17 
power of assimilating their colour to that of the bottom on 
which they lie. And this change is effected very rapidly. 
Not very long ago, I placed in a tank, in the Zoological 
Gardens, some small plaice, which I had caught on the flat 
shore near Reculvers. They were then indistinguishable in 
tint from the sand from which they had been removed, but 
the bottom of their new home was strewn with cockle-shells 
which had become partially covered by the growth of a dark 
green conferva, and appeared in “the imperfect light as if 
irregularly chequered with black and white. Within a quarter 
of an hour of their being put in this tank, all the young plaice 
had lost their sandy hue, and had assumed the mottled ap- 
pearance of the bottom on which they were resting. The 
colours of some fishes vary also at different seasons. An old 
kelt salmon, after the spawning season is over, is hard to recog- 
nise as the brave, brilliant fish he was when, fresh from the sea 
and dressed in his wedding suit, he took his course up the 
river, despising the adverse force of the swiftly-running 
stream, and overleaping with vigorous ardour the obstacle 
of weir or waterfall which opposed his progress on his way to 
meet his bride. He returns a hook-nosed, draggle-tailed, 
lean, and almost loathsome brute; with his colour changed 
to a lurid, unwholesome red, as if some marauder had 
robbed him of his bright silver armour, and sent him home 
in an old damaged suit of rusty iron. I instance this as 
indicating the changes that take place in the structure and 
coloration of fish-scales as a useful subject for microscopical 
investigation. Let us now examine the respiratory organs of 
some of these fishes. The blood of all animals has, as I need 
not tell you, to be purified and renovated by contact with 
oxygen. In the mammalia, in birds, and in the mature 
reptile, this office is performed by the lungs, and the oxygen 
is obtained from the air; but as fishes breathe water, and not 
air, they are furnished with a respiratory apparatus—the 
gills—by which their blood in the course of its circulation is 
purified by being brought under the influence of the oxygen 
contained in the water. Most lovely objects for the micro- 
scope are the gills of fishes, either in their natural state, or 
injected as permanent specimens; and wonderfully beautiful 
the minute network of vessels in their laminz, so constructed 
and arranged as to present the greatest possible amount of 
surface to the action of the water. But there is one point 
concerning them to which I referred just now, and which has 
always struck me as very remarkable. In examining the gills 
of the salmon and the eel, I have found no structural difference 
between them so marked and important as to form a satis- 
factorily characteristic distinction separating them from each 
