half gill on 

 each side, and the hyoid arch a half on its posterior surface, 



