488 FISHES. 



, 1 J ^f unknown significance 

 live surface ; a small rectal gland ot unicnow i 5 

 is attached to the terminal or rectal portion ot the gut , tne 

 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 cloaca^||jrture. Excepting 

 mouth-cavity and cloaca, the gut is^ij^j^y endoderm. 



Respiratory System. 

 The first apparent gill clefts — the spiracles — open dorsally 

 behind the eyes. Each contains a rudimentary gill on the 



Fic. 160. — Upper ]:iart of the dorsal aorta in the .'?kate. 

 (After Monro.) 



d.a.. Dorsal aorta; c.^ cccliac artery; 7«., superior mesenteric; 

 s.cL^ subclavian ; c.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 



