4 88 



FISHES. 



tive surface ; a small rectal gland of unknown significance 

 is attached to the terminal or rectal portion of the gut ; the 

 end of the gullet and the anterior portion of the stomach 

 and the rectum are supported by folds of peritoneum, the 

 membrane which lines the body cavity, the rest of the gut 

 lies freely ; into the terminal chamber or cloaca the rectum, 

 the ureters, and the genital ducts all open ; an abdominal 

 pore opens on each side of the cloacal aperture. 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 



FIG. 1 60. Upper part of the dorsal aorta in the Skate. 

 (After MONRO.) 



d.a., Dorsal aorta; c., coeliac artery; m., superior mesenteric ; 

 s.cl.) subclavian ; e.b., efferent branchial vessels, three formed from 

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



anterior wall, supported by a spiracular cartilage. Through 

 the spiracles water may enter or leave the mouth. 



There are five pairs of gill cavities, separated by parti- 

 tions, and with ventral apertures. The first cavity is 

 bounded anteriorly by the hyoid arch, posteriorly by the 

 first branchial arch. The hyoid arch bears branchial fila- 

 ments on its posterior surface; the first four branchial 

 arches bear gill filaments on both surfaces ; the fifth 



