CCELOM AND ALIMENTARY SYSTEM J25 



Digestion follows the same general course as in mammals 

 (p. 444) ; the food remains in the stomach, which is even more 

 acid than in man, for two or three days or even longer. 



RESPIRATION 



The hemibranchs meet across the gill pouch and divide it into 

 two chambers ; by the action of muscles water is forced through 

 the lattice of the gills from the buccal cavity to the exterior. 

 When the floor of the pharynx is lowered water is drawn in through 

 the mouth and spiracles, while the flexible front edge of each 

 gill cleft is caused, by the lower pressure within, to flap back so 

 as to prevent the entry of water that way. When the floor is 

 raised the lips prevent the escape of water through the mouth, 

 contraction of the oesophagus keeps that also closed, and the 

 water, being under pressure, opens the clefts and passes out 

 over the gills. Water is also sucked into the outer part of the gill 

 chamber by contraction of muscles in its walls ; when the chamber 

 contracts back flow of water to the pharynx is prevented by the 

 high resistance of the gills. Through the thin membrane which is 

 all that separates the blood in the gills from the water, the gases 

 of respiration are exchanged. 



BLOOD VESSELS : HEART 



The heart of a dogfish lies in the pericardium at the level of 

 the last gill cleft. It is a median structure with muscular walls, 

 and consists essentially of an irregular tube, bent twice like an S 

 (Fig. 246) and composed of four successive chambers. The hinder- 

 most chamber is the thin-walled sinus venosus, which is triangular 

 as seen from below, and hes with its base against the hinder wall 

 of the pericardium. In front of it comes the thicker-walled auricle. 

 This is also triangular, with its apex forwards, and has its hinder 

 angles widened into pouches. The S then curves downwards, as 

 the very thick-walled, conical ventricle, which lies below and 

 somewhat behind the auricle. From it the narrow conus arteriosus 

 passes forwards through the front wall of the pericardium to 

 become the ventral aorta, which is merely the foremost part 

 of the single vessel whose thickening and twisting produces 

 the heart behind. Thus the heart, or contractile blood vessel, of 

 the dogfish, hke that of all other vertebrate animals, is ventral 



