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PHYLUM CHORDATA I CLASS PISCES — FISHES 



the gut. The end of the gullet and the anterior portion of 

 the stomach and the rectum are supported by folds of peri- 

 toneum — the membrane which lines the body cavity ; the 

 rest of the gut lies freely. Rectum, ureters, and genital 

 ducts all open into 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- 



FiG. 339. — Heart and adjacent vessels of skate. — In part 



after Monro. 



v., Ventricle ; c.a., conus arteriosus ; pA., posterior innominate ; 

 v.a., ventral aorta ; a.i., anterior innominate ; Th., thyroid ; 

 m., mouth ; a., auricle ; s.v., sinus venosus ; s.c, precaval 

 sinus or sinus of Cuvier ; h.s., hepatic sinus ; j., jugular ; br., 

 brachials ; cd., cardinal ; epg., epigastric. 



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 con- 

 tains a rudimentary gill on the anterior wall, supported 

 by a spiracular cartilage. Through the spiracles water 

 may enter or leave the mouth. 



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

 complete partitions (Elasmobranch), and with ventral 

 apertures. The first is bounded anteriorly by the hyoid 



