560 PISCES FISHES. 



cavity ; one radius of the blastoderm becomes thicker than the rest, ar.d 

 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 ot 

 the gastrula, much disguised by the presence of yolk. 



The newly hatched larva is still mouthless, and lives for awhile on 

 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 

 LIFE 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 to 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 

 Exoccetus} 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 Diodon and Tetrodon, 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 colour 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 (Callionymus 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 



