THE ELASMOBRANCH FISHES 157 



experiments on Squat ina, above mentioned, I have also observed the phenome- 

 non of the outgoing current, but that the spiracle is not of great importance 

 as an aperture for the intake of the current is shown by my experiment. 



Something of the same rhythmic current occurs in the rays, although it may 

 be produced differently. In s{)ecimens of the small sting-ray, Urolophus, which 

 are buried in the sand with only the spiracular clefts and the outline of the 

 bodj' discernible, the s])iracular clefts open and close regularly. The only evi- 

 dence of the outgoing current, however, is seen in a regular geyser of sand 

 grains arising to an inch or two in height at the anterior margin of the pec- 

 toral fin. 



CIECULATION OF BLOOD IN FILAMENTS 



In a later study of the blood system ( Chapter VII, p. 161 ) it will be found that 

 the afferent arteries bear non-oxygenated blood from the heart to all the 

 demibranchs and give off to them smaller branches which run outward toward 

 the tips of the filaments; the branches in turn give off thin-walled arterioles 

 which in their course break up into capillaries. It is in these capillaries that 

 the exchange of oxj^gen and carbon dioxide takes place. From the capillaries 

 efferent branchial arterioles convey the oxygenated blood toward the base of 

 the filaments into either an anterior or a posterior efferent-collector (efc). By 

 means of these collectors oxygenated blood is removed from the region of the 

 gills. In other words, while a single afferent artery supplies both anterior and 

 posterior series of filaments with non-oxygenated blood, two efferent-col- 

 lectors, one at the base of each demibranch, carry away the blood which has 

 been oxygenated. 



RESPIEATION OR THE EXCHANGE OF GASES 



The respiratory current thus brought into the gill pockets is separated from 

 the blood contained in the capillaries of the filaments only by the thin capil- 

 lary wall. The free oxygen of the water passes by osmosis through this wall 

 into the blood to be distributed to the body, and the carbon dioxide brought to 

 the capillaries from the body tissues passes outward into the water to be 

 eliminated by the respiratory current. The exchange of gases takes place with 

 extreme rapidity as is evident from the fact that the blood makes its complete 

 transit of the capillaries in a very short time. 



