The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

spect they are by no means unique, for there are aquatic mammals and there have been many kinds of aquatic reptiles; but these latter animals are exceptional members of essentially terrestrial classes, and their structure is the structure of a land animal modified for a watery existence. They are air-breathers, and have to rise to the surface from time to time in order to renew the air in the lungs. Their paddles are typical terrestrial limbs masked only superficially by a finny disguise. Fish, on the other hand, are aquatic through and through. They breathe water, and their fins do not show the five-toed plan that underlies the limbs of land-living vertebrates. Their gills are paralleled by larval amphibians and even by a few adult members of that group; but most amphibians have lungs when they grow up, and their jointed limbs always end in unmistakably terrestrial hands and feet. The most distinctive feature of the fish is the fin—a broad, flat expanse ending in a plate of characteristic “ fin-rays.”’ They may be defined as finny vertebrates.

There are two sub-classes of fish—the gristly and the bony fish. In the former the skeleton is composed in the main of gristle ; this is strengthened by the deposition of lime-salts in various places—always in the teeth and scales, and often in the vertebree and jaws. In the second sub-class there is bone in the skeleton, and especially in the skull ; it is as if the skull of a gristly fish had been plastered over with a layer of flat bones. The easiest way of distinguishing the two sub-classes is by looking at the gillshits. In the gristly fish (with one or two rare exceptions) the five or more gill-slits can be seen as separate openings, while in the bony fish they are covered by a flap of bone, the gill-cover, and they open together by a single cleft at its back margin.

The gristly fish include sharks and dogfish, the flat, bottom-living skates and rays, and the weird and rarely-seen ‘ Chimeras.” They include the largest of fish, the basking sharks, up to forty feet long. The eggs of dogfish, rays and the like are well-known seaside objects, often thrown up on the beach by storms and called “ sailors’ purses ”” or “ mermaids’ purses.” They have tough horny shells and contain a large store of yolk, and the embryo may develop inside them for over a year before hatching. Some species, like the “spur dog,” are viviparous. But they are not now the prevalent kind of fish ; it is to the second class that the great majority of fish belong.

The bony fish vary enormously in form and habit. Some, beautifully stream-lined,

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THE SCIENCE OF LIFE

CHAPTER 1

swim swiftly and with endurance ; a good example is the mackerel, whose clear blue colour, light below and darkly rippling above, makes him almost invisible as he slides through the sea. Others, curiously flattened, lie on the bottom, snapping with their wry mouths at unsuspicious passers-by, and only occasionally swimming to a new pitch ; the plaice and turbot, for example, have an astonishing power of changing colour and assuming the pattern of the bottom on which they lie. Some, like the carp, have small, fastidious mouths and swallow worms and such-like delicatessen; others, like the pike, have enormous jaws and cruel teeth and destroy fishes nearly as large as themselves. There are fish with mouths that shoot out like the lattice-work “ ticklers ” of our fairs and snatch unwary creatures ; there are fish with lures to attract their prey within reach of the hungry jaws. The flying-fish have large, broad fins so that they can skim for a considerable distance through the air. There are cylindrical fish, ribbonlike fish, globular fish ; in the abyss there are fish more grotesque, more horrible than any mythological monster. There are freshwater fish and salt-water fish, and there are some, like the salmon, that travel regularly to and fro between sea and river. There are also fish that have structures very like lungs and that can, if they choose, breathe air. ‘They are amphibious fish and they are found in tropical rivers subject to summer drought. Then they cease to use their gills, encase themselves in mud and lie breathing gently with their lungs until the waters return.

§ 3f

Cyclostomes, a Class of Degraded Antiques

And now we come to another group less numerous now than it was in the remote past. These are the Cyclostomes. Of this class only a few genera now survive. Once they seem to have been the only vertebrata in existence. The living forms are the lampreys and the hag-fish or slime-eels. They live, some in fresh water and some in the sea. Their bodies are long and eel-like. They breathe by means of gills as a fish does. But in a great number of respects they .are extraordinary. They have no trace of paired limbs, finny or otherwise ; the skeletal supports of their gills are unlike those of any of the true fishes; they have only one middle nostril and only one nasal cavity ; instead of three semicircular canals in their ears which all other fishes possess