506 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [16] 
cover.them. Infected eggs are hopelessly lost, and in the attempt to 
clean them detached germs of these parasites might transfer the malady 
to eggs which so far have been spared. 
Another very dangerous enemy of the spawn is found in the family 
of the diatoms, some of which attach themselves in enormous quantities 
to the stones and gravel at the bottom of the apparatus and cover it 
with a brownish or yellowish- 
J green covering; they there- 
) upon attack the eggs, exclude 
Fic. 20. them from the air, and thus 
cause the death of the spawn, no matter to what degree of development 
it has attained. The species most to be feared are (Fig. 21) Meridion 
circulare (f), Synedra angustata (a), parvula (b), acicularis (c), VAUCHE- 
RLA, palea (d), mucida, and the diatoma pectinale. We possess two very 
excellent remedies against these plagues of the pisciculturist. These 
are: rapidly running water and the exclusion of light; but whilst the 
first can only be applied to fish of the salmonoid species, the second 
will, under all circumstances, prove effective, is not at all hurtful to the 
spawn, and can be applied anywhere. The exclusion of light hinders the 
propagation of the diatoms and conferve, whilst the eggs can be suc- 
cessfully hatched even in the densest 
darkness.* Several authors also rec- 
ommend in such cases to transfer the 
eggs to other vessels. The crooked 
and straight pipes used for this pur- 
pose can only be employed in small 4% 
establishments, and they may -be re- 
placed by the first glass tube near at 
hand, provided that it can be hermeti- 
cally closed with the thumb when the 
eggs have entered its lower portion. It 
will be necessary, however, to have due 
regard to the state of development of 
the spawn, and only to employ this remedy when it is absolutely neces- 
sary, and even then only when the eyes of the embryo are visible. 
The intervention of man is also required when the eggs are attacked by 
the larvie of insects, and particularly by those of Disticus marginalis 
and Hydrophilus pisceus ; nothing but constant watchfulness can prevent 
the ravages which they would cause. Another small insect, probably 
in its larva state (Ascarides minor ?), and which probably comes from 
fish used in the operation, is very dangerous to the eggs at the time 
when the embryo has almost reached its full development. It pierces 
the outer shell of the egg and devours its contents. As the presence 
of this animalcula can only be recognized by the shells of the eggs float- 
ing near the surface, there is no time to think of its destruction. 
xh: le air nen: ates : j 
This observation is based on positive and repeated experiments. The hatching 
process takes place in a regular manner, but is somewhat delayed. 
