402 
BULLETIN OF BUREAU OF FISHERIES 
low oxygen or in the stronger hypertonic solutions of various salts and other chemicals, 
regardless of the formation of any sort of mucoid precipitate. 
In connection with gill damage, it must be pointed out that the weaker con- 
centrations of various pollutants which do not damage the gills rapidly enough to 
cause speedy respiratory and circulatory failures must also be considered as pollu- 
tion hazards if these solutions be even slowly toxic to the cells of the gill, for Smith 
(1929) has demonstrated that the gills of fresh-water fishes (carp and gold- 
fish) are important excretory structures, removing from 6 to 10 times as much nitrog- 
enous wastes as the kidneys. Keys and Willmer (1932) have found special cells 
associated with chloride secretion in the gills of various fishes, including fresh-water 
species. Damage to the gills from pollutant substances in the water can result, 
therefore, in the impairment of other functions, particularly those of salt balance and 
excretion in addition to respiration and circulation as already described, without 
actual toxic action on the internal vital organs of the fish. 
POLLUTANTS ENTERING THE BODY OF THE FISH AND EXERTING 
TRUE TOXIC ACTION 
The gills, the lining of the mouth, and the skin are the main portals of entry 
through which toxic substances can be absorbed into the body of the fish. However, 
Bond (1933) concludes that the bony fishes and many fresh- water invertebrates have 
little or no permeability to a variety of substances. The writer has been able to 
confirm this relative nonpermeability of the external structures of various fresh-water 
fishes, particularly the gills, for a number of substances. The active agents in vari- 
ous stream pollutants, therefore, may be still further subdivided into those which 
can enter the body of the fish through external structures as various volatile com- 
pounds from crude oil, dilute chlorine, ether, chloroform, methyl mercaptan, and 
formaldehyde, and those which do not enter or at least enter very slowly through the 
external structures as certain arsenic compounds. 
The main internal channel for the absorption of injurious and toxic substances 
is the gastrointestinal tract. The writer, using colloidal dye methods, has found 
that many fresh-water fishes refuse to swallow water containing various types of 
effluents for sometime after being placed in such water, for as has been pointed out 
by Smith (1930) most of the water required for the formation of urine in the fresh- 
water fish is probably taken in through the lining of the mouth. However, after 
varying intervals, usually 48 hours or more, fresh-water fish swallow some of the sur- 
rounding water even if not fed, suggesting that the water needs of the body are not 
entirely supplied through absorption by the external surface or the lining of the mouth. 
The swallowing of polluted water allows any of the injurious or toxic substances which 
this water may contain direct access to the mucosa of the gastrointestinal tract. 
Harmful substances may, therefore, either damage the lining of the intestinal tract 
or may be absorbed from the gastrointestinal tract, and in this way gain access to the 
internal vital organs. Consequently, another group of pollutants can be designated 
as those acting on the fish after ingestion into the gastrointestinal tract. It is through 
this portal that many cumulative poisons enter the body of the fish during long-time 
exposures to concentrations of pollutants which may at the time seem harmless. 
