In fact the distinction between the two divisions of Lepidoptera is one of convenience rather than of science, for it marks difference of habits rather than of structure. Instead of a naked pupa, that of the moth is inclosed in some sort of envelope called a "cocoon." This may be an earthen cell underground, or a woolly tuft fastened to some such support as the bark of a tree, or a leaf rolled and tied by silken threads into a tube, or a burrow in dead wood, or a paperlike case fastened to a twig; but in every case some special provision is made for the easy emergence of the imago when the time comes for its birth as a moth. The moths themselves do no harm. Their few weeks of life are devoted entirely to mating and putting their eggs in just those places where the larvæ they will never see can have the food proper for them and the best chance for life—a matter of marvelous instincts and adaptations. Few of them, except the hawk moths, eat at all. That is done in the caterpillar stage, when many sorts become destructive of the labor and hopes of the farmer and gardener and orchardist, or make havoc in stores of grain and meal, and in garments of wool and fur, carpets, and cabinets of natural history specimens.
Most of the moths are small, inconspicuous, grayish or brownish creatures whose markings, very lovely when closely examined, so closely resemble in their mottlings the places where the moths rest during the day, that they are comparatively safe from the birds, monkeys and other enemies that seek to catch and eat them. Some, however, are of large size and brighter hue. Thus the silkworm moths of the Orient (and of our own land) may measure four or five inches across the outstretched wings, as does the cecropia and others that flit about evening lights; and a near relative among us is the exquisite, long-tailed, luna moth, which is pale green with chestnut edgings; many others in this group are almost as "richy bedight" as butterflies. It is these that make the large papery cocoons so easily seen in the fall in trees and bushes.
A remarkable family (Bombycidæ) is that of the hawk moths, which much resemble in shape and action humming birds. They are day flyers, but most active in the morning and evening twilights, and hover on whirring wings before a flower, while with their long, tubular tongues they suck its nectar, for these moths feed as well as do their fat, uprearing, bulldoglike caterpillars, to which they owe another common name for the family—that of sphinx moths. Their pupæ are lodged under, on, or near the ground in a loose cocoon, and are to be recognized by an appendage, curled around like a jug handle, in which lies the chrysalis' long tongue.
Flies, scientifically speaking, are only those insects of the order of Diptera, distinguished by having only one pair of fully developed wings. They pass through a complete metamorphosis, and the larva is in all cases a "grub" or "maggot" destitute of legs. It is rarely enclosed in a cocoon but lies buried in the ground, floats in the water, or is protected by the last larval skin which, separating from the pupa skin, remains around it as a hard case. Flies and their larvæ live in the most diverse manner. Some flies attack backboned animals and suck their blood, some prey on smaller insects, some suck honey, and some find their food in decaying animal and vegetable matter. A large number of dipterous larvæ eat refuse, many feed inside growing vegetable tissues, and some prey, or are parasitic, on other insects. More than 10,000 species of true flies have already been named in the United States alone. The order contains all the different species and varieties of fleas, mosquitoes, sand flies, gnats, midges and gall flies. Then come the blood-sucking gadflies, and half a dozen families allied to them; the scavenging syrphus flies, the bots that trouble cattle, the house flies and stable flies of deservedly bad repute; and, lastly, the horseflies, bee parasites, and botflies. The popular interest in these insects is confined to the flies of our houses and stables, and to the mosquitoes. In fact it is in the relation that the flies mentioned, and some others, bear to public health and comfort, that this group of insects is important at all to any but the special student.
The beetles (order Coleoptera) make up a very distinct and natural group of insects, characterized by the horny or leathery texture of their forewings, or "elytra," which serve as cases for the folding membranous hind wings alone used in flight. These elytra, when closed, usually cover the whole hind body. They are strengthened with ridges around their edges, and marked with a series of longitudinal furrows and often also with impressed dots. The hind wings are sometimes very small or wanting; in such cases the elytra are often fused together along their middle edges (suture). The head is usually extended from behind forward, having therefore a large crown and a small face; the feelers are very inconstant in form; the mandibles are always developed as strong biting jaws; the prothorax is free and movable; its tergite (pronotum) is a very prominent feature in all beetles, reaching back to the origin of the elytra.
The beetles undergo a complete metamorphosis, and the larvæ, called "grubs," have various shapes, while the pupa is "free," that is, closely similar in development and appearance to the adult. Beetles are world-wide in distribution and more than 100,000 species have been catalogued. They are divided into a great number of families, among which those mentioned below contain the most noteworthy forms.
The tiger beetles are large-headed, predacious forms, most numerous in the tropics, which live in holes in the soil and rush out to seize passing prey. The ground beetles (Carabidæ) are a very extensive family, represented in all parts of the world, and are insect hunters, destroying hosts of injurious insects. Most of them are black or brown. The Dyticidæ and Hydrophilidæ are aquatic families, including some of the largest and fiercest of carnivorous beetles, the terrors of ponds and marshes, where they prey not only on other insects and their young, but on tadpoles, small fishes, etc.; and their grubs are quite as savage. The rove beetles (Staphylinidæ) are a very large family of narrow, elongated species, which are very active; they feed mostly on small insects, worms and snails. The carrion beetles belong to the family Silphidæ, the smaller among which live in moss and under tree bark, and the larger genera contain the noted "burying beetles." Some groups of very minute, ground-keeping species lead to the familiar "ladybirds" (Coccinellidæ), a large and world-wide family of small, rounded beetles, usually brightly spotted, which frequent plants of all sorts, and feed chiefly on aphids. Some quaint superstitions pertain to these pretty insects, that should be attracted rather than repelled when they visit window gardens and greenhouses, which they will endeavor to clear of the "greenfly" and similar injurious plant lice. Passing over several inconspicuous families we come to the dermestids, very small, dark-colored beetles of elliptical outline, some of whose genera are among the worst of household pests, and have been spread by commerce throughout the civilized world.
Some of the dermestids are troublesome as museum pests; others attack food in the pantry, store, or warehouse. "Drugs do not escape their attack, species devouring even cantharides and tobacco; woolen and silk goods, feathers and furs, are ruined if left long exposed to their depredations; and one species is accused of biting young doves.... Anthrenus scrophulariæ, probably introduced into America from Europe, has received the names carpet beetle and buffalo bug, on account of its habit, both as larvæ and imago, of destroying carpets. This beetle measures about four-fifths of an inch in length, and is black, brick-red and white, the last crossing the back in two zigzag lines. The point of attack is the nailed-down edge or the lines of the seams."
Who has not been amused at the labors of the big black beetles that one meets in summer on dusty paths rolling balls of fibrous material. These "dung beetles" are the American cousin of the scarab of the ancient Egyptians, which typified to them many mystical ideas connected with life, present and eternal. With its shovellike head and broad forelegs the beetle gathers and compacts the material it wants, and begins to roll it, sometimes with the help, more often against the struggles, of another beetle toward a prepared nest-hole. Arrived there an egg may be inserted into it, and then the rounded mass is left as food for the grub to be hatched from the egg; if no egg is inserted, the ball becomes simply a mass of stored food to be eaten by its maker. Processes vary among the 7,000 or more known species of this cosmopolitan family.
Not all of this great family are dung beetles, however, or scarablike. Here belong the May bugs and June beetles that come blundering around lighted country residences in the evenings; and it is their fat white grubs that, hatched from eggs buried in the ground, devour the roots of the grass and other plants, spoiling the lawns and strawberry beds. The robin is their most effective enemy. Among the lesser genera are those of the rose bugs, hated pests of the horticulturist and fruit grower. In that section of the family known as the cetonians are found the giants of the race, the West African "goliaths," four inches long; the tropical American Hercules beetle, exceeding six inches long, half of which belongs to the forward-reaching horn of its helmet, the South American elephant beetle which is even more bulky, and several other giants, the males of which have the head ornamented with fearsome protuberances.
Other families of beetles are the Buprestidæ, whose larvæ are injurious to trees by boring into their wood; the Elateridæ, or snap beetles, which arch their bodies and leap when they happen to fall on their backs, and among which are found the many varieties of brilliant "fireflies" for which the American tropics are famous. The larvæ of the elaters mostly live in decaying wood, and are the justly hated "wireworms" of our gardens. Then there are the Meloidæ, that include the blister beetles, or oil beetles, one of which is the cantharides of the pharmacopœia; and there are a great many more.
A long shelf is required in the naturalist's library for the books relating to the Hymenoptera of America alone—our wasps, bees, ants, and their smaller relatives, which engage everybody's attention by their social habits and amazing display of instincts. Besides these three principal and familiar groups the Hymenoptera include a host of other insects of great but inconspicuous importance. In large part these are parasitic on other insects or their larvæ, or even on their eggs, and some are the most minute insects known, virtually invisible to the unaided eye. Scarcely larger are the makers (Cynipidæ) of the galls so commonly seen on trees and plants in which they breed. Another group (Chalcidoidea) cause the swellings that disfigure plants by placing their young within their tissues, such as the "joint worms" that ruin grain; and here, again, many species are parasitic on grubs. Then there are the sawflies (Tenthredinidæ), resembling bees, whose ovipositors are like a pair of saws with which these insects are able to bore holes into wood, within which the egg is placed and the young larva burrows; of these are many and various kinds, all injurious to trees, garden shrubs and plants, each kind restricted to a particular sort of plant.
Perhaps even more numerous are the ichneumon flies, whose service in the world seems to be to keep the insect hosts down to the number possible to exist and at the same time to allow men and other animals to live. Their method of life is to deposit their eggs on or in the bodies of other insects, usually in the larval stage, where they hatch and thrive by the slow death of the host. The ichneumon flies are the dread of all other insects, most of whose adaptations for self-preservation are directed against this insidious and universal enemy to insect life.
None of the foregoing Hymenoptera live in colonies or by social methods. That plan belongs to the four most advanced divisions—wasps, bees, termites, and ants. Even among the wasps and bees, however, the larger number of species live alone or in single families, each female constructing a solitary receptacle for her purpose underground, in soft wood or otherwise. Most species store with the egg placed there half-dead insects, or pollen, etc., as food for the grub, which receives no further attention; but a few, such as the big digger wasp (Bembex) take food to the grubs daily. Another class of both wasps and bees form nests of several cells containing eggs, and thus in spring families are originated by fertilized females that have survived the winter. As the larvæ develop in succession they are fed by the mother, and presently mature sufficiently to aid her in caring for the younger grubs. Out of such family nests, or "combs" of paper cells, often attached to the ceilings of sheds and porches of rural houses, have apparently developed the mutually helpful societies of bees and ants, which are often of surprising extent and permanency.
The prosperity of these social insect communities, whose instincts, habits, and products amaze us, is due to an organized division of labor in the community between three classes of "citizens"—(1) the comparatively few males, whose whole duty is to fertilize the queen mother and supply the community with progeny; (2) the selected and specially nourished "queen"; (3) a vast number of nonreproductive females, the "workers," that build and guard the nest, gather and preserve stores of food (honey), and nurse and rear the young. In some groups the duties of the workers are subdivided among classes that differ in size and equipment. It is these female workers, or their correlatives among the solitary bees and wasps, that sting, their useless ovipositors having been transformed by the addition of poison into deadly weapons by which they procure their prey, or defend themselves, or both. It is this division of labor, and attendant habits, that especially characterize the higher Hymenoptera, and give to the order the supreme rank it occupies among insects.
We have been considering up to this point one of the two primary and natural divisions of the animal kingdom—that into Invertebrates and Vertebrates. Although these are terms made familiar by long usage, and refer to the absence or presence of a backbone composed of jointed sections (vertebræ), a truer conception of the distinction is had by regarding the first as animals whose skeleton, or frame, that gives support to the muscles and other soft parts of the body, is exterior; and the second as animals whose skeleton is interior. The one is, in scientific language, an "exoskeleton," or more or less hardened outer shell from the inner surface of which the organs grow and maintain their attachment and leverage for work or protection; and the other an "endoskeleton," around which the organs and integument are accumulated by growth, and by means of which the animal's strength is maintained, the interior bones—of which the chief is the spinal column, or backbone—giving a firm fulcrum for the operation of the muscles and a support and protection for the vital organs.
All the vertebrates are included in a single phylum—Chordata. This term has supplanted in zoölogy the old term Vertebrata (now reserved as a class name only), because it is more comprehensive and precise. Professor Harmer says:
"The axis of the backbone of all vertebrates is formed by an elastic rod known as the 'notochord,' which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the phylum Chordata. The members of this phylum are further distinguished from other animals by several important features. Of these one of the most important appears to be the existence of lateral outgrowths of the pharynx, which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill slits, and in the fishes their walls give rise to vascular folds or gills. With the assumption of a terrestrial life the higher vertebrates lost their gills as functional organs, respiration being then performed by entirely different organs, the lungs. But even in these cases, the gill slits appear in the embryo.... Another fundamental characteristic of the Chordata is given by the central nervous system, which lies entirely above the alimentary canal, just dorsal to the notochord. Not only does this position of the nerve centers distinguish the Chordata from the Invertebrates, but a further point of difference is found in the development."
This definition requires the inclusion of various creatures very unlike "vertebrates," and the phylum therefore embraces three subdivisions: 1. Adelochorda—marine wormlike creatures having a notochord in the anterior of the body, and gill slits, both persistent; 2. Urochorda—the ascidians or tunicates, small marine creatures, some fixed along shores, others free-swimming and in some cases united into swimming colonies (e. g., the salpæ), the tadpolelike larvæ of which show a notochord in the tail; and 3. Vertebrata.
This last great subphylum is divisible into seven grand natural groups with the rank of classes, namely:
1. Acrania—Lancelets (Amphioxus).
2. Cyclostomata—Lampreys; hags.
3. Pisces—Fishes.
4. Amphibia—Amphibians.
5. Reptilia—Reptiles.
6. Aves—Birds.
7. Mammalia—Mammals.
The first of these seven classes, the Acrania, has usually, heretofore, been set apart as a subdivision equal in rank to the subphyla Adelochorda and Urochorda, and the remaining six classes were grouped into a coordinate subphylum Craniata, denoting that they alone have a distinct head (cranium); the reason was that its members, the lancelets, have no spine, but only a notochord, which, however, extends from end to end of the body above the digestive organs, and persists in the adult and throughout life. The lancelets (amphioxus) are small, fish-shaped creatures that burrow in the sand of the seashore, usually leaving only the head exposed, and sucking in a continuous current of water which brings with it minute food. They breathe through gill slits. The reproduction is bisexual, and by eggs.
The significance of the Acrania in this phylum is that they represent a very early ancestral stage of the stock from which the higher vertebrates (Craniata) have developed, and from which they themselves, of course, have also diverged to a certain degree; and it is because they retain many primitive characteristics that the study of their life histories has engaged the attention of so many eminent zoölogists and has thrown so much light on the evolutionary history of the "higher animals," or vertebrates.
Popularly included among fishes, the lampreys and hags of the class Cyclostomata (roundmouths) differ from true fishes by the possession of a suctorial mouth devoid of functional jaws, by the single olfactory organ, and by the absence of lateral appendages, or paired fins. They have an eellike form and method of travel, and some species are a yard in length. They are bisexual, discharging both eggs and milt into the water to become fertilized by accidental contact. Lampreys ascend the rivers to spawn, however, and there make little heaps of pebbles, carried and piled with the mouth, in which the eggs find some protection from the many egg-eaters in all streams. Most, if not all, of the migratory parents die after spawning. From the eggs hatch larvæ that undergo a metamorphosis. Lampreys live on small crustaceans, worms, and so forth, eat carrion, and also attack living fishes. The tongue, like the interior of the mouth, is armed with teeth. They are in the habit of attaching themselves to stones in order to hold themselves against a river current, breathing meanwhile by taking water directly into the pouchlike gill chambers and expelling it, instead of sucking it through the mouth and passing it out of the gill slits. In ancient Rome the big sea lampreys of the Mediterranean were eaten as a delicacy, and even cultivated in landlocked ponds, and they are still highly prized in some parts of Europe.
The hags are an even more primitive group of cyclostomes that live in the mud of shallow seas and are too abundant on both our coasts, where they are a pest of the fisheries. Their general habits are similar to those of lampreys, but wherever possible they attach themselves to fish on which they feed. The hag is particularly destructive to fishes caught on "set lines" of hooks, or in nets, and the loss thus resulting on the coasts of California, in Japan, and in some European fisheries is very serious. As these cyclostomes have no scales or other hard parts to be preserved except a few teeth, no fossil remains are certainly known, but it is the opinion of paleontologists that otherwise the class might be traced to the earliest Paleozoic time.
In beginning, with the fishes, an account of the typical vertebrates, it will be well to point out the structural features in which all agree. Vertebrates are bilaterally symmetrical animals, with an internal skeleton, the axis of which is composed of similar segments (vertebræ) and divides the body into a dorsal and a ventral portion. This skeleton is first formed in cartilage, and remains so, or it may become more or less hardened by deposits of lime, or completely transformed into bone. The anterior end of the vertebral column (backbone) carries a capsule (the skull) inclosing the brain. When limbs are present there are never more than two pairs. The nervous system consists of a brain and spinal cord from which trunk nerves arise and ramify throughout the body. The blood is first driven to the gills, or to the lungs, as the case may be, by means of a heart having either one or two auricles, and after it has traversed the body through arteries and veins it returns to the heart. The stomach, liver, and other viscera, lie in the ventral part of the body. The skin produces a protective covering characteristic in each division of the class, as scales for fishes, feathers for birds, and so forth.
Fishes are vertebrates fitted to live in water. Their typically fusiform shape is that best adapted to progress through the rather dense medium they inhabit; and their limbs are swimming organs, or "fins." These are of two kinds, "paired" and "median." The former are the pectorals, one on each side of the forward part of the body, and the pelvic, or ventral fins on the belly and near together; these four serve, like the bilge keels of a ship, to maintain stability—prevent rolling over—rather than for progression. The median fin is vertical, and extends around the tail from the middle of the back to the end, when it is complete; but in most cases it is represented by an upright fin, the "dorsal" on the back, by the "caudal" fin fringing the tail, and by the "anal" fin at the vent. The powerful caudal fin is the principal agent in swimming, aided by undulatory movements of the dorsal and ventral fins; and it has a twisting action that drives the animal forward as does the rolling of the oar in "sculling" a boat. The median fins are developed from the skin, and are supported by a skeleton system of their own, not connected with the spine. In most fishes the upper and lower halves of the caudal fin are alike, and the tail is symmetrical, but in sharks and some others the end of the spine curves upward and the lower wing of the tail is much larger than the upper; in the former case the caudal fin is said to be "homocercal," and in the latter "heterocercal."
The fins of fishes are in many species modified and adapted to purposes remote from swimming or balancing. Thus it is the first dorsal fin of the remora that has become the sucker on its crown; in the angler the first rays of the back fin are lengthened and lobed to form its "lure," and elongation of various fin rays as feelers, or light bearers, etc., may be found elsewhere. The pectorals are enormously enlarged to make wings for the flying fish and the gurnard, and to give a substitute for legs to the Oriental gobies that like to go ashore, while the ventrals are transformed in certain fishes of swift streams into organs by which they can fasten themselves to the bottom or climb against a cataract.
The skin of fishes is rather thick and tough, and abounds in glands that secrete mucus, and in cells that secrete the hardening, or protective, denticles and scales that form the coat of most species, and which differ widely.
Louis Agassiz distinguished four kinds of scales—placoid, ganoid, cycloid and ctenoid. The first named occur only in the selachians (sharks and rays) and are variously shaped particles of lime that prick through the skin, which makes excellent polishing material when prepared as "shagreen." These "denticles" in the skin become teeth in the mouth without change of structure, and the great spurs with which the "saw" of the sawfish is armed are only extreme instances of this special adaptation.
Ganoid scales are such as formed the armor of the great extinct tribe of ganoid fishes, a remnant of which survives in our gar pikes, or billfish. In some of the fossils they are roundish, and overlap, but in modern ganoids they are rhombic in shape and plate the body edge to edge, connected by toothlike processes that articulate with the adjacent scales, and permit flexibility in the body. The outer face of the scales is enameled, like teeth, beneath which is a layer of bone substance and the teeth in the mouth are only modified scales.
Cycloid and ctenoid scales are those of ordinary fishes, and are precisely alike, except that the hinder, or attached, end of the latter is split into a comblike fringe. They have a rounded or often polygonal form, are composed of lime, and are translucent, thin, elastic, and overlap like shingles on a roof. The scales of fishes increase in size with the animal's growth by additions to the exposed rim, and as these accessions may be observed, by counting them the age of the fish may be computed, when checked by certain other considerations.
The colors of fishes are produced by pigment cells, both in the skin and on the outside of the scales; and by a peculiar tissue composed of secretion products called "iridocytes." These, by their various ways of reflecting light, and by the color elements contained in them, give rise to the different hues of fishes.
Fish show their inferiority as a class by retaining the method of respiration by means of gills characteristic of the aquatic invertebrates. The gills are composed of bright red tassels set on hoops that encircle the throat, and are usually covered by a movable flap—the "gill cover." Under this flap, the neck of the fish is perforated by crescentic slits. The fish normally breathes by taking gulps of water into the mouth and throat, and squeezing it out through the gill slits; during its rhythmical passage over the thin gills the oxygen of the dissolved air is absorbed by the hæmoglobin of the red blood, and is carried away to incessantly revivify the body; and at the same time carbon dioxide is set free and got rid of in the outgoing stream.
An organ peculiar to fishes is the air bladder—a sac lying under the backbone and communicating by a duct with the stomach. It is not only of service in respect to buoyancy, but is accessory to respiration. In spite of its name, however, it does not contain air, but a gas rich in oxygen and nitrogen which is secreted by certain arteries and is carried away when needed by other blood vessels, as fat and starchy substances are stored elsewhere and may be drawn upon when food falls short. Nevertheless, the chief function of the "swim bladder," which is exceedingly varied in shape, is to render the fish of the same weight as the water in which it lives. In this condition of equilibrium the fish swims with a minimum of muscular effort. A consequence of the organization, however, is to restrict the vertical range of each fish and kind of fish, because any considerable movement up or down means a change of pressure. This will bring about the expansion or contraction of the volume of gas in the air bladder and thus alter the specific gravity of the animal. Such automatic adjustment is limited, however, and practically prevents a fish rising or falling far above or below the depth to which it was born; and the fatal effects of violent change are seen in those fishes brought up in explorers' dredges from great depths, the air bladders of which are invariably so distended as to kill the animal. Nevertheless, some species seem able to migrate from and to great depths; and temperature is perhaps a greater factor in vertical distribution than the air bladder, the adjustments of which must be slow. The great body of fish life in the sea resides within about 300 fathoms of the surface.
Fishes have a brain and a system of nerves and sense organs varying according to rank, and outlining the higher developments of the nervous system as found in mammals. Of the sense organs the most peculiar are the small sensitive bodies scattered in various parts of the skin, fins and mouth, called "end buds," each at the terminus of a nerve fibril. These buds seem to carry the sense of feeling, and are said to be represented in mammals by the taste buds in our tongues. They are aggregated in a narrow band along the side of the fish, and in a maze on the side of the head, called the "lateral line," the course of which is plainly visible on many fishes, as for example, on the sunfish of brooks and ponds. This lateral line consists of canals in the skin, opening to the surface by pores, and reached by branches of large nerves. The use of the lateral line to the fish is not well known, but it is believed that its cells are of service in balancing the body. As blind fishes are able to avoid obstacles with the greatest ease when swimming, it is possible, in the opinion of Dr. Bridge, that these organs enable their possessors to appreciate undulatory movements in the water in the shape of reflex waves from contiguous surfaces or objects.
One feature of the lateral line on the head are the "auditory organs," varying with the kinds of fish, which contain semicircular canals, with otoliths, in the more or less complete form of an internal ear. Each is reached by the auditory nerve from the brain and is also connected with the air bladder in many cases. Whether this is a true organ of hearing in the ordinary sense, or whether it serves some other purpose, as, for instance, the regulation of the distension of the air bladder, is not known. The old question of whether fishes hear sounds made above the water is not yet answered scientifically; but it is probable that they can feel the jar of sounds made in the water, which is equivalent to hearing, as far as it goes. Fishermen have a saying that if you swear you won't catch any fish—a good precept, anyhow; but more effective is the care anglers take not to step heavily, nor to make loud, jarring noises, near the bank of the stream in which they mean to cast their lines.
The great majority of fishes have good eyesight, and the eyes themselves are similar in structure to those of the higher land animals; but it seems probable that the range of vision is short. The eyeballs are usually large in proportion to the size of the head—sometimes strikingly so—and are movable; while the situation in the head is naturally such as to give the most advantageous vision according to the habit of life. Thus those of sharks, and other predatory sorts that live by the chase, are well forward; while those of bottom-feeders, and especially rays, flatfish, anglers and the like, are in the top of the head, looking upward. Nocturnal species have the largest eyes, but the unfortunate cave fishes, whose whole life is, and has been for unnumbered generations, passed in the total darkness of caverns and underground streams, have lost the use of their eyes altogether, and the organs themselves have disappeared by atrophy.
Blindness is found also in oceanic families that dwell far below the penetration of daylight; yet many fish of the Stygian depths, which, so far as we know, never leave that region of utter blackness, possess big and apparently efficient eyes. Most of the blind or nearly blind sea fishes thus far obtained have been in hauls from a depth of about 1,200 fathoms. It is believed that the ability to see in deep-sea fishes is connected with the light-giving (phosphorescent) organs possessed by many of them, and with the fact that animals of all sorts on the sea bottom in deep water are luminous, and so reveal themselves to the predatory creatures that feed on them, while the fishes' own "lanterns" enable them to chase moving prey, avoid enemies, and find mates.
Fishes have efficient olfactory organs situated near the snout, and in the higher families they are in pairs and become true, but internal, nostrils. The sense of smell is strong, and perhaps more useful on the whole than the sense of sight, especially among the carnivorous species. Sharks seem to follow their prey by scent like hounds.
All these senses serve instincts related to the necessities of the individual and the race in each kind of fish. This is sometimes manifested in what appears to us as cunning means of safety or of provision for young; but discriminative intelligence is small in fishes, which probably are able to learn little more than that at certain places and times food may be had, as is illustrated in cultivated fish ponds, where the captives from infancy onward are fed regularly. Anglers tell of old trout that refuse year after year to be beguiled by their experiments in flies; but it is doubtful whether this is anything more than an increased wariness due to frequent disturbance. The remora is, or has been, used by the Caribs of the West Indies and the negroes of Zanzibar for catching sea turtles, a line being fastened to a captive and comparatively tame remora carried in the boat, and the fish turned loose as soon as a turtle is seen at the surface. The remora will make a bee line for the turtle and attach itself firmly to the shell so tenaciously that both animals may be dragged to the boat. It is to be noted that the fishermen see the turtle near by before they dispatch their living grapple, and it is doubtful whether the remora has any notion of what it is doing. It simply obeys repeatedly an instinct. This very low degree of intelligence is doubtless owing to the almost invariable environment of piscine lives, in which virtually nothing occurs to suggest any change in traditional habits or arouse into activity any rudiments of mind a fish may possess. Mental inertness is characteristic of aquatic animals of all kinds, as contrasted with the correlated activity of body and mind of land animals stimulated by varied and changeable surroundings.
The breeding habits of fishes furnish one of the most interesting chapters in their natural history, and many surprising facts have been learned within a few years in regard to the reproduction of marine species, of great value to the sea fisheries.
In all fishes the sexes are separate. As a rule females are larger than males, and more numerous. The size of the egg in any group depends on the amount of food yolk stored for the sustenance of the young, which must thrive by its absorption until it is able to eat by its mouth. The largest are the eggs of sharks, etc. (Elasmobranchii), which resemble fowls' eggs. The European dogfish, perhaps two feet long, has eggs an inch in length, each in a flattened leathery "purse" having tendrils at the ends that twine about weeds and anchor it like a rocking cradle. The similar egg capsules of skates, dropped on the sand, are common objects on all beaches. Elasmobranch eggs are deposited at intervals throughout the year and, as they are exposed to comparatively little danger, are few in number. In most other orders spawning, as the egg laying of fishes (and aquatic amphibians) is termed, is limited to a short period, the eggs are small, and the number of eggs produced is often enormous—five or six millions in a large cod, for example.
In the majority of Teleostomi—a group name embracing all the modern bony fishes—the eggs are voided broadcast into the water, the males at the same time emitting clouds of milt. These eggs are of two kinds, one that sinks and, often being glutinous, sticks to some object on or near the bottom, and is called "demersal"; and another that contains an oil bubble, making it so buoyant that it floats, and these latter are called "pelagic." The fertilization of such spawn must be accidental, but as the milt and the eggs sink or drift together the number that come into fertilizing contact is no doubt considerable. Nevertheless, an extremely small percentage ever reach the point of hatching, and still fewer survive to become mature, for in addition to unfavorable circumstances of water and temperature, every living thing, almost, in the ocean, including the parent fish themselves, is a devourer of the eggs and young of fish; and it has been said that the vast number of eggs dispensed by certain species, only a single pair of which on the average survives to maturity, is one of nature's methods of providing food for the inferior forms of marine life.
Only a rapid systematic sketch of the class Pisces, fishes, is possible, distinguishing the main divisions, alluding to their history, and touching here and there the most characteristic genera and species of the thousands that have been described by ichthyologists. The primary division is into three subclasses:
1. Elasmobranchii—Sharks, skates, rays, etc., having a cartilaginous skeleton.
2. Teleostomi—Ordinary fishes, having a bony skeleton.
3. Dipnoi—Lepidosiren, and many extinct, primitive families.
In the Elasmobranchii, or selachians, the skeleton consists of cartilage, as in the embryos of all fishes—a sign of their primitive and inferior rank; but parts of it in various species become hardened by depositions of lime, especially in the vertebræ, in spines and teeth, parts often well preserved as fossils. Sharks' teeth are among the best known of fossils, and before science established their true character were commonly called "birds' tongues," or "snakes' tongues." The sharks, first to be considered, are a very ancient race, originating in early Paleozoic times. Of the many curious extinct forms that terrorized the Silurian, Devonian, and Carboniferous seas, a few representatives still exist in the South Pacific, notably the cow sharks; an eellike Japanese species with frilled gills that Dr. Garman thought might easily fill the rôle of "sea serpent"; and the quaint bullhead, or Port Jackson sharks of Australian waters; all these are of small size and the last named represents the principal race in Mesozoic seas. Their flat teeth form a sort of pavement of the mouth, enabling them to crush the mollusks and crabs on which they chiefly feed.
Next in a rising order of classification, and of somewhat more recent origin geologically, is the European family of dogfishes (Scyllidæ), which includes also the "ground sharks" of warm seas—deep-water fish eight to twelve feet long, that creep about near the bottom in search of prey. Next come the large pelagic sharks of the family Carchariidæ, which contains about sixty species, scattered over all the seas, and one confined in the fresh water of Lake Nicaragua. One section, that of the "topes" and "hounds" of temperate and warm seas, are bottom-feeders, and have pavementlike teeth adapted to crushing and grinding the shellfish on which they subsist; but most of the family are swift and powerful hunters of fishes in the open sea, such as the dreaded tiger shark of the West Indies, which is variegated in color and sometimes twenty feet long, and the equally big blue, white, dusky, and other ferocious bandits ranging not only the tropical seas, but more common in northern oceans than is generally supposed. The teeth in these and other hunting, fish-catching sharks, are shaped somewhat like arrowheads, in some cases smooth-edged and single-pointed, in others with sharply notched edges and side cusps. They are set in the flesh of the jaw, unattached to the underlying bones, in concentric rows, one close behind the other, all round the front of the mouth, both above and below, and look and act like a set of saws, a sidewise movement of the jaws sawing through an object seized in a single bite. As fast as the front row of teeth are lost they are replaced by those of the row immediately in the rear. But all gradations exist between these and the mosaic of "pavement" teeth in the topes. The mouth of these hunting sharks is on the underside of the head, and they must turn on their backs to seize anything floating or swimming near the surface.
This is the group that furnishes the "man-eater" stories—tales that have been substantiated by so many terrifying examples that no precaution of safety against them should neglected, even on our northern coasts, where the ravenous blue shark, or the dusky species, may appear at any time, even in harbors. A few years ago a man was seized by one of these sharks in a little inlet of New York Bay, at Freeport, New Jersey.
Blue sharks are nocturnal in habit, and are sometimes seen asleep or resting in the daytime, with the tips of the two dorsal fins, characteristic of this family, in sight above the surface of the water. "So gentle are they in their movements," says one authority, "that, unlike many other monsters of the deep, they do not disturb the luminous creatures, which at the same time will be lighting every wavelet with their phosphorescence. Blue sharks are not very particular as to what fish they take as food, though those which are gregarious in their habits, like mackerel, pilchards, and herring, are most commonly hunted by them." A curious relative of these "man-eaters" is the hammerhead, in which the sides of the head are extended in two great lobes, with eyes at their extremities; this kind of shark is greatly feared in the East Indian seas. In spite of it, and the prevalence of other huge and voracious sharks, the Arabs about the entrance to the Red Sea, and the natives of other Oriental shores, will swim and dive in the open sea, apparently without fear, where Europeans would be devoured almost instantly. Another peculiar shark is the thresher, well known in the North Atlantic as elsewhere for its strategic maneuvers. It grows to a length of fifteen feet, of which the tail forms at least one half. Quite inoffensive to man, the thresher feeds on the shoals of smaller teleosts, such as pilchards, herrings, and sprats. When feeding it swims in gradually diminishing circles around the shoal, splashing the water with its long tail, and keeping the victims so crowded together that they become an easy prey. Hammerheads and their relatives, the "bonnet" sharks, frequently visit both our shores in summer.
The porbeagles are big, fierce sharks of the family Lamniidæ, the giant of which is Rondelet's shark, known to attain a length of more than forty feet. The triangular, saw-edged teeth of such a one measure nearly an inch across the base; but similar fossil teeth, and also others dredged from the bottom of the South Pacific, are much larger, indicating sharks beside which Rondelet's would be small, and in all probability these monsters survived to a comparatively recent date. A remarkable lamnoid shark of Japanese deep waters has the snout produced into a long, flat, flexible, leaflike blade.
Closely related sharks, almost as big, are well known in the North Atlantic. Two of them, the "bone" and the "basking" sharks, are killed by fishermen whenever encountered for the sake of the oil in their livers. The name of the second refers to its habit of loafing and sleeping on the surface on fine days, when a boat may go so near it that a harpoon may be planted in its hide before it will move. The real "basking shark," however, is a gigantic species of Rhinodon, of the Indian and South Pacific oceans, with a very bulky body that may exceed forty-five feet in length. Both of these ponderous fishes are sluggish, and are not dangerous to man, except that a blow of the tail may smash a boat when an attempt is made to harpoon them at close quarters. They feed on small fishes that go in shoals, and also, perhaps, on seaweeds.
The last sharks to be mentioned are American dogfish of the family Squalidæ (another family, Scyllidæ, are known as "dogfish" and "hounds" in Great Britain), which are numerous and greatly hated along both our northern coasts. The common gray dogfish of the North Atlantic and California coasts is the spiny one (Squalus acanthias), the larger females of which will weigh about eight pounds. It makes its home in deep water off the New England coast, approaching the shore when the mackerel come in and disappearing when they depart; but dogfish are to be found all summer in shoal places such as George's Bank, and irregularly in shore inlets. In the late autumn they become numerous on the Grand Banks, and stay there until the winter's cold drives them away into deeper water. Everywhere these small sharks are a nuisance to the fishermen, by tearing nets and by eating the cod, etc., hooked on the trawl lines. Formerly they were regularly hunted for the oil in their livers, which is especially valuable for certain purposes, as in harness making, but the price of this oil is now low, and the fishery has declined.
The economic use of sharks is not great except as producers of oil. The flesh is good food, but not popular. In China sharks' fins are a favorite substance for delicate soups and sauces, and a very large trade in catching sharks for their fins is carried on near Bombay, and in East Indian waters.
The ugly angel shark, with its squat, toadlike body, big, winglike side fins and thick tail, occupies an intermediate place between the sharks and the rays. It creeps along the bottom, and is remarkably voracious. The chimæra is another queer "monster" of the deep.
The rays (order Raiæ) differ from the sharks superficially rather than in structure, where the most important difference is the position of the gill clefts, which are lateral in the sharks and ventral in the rays and skates, as the smaller members of the order are called. The majority of them have a flattened, depressed body, from which the broad, expanded pectoral fins are scarcely distinct, while the tail is usually long and slender, in one family so much so that they are known as "whip rays;" and in some a horny point at the tip is connected with a venom gland so that its pricking is poisonous, and these are called "sting rays." All the rays are carnivorous, but only the sharklike forms (sawfishes and the Rhinobatidæ) actively pursue their prey. The true rays live on the bottom and feed on shellfish and small fishes. Most of them bring forth a few young alive, but many lay eggs in squarish, oblong, leathery cases with tendrils at the corners by which they become moored to eelgrass, etc; they are frequently cast up on beaches, and go by the name of "sea purses," In the earlier stages the young ray is much like a shark, and the enormous development of the pectoral fins does not occur until nearly the time of hatching.