372 ADAPTATIONS TO MEDIA AND SUBSTRATES 



It would seem that there must also be a difference in the source of 

 the water of the humors in fresh- and salt-water teleosts. The former are 

 known to admit water readily everywhere through the skin, and to pro- 

 duce large volumes of urine in consequence. The marine teleosts, de- 

 prived of this use of osmosis, must fight for their internal water. They 

 swallow sea-water and absorb it from within, excreting the excess salts 

 by means of special cells in the gills. 



No consistent differences have been reported between the eyes of 

 freshwater and marine teleosts. If in the former the intra-ocular fluid 

 and its pressure are recruited by osmosis through the cornea, it becomes 

 a mystery where these come from in the marine form. If the latter gives 

 off aqueous from the iris, falciform process, or hyaloid vessels, does this 

 occur — and if not, what prevents it — in the freshwater eye? And, there 

 are both anadromous (e.g., the Pacific salmon) and catadromous (e.g., 

 the common eel) teleosts, whose eyes appear the same as regards possible 

 secretory structures whether they are in fresh water or salt. 



The elasmobranchs are mostly strictly marine though some, like the 

 sawfish (Pristis) may enter perfectly fresh water; and several species are 

 landlocked. All are known to maintain a high level of urea (2%) in 

 their blood to give it a slightly higher osmotic pressure than the sea-water, 

 so that they have as easy a time to maintain their general water balance 

 as does the freshwater teleost; and like the latter they do not need to 

 drink. They form little urine except when in fresh water, when they pro- 

 duce 50-100 times as much. In Squalus, at least, the intra-ocular fluids 

 have been found to have a still higher osmotic pressure than that of the 

 blood. It is therefore easy to believe that the intra-ocular water can come 

 in through the cornea and that the intra-ocular pressure can be automat- 

 ically regulated by controlled osmotic pressure; but it is a disturbing fact 

 that the elasmobranchs are the only fishes which have ciliary folds (Fig, 

 104, p. 259), This makes it look, at first glance, as if they secreted their 

 aqueous just as a land animal must. The ciliary folds are low and no 

 more heavily vascularized than epithelial folds generally are, however, 

 and moreover are blanketed (except where they continue, even lower, 

 onto the iris) by the thick peripheral rim of the gelatinous 'zonule'. They 

 do not appear to be advantageously organized for secretory purposes. Is 

 their function purely mechanical, to increase the surface of attachment 

 of the zonule? Franz believes so, since he found them best developed in 

 the species with the most powerful lens-muscles. Are they even absorp- 

 tive, the cornea being unable to control the amount of incoming water, 



