than in any of the invertebrates. It also differs in its general plan from that of most adult vertebrates, owing to the peculiar method of respiration. In almost every case the vessels returning from all parts of the body unite into one vein leading into the heart, which consists of only one auricle and ventricle (Fig. 107). From the heart the blood

FIG. 107.-Dissection of a bony fish, the trout (Salmo). a.bl., air-bladder; an., anal opening; au., auricle; gl.st., gills; gul., esophagus; int., intestine; kd., kidney; lr., liver; l.ov., ovary; opt.l., brain; py.c., pyloric cœca; sp.c., spinal cord; spl., spleen; st., stomach; v, ventricle.

is forced through the gills, with all their delicate filaments, and now, laden with oxygen and nutritious substances, already absorbed from the coats of the digestive tract, it travels on to all parts of the body, continually unloading its cargo in needy districts and waste matters in the kidneys before returning once more to the heart.

169. The senses of fishes.-The habits of fishes indicate that they know considerable of what is going on in the outside world, and their well-developed sense-organs show the degree of their sensitiveness. A share of this information comes through the sense of touch, which is distributed all over the surface of the body, chiefly in the more exposed regions sometimes especially provided with 'fleshy feelers, like those on the chin of the catfish.

The sense of smell appears to be fairly developed, as is that of hearing; but there is no evidence of a sense of taste. A few fishes chew their food, and may possibly taste it, but there are others that swallow it whole, and in all there are relatively a few nerves going to the tongue or floor of the mouth.

The eyes of most fishes are highly developed, and are of the greatest use at all times. Exceptions to the rule are found in certain species which live in caves or in the dark abysses of the ocean. In some of these the eyes have disappeared amost completely, and the sense of touch becomes correspondingly more acute; in other deep-sea forms they have grown to a large size, enabling them to distinguish objects in the gloom, like the owls and other nocturnal animals. Embedded in the skin of some of these deep-sea fishes, and certain nocturnal ones, are peculiar spots, composed of a glandular substance, which produces a bright glow like that of the fireflies. These may be located on the head or arranged in patterns over various parts of the body, and may serve to light the fish on its way and enable it to see its food to better advantage, or it may act as a lure to many fishes that become victims to their own curiosity. In those fishes which are active most of the time the eyes are located on the sides of the head, and in those which remain at or near the bottom they are turned toward the top; in every case where they can be used to the best advantage.

170. Breeding habits.-Among fishes the egg-laying time usually comes with the spring, when the males of several species become more resplendent, and sometimes engage in struggles for their respective mates. In others this ceremony is performed without show of hostility. Some make nests, while others lay their eggs loosely in the water.

In the salmon family, the eggs are laid in cooling waters, as in rivers or brooks. The young make their way downward, often entering the sea. When the young in the sea

become mature they emigrate in great companies, and make their way hundreds, perhaps thousands, of miles to the rivers in which they spent their youth. Up these streams they rush in crowds, leaping over waterfalls and rapids, and, dashed and battered on the rocks, many, and in some species all, die from injuries or exhaustion after the breeding season is passed. The eggs, like those of the chubs, suckers, sunfishes, and catfishes, are usually buried in shallow holes in the sand, and the males of most fishes keep a faithful watch over the young until they are able to live in safety. In some of the sticklebacks and in several marine species elaborate nests are composed of grass or seaweeds; some of the catfishes carry the eggs until they hatch in their mouths or else in folds of spongy skin on the under side of the body; in the pipefishes and sea-horses a slender sac along the lower surface of the male acts as a broodpouch, in which the female places the eggs to remain until developed; and some fishes, such as the surf-fishes and a number of the sharks, bring forth their young alive. On the other hand, the young of many of the herrings, salmon, cod, perch, and numerous other fishes are abandoned at their birth, and fall a prey to many animals, even the parents often devouring their own eggs.

In the former cases, where the young are protected, only a relatively few eggs are produced: where they are abandoned the female often lays many millions. In every case the number of eggs is in direct relation to the chances the young have of reaching maturity, a few out of each brood surviving to perpetuate the race.

171. Development and past history. The eggs of the higher bony fishes are usually small (one-tenth to one-third of an inch in diameter), and the young when they hatch are accordingly little; in the sharks the eggs are larger, the size of a hen's egg or even larger, and the young when born are relatively large and powerful. These differences, however, do not greatly affect the early development, for

in every case the head and then the trunk soon become formed, gills arise, the nervous system appears, which is invariably supported by a skeleton in the form of a gristly rod-the notochord. In the lower forms of fishes this persists throughout life; but in the sharks and skates it becomes replaced in the adult by another and higher type of skeleton, which is much more specialized with the bony fishes.

Those who study the fossils on the rocks tell us that the first fishes were very simple, and many believe that their skeleton, like that of the little growing fish, consisted only of a notochord. Many of these old forms died out long ago, while others gradually changed in one way and another to adapt themselves to their surroundings, the constant need of adaptation having resulted in the multitude of present-day types. Some of the sharks have probably changed relatively only to a slight extent; others, like the garpike, are much more altered; and the bony fishes are far from their original estate, though their development has been rather toward a greater specialization for aquatic life than an advance upward. The little fish in its growth from the egg thus repeats the history of its ancestral development; but as though in haste to reach the adult condition, it omits many important details. Moreover, the record in the rocks is not complete, and we have many things yet to learn of the ancient fishes and their development from age to age to the present day.

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CHAPTER XV

THE AMPHIBIANS

IN many respects the amphibians-toads, frogs, and salamanders-resemble the fishes, especially the lung-fishes (Dipnoi). The modern amphibians are essentially fishes in their early life, but in developing legs and otherwise changing their bodily form they become adapted for a life on land under conditions differing from those of the fishes. Judging from this class of facts, we may assume that fishlike ancestors, by the development of the lungs, became fitted for a life on land, and that from these the amphibians of our times have been derived.

172. Development. The eggs of the Amphibia are laid during the spring months in fresh-water streams and ponds. They are globular, about as large as shot, and are embedded in a gelatinous envelope (Fig. 108). They are either deposited singly or in clumps, or festooned in long strings over the water-weeds. During the next few days development proceeds rapidly under favorable conditions, resulting in an elongated body with simple head and tail. In this condition they are hatched as tadpoles. As yet they are blind and mouthless, but lips and horny jaws soon appear, along with highly developed eyes, ears, and nose. External fluffy gills arise on the sides of the head, and slits form in the walls of the throat, between which gills are attached, and over which folds of skin develop, as in the fishes. A fin-fold like that of the lancelet or lamprey appears on the tail. The brain and spinal cord, extending along the line of the back, are supported by a gristly notochord, and complete and com