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PISCES 



In higher vertebrates the spiracle is used in connection with 

 audition and while other gill clefts disappear entirely, it gives rise 

 to the tympanic cavity and the Eustachian tube. There are five 

 pairs of gill pouches, opening internally to the pharynx and externally 

 by gill slits. 



Bile due t — -/- — • 



Gall bladder 



Liver 



■Cordioc stomach 



Spiral valve 



Rectal gland 



Fig. 109. Digestive system of skate. (Drawn by W. J. Moore.) 



Water enters the mouth and passes the interior clefts into the 

 branchial pouches, then outward by the exterior clefts. Gill 

 pouches are developed from the pharynx and so the respiratory 

 epithelium is endodermal. Cartilaginous visceral arches alternate 

 with the gill clefts. A gill consists of two hemibranchs or half gills. 

 The Dipnoi utilize the gills and the modified swim bladder which 

 functions as a lung. 



Circulatory System. — The heart is four-chambered consisting of 

 one auricle, one ventricle, with an auriculo-ventricular valve with 

 two lips, the sinus venosus, situated dorsally and posteriorly and 



