560 PISCES—FISHES, 
cavity ; one radius of the blastoderm becomes thicker than the rest, and 
forms the first hint of the embryo ; an inward growth from the edge of 
the blastoderm forms an invaginated layer—the dorsal hypoblast or roof 
of the gut; the periblast forms the floor of the gut, and afterwards aids. 
the mesoblast, which appears between epiblast and hypoblast; the 
medullary canal is formed as usual in the dorsal epiblast. It is likely 
that the edge of the blastoderm represents the blastopore or mouth of 
the gastrula, much disguised by the presence of yolk. 
The newly hatched larva is still mouthless, and lives for awhile om 
the residue of yolk, which, by its buoyancy, causes the young fish to be 
suspended in the water back downwards. 
GENERAL NOTES ON THE FUNCTIONS, HABITS, AND 
Lire Histories oF FISHES 
Movement.—A fish may well compare with a bird in its mastery 
of the medium in which it lives. Thus a salmon travels at the rate of 
about eight yards in a second, or over sixteen miles an hour. The 
motion depends mainly on the powerful muscles which produce the 
lateral strokes of the tail and posterior part of the body. It may be 
roughly compared to the motion of a boat propelled by an oar from the 
stern, So energetic are the strokes that a fish is often able to leap 
from the water to a considerable height. In some cases undulating 
movements of the unpaired fins, and even the rapid backward outrush 
of water from under the gill-cover, seem io help in movement. The 
paired fins are chiefly used in ascending and descending, in steering and 
balancing. The large pectoral fins of the flying-fish (Dactylopterus and 
Exocetus) are used rather as parachutes than as wings during the long 
skimming leaps. 
Shape in relation to habit.—The characteristic form of the 
body, as seen in herring or trout, is an elongated laterally compressed 
spindle, thinning off behind. like a wedge. In most cases the trunk 
passes quite gradually into head and tail. This torpedo-like form is. 
well adapted for rapid progression. Flat-fishes, whether flattened 
from above downwards, like the skate, or from side to side like the 
plaice and sole, usually live more or less on the bottom; eel-like 
forms often wallow in the mud, or creep in and out of crevices ;. 
globe-fishes, like Dzodon and Zetrodon, often float passively, 
Colour.—The colours of fishes are often very bright. They 
depend partly on the presence of pigment cells in the skin, partly on 
the physical structure of the scales. The common silvery colout is. 
due to small crystals of guanin in the skin. In many cases the colours 
of the male are brighter than those of his mate, as in the gemmeous 
dragonet (Caddonymus lyra) and the stickleback (Gasterosteus), and 
this is especially true at the breeding season. The colours of many 
fishes change with their surroundings. In the plaice and some others. 
the change is rapid. Surrounding colour affects the eye, the influence 
passes from eye to brain, and from the brain down the sympathetic 
nervous system, thence by peripheral nerves to the skin, where the 
