STRUCTURE OF THE GILuLS. 191 
regularity of structure, such as no human mosaic could ever 
equal, 
Many of our European fishes are richly decorated with vivid 
colours, but their scaly raiment is generally far from equalling 
the gorgeous magnificence of the fishes of the tropical seas. 
If in the birds of the equatorial zone a part of the plumage 
sparkles with a gem-like brilliancy, all the colours of the rain- 
bow combine to decorate the raiment of the tropical fishes, and 
no human art can reproduce the beauty of their metallic lustre, 
which at every movement in the crystalline waters exhibits to 
the enchanted eye new combinations and reflections of the 
most splendid tints. 
The gaudiest fishes live among the coral reefs. In the tepid 
waters, where the zoophytes, those sensitive flowers of the ocean, 
build their submarine palaces, we find the brilliant Chetodons, 
the gorgeous Balistinz, and the azure Glyphysodons gliding 
from coral branch to coral branch like the playful Colibris, that 
over the Brazilian fields dart from one lustrous petal to another. 
Oxygen is as necessary to fishes and other marine creatures as 
it is to the terrestrial animals, but as they are obliged to draw 
it from a denser element, which absorbs but a small volume of 
air, their gills are necessarily differently constructed from the 
lungs of the creatures breathing in the atmosphere. In most 
species, comprising all the bony fishes, and the sturgeons, 
among those which have a cartilaginous skeleton, we find on 
either side of the throat five apertures, separated from each 
other by four crooked, parallel and unequal bones, and leading 
to a cavity, which is closed on the outside by an operculum or 
cover. In this cavity, and attached to the bones, are situated the 
delicate membranes, bearded like feathers, which serve to aérate 
the blood. The water constantly flows through the gills in one 
direction, entering by the branchial apertures of the throat, and 
emerging through the operculum. This is, in more than one 
respect, a most wise provision of Nature; for if the fishes were 
obliged to receive and reject the water by the same aperture, 
as we do the air, each expiration would evidently drive them 
backwards, and consequently retard their movements. It is 
also evident that the delicate fringes or folds of the gills would 
soon get into disorder if the water were carried through them in 
two opposite directions. 
