vii. 9 BLOOD OF TELEOSTS 203 



In fishes in fresh water the blood is more dilute, about o-6 per cent. 

 NaCl, but is, of course, more concentrated than the surrounding 

 medium, which contains only traces of inorganic ions. Freshwater 

 fishes are able to take up salts from the water through the gill surfaces. 

 The kidney apparatus, with its filtration system of glomeruli and 

 tubules for salt reabsorption, was probably developed for life in fresh 

 water and still serves in this way in the freshwater forms. Various 

 special devices are adopted in fresh water for minimizing the tendency 



Fig. 122. Diagram of the branchial circulation of a teleostean fish. 

 ab. artery to air-bladder; a/ 3-6 , four afferent vessels from ventral aorta; ca. carotid artery; 

 cc. circulus cephalicus; cl. coeliac artery; d. ductus Cuvieri; da. dorsal aorta; ef 3 . efferent 

 vessel of first branchial arch; ep. epibranchial artery; ha. hyoidean artery (afferent vessel of 

 pseudobranch); hp. hepatic vein; ht. heart; mis. mesenteric artery; oa. ophthalmic artery 

 (efferent vessel of pseudobranch); ps. pseudobranch (hyoidean gill, possibly with spiracular 

 gill); s. position of spiracle (closed); va. ventral aorta; I-V. five branchial slits. (From 

 Goodrich, Vertebrata, A. & C. Black, Ltd., after Parker.) 



to gain water and lose salt. The skin is little vascularized and probably 

 makes an almost waterproof layer. The production of mucus assists 

 in this waterproofing, and abundant mucus is secreted when an eel is 

 transferred from salt to fresh water: the full change cannot be made 

 suddenly without killing the fish. 



In marine teleosts the problem is the opposite one of keeping water 

 in, or keeping out salt. The usual kidney mechanism is clearly ill 

 suited for this and it is found that the glomeruli are few, or often 

 completely absent from the kidneys. This no doubt reduces the loss 

 of water, but is not enough by itself to solve the problem, which is 

 met by taking in water and salts and excreting the salts. For this 

 purpose special chloride-secreting cells are present in the gills and it 

 has been shown that the amount of oxygen they use, and hence the 

 work they do in diluting the blood, is proportional to the difference of 

 concentration between the inside and the outside. A marine fish is 

 able to drink and absorb sea water in spite of the fact that this is more 



