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CHAPTER XIV
MARINE VERTEBRATES

The vertebrates form the highest sub-kingdom of animal life—the sub-kingdom to which we ourselves belong, the chief distinguishing characteristic of the group being the presence of an internal skeleton, the principal part of which consists of a rod or column of cartilaginous or bony material running along the dorsal side of the body, known as the vertebral column.

This column is usually composed of a number of elements called vertebræ, each of which gives off two processes that unite and form an arch on its dorsal side, while all the arches form a tube through which passes the central portion of the nervous system.

Below, or on the ventral side of the column, is the body-cavity containing the organs of digestion and circulation; so that if we make a transverse section of a vertebrate animal we find that there are two distinct tubes or cavities—a neural or cerebro-spinal cavity on the dorsal side of the vertebral column, formed by extensions from the substance of the latter, and enclosing the chief portion of the nervous system; and a body-cavity on the ventral side containing the viscera or internal organs.

The above features are highly important, and will always prove quite sufficient to enable us to decide whether any particular animal is a vertebrate or an invertebrate, for it will be remembered that the body of the latter has only one cavity, containing the nervous system as well as the viscera, and that the nervous system is commonly placed along the ventral side, but never along the dorsal. In addition to this the vertebrates never have more than two pairs of limbs, and these are always directed from the nervous system; and the jaws, which are appendages that move in the horizontal plane in invertebrates, are, in the higher animals, portions of the framework of the head and move vertically. In vertebrates, too, there is always a complete blood system, consisting of a heart with two, three, or four cavities, a system of arteries to convey the blood to the different parts of the body, veins to return the blood to the heart, and networks of fine capillaries connecting the former with the latter.

All vertebrates, at an early stage of their existence, have a cartilaginous rod running through the dorsal portion of the body, called the notocord. In some of the lowest animals of the division this rod persists without any important alterations in structure, while in the higher forms it gives place to the series of cartilaginous or bony elements above referred to as the vertebræ; and the arrangement of the vertebrates into their relative positions in the scale of life is based largely on the degree of development of the vertebral column from the notocord. Another interesting feature in the development of a vertebrate is the formation of five or more transverse, archlike thickenings on each side of the digestive tube, just behind the head; and, in the spaces between them, of a series of slits forming a communication between the pharynx and the exterior. These arches and clefts have but a brief existence in many vertebrates, while in others they persist throughout life; and, like other points referred to, they assist us in recognising the relations of the vertebrates to one another.

The vertebrates are divided into the following classes:—

1. Cyclostomata—Lampreys.
2. Pisces—Fishes.
3. Amphibia—Frogs, Toads, Newts, &c.
4. Reptilia—Snakes, Lizards, Tortoises, &c.
5. Aves—Birds.
6. Mammalia—Mammals.

The first of these includes only a few species, one of which is found in our seas, and will receive a short notice here. The fishes will, of course, demand a fair share of our attention. Amphibians and reptiles have no British marine representatives, and are therefore quite excluded from this work. As to the birds, although there are so many that live entirely on the sea and in its immediate neighbourhood, these have been so ably dealt with by Mr. Hudson in one of the books of this series that it would be superfluous to mention them. The mammals include a considerable number of marine species, but as only one of these—the Porpoise—is really commonly observed round our coasts, it alone will be selected for description.

Lampreys and their few allies were formerly classified with fishes, but are now made to form a small class by themselves; and there is abundant reason for the separation. It will be remembered that vertebrates, in the early stages of their development, are characterised by a cartilaginous rod running through the dorsal region of the body, below the central cord of the nervous system, and that they possess a series of slits opening into the sides of the pharynx. Now, while these characteristics are usually only transitory in the vertebrates, the Lampreys and their relatives are the only animals in which they persist throughout life, and it is for this reason that they are exalted to the dignity of a class under the title Cyclostomata.

This name signifies ‘round-mouthed,’ while the Lampreys themselves form the still smaller division Marsipobranchii, which means ‘pouch-gilled,’ these two being among the most evident characters of the creatures concerned. They have no true jaws, the circular mouth being supported by a ring of cartilage, and provided with a rasp-like tongue that enables them to divide their food. They have no true bone in their bodies, the simple skeleton, without limbs and ribs, being entirely cartilaginous, and the rudimentary skull is not movable on the dorsal cartilage. Their bodies are elongated and eel-like, with a single medial fin, supported by fine cartilaginous rays, and with seven little slits on each side of the neck, communicating with as many gills in the form of little pouches. The mouth is suctorial, presenting, when open, a circular adhesive disc, by which the animals can attach themselves to any solid object, but assumes the form of a mere slit when closed. The young differ from the adult in a few points of structure. Thus they have no eyes, and the long fin, divided in the adult, is continuous. With the above characteristics in mind, there will be no danger of confusing the lampreys with the eels and other similar fishes.

There are three or four British lampreys, two or three of which inhabit fresh water. Their habits do not seem to be well understood, but it appears certain that the Sea Lamprey (Petromyzum marinus), which reaches a length of from one to two feet, ascends rivers to spawn, while the smaller River Lamprey (P. fluviatilis) has been caught in the sea; and it is probable that the migrations of both, together with the sojourn of the young of the former for a longer or shorter period in fresh waters, have been the cause of the widespread confusion between species.

Lampreys are carnivorous creatures, and attach themselves to fishes by their suctorial mouths, and rasp away the flesh. They have also been known to attack bathers.

Passing now to the true fishes, we must first study the general features of the group by which they are to be distinguished from other animals. Since there are so many creatures outside this class that are more or less fishlike in some respects, it becomes no easy matter to give a concise definition of a fish, and the shortest satisfactory description must necessarily include several points of structure. Thus, we may define a fish as a cold-blooded vertebrate that does not undergo metamorphoses, with limbs modified into fins, possessing also median fins on the dorsal and ventral surfaces, having distinct jaws, a heart with two chambers, and breathing by gills. To this we may add that the young are generally produced from eggs, and that the skin is covered with scales or bony plates, or is naked.

But let us now look more closely into the structure of fishes, so that we may be enabled to see how marvellously they are adapted to their aquatic life, and in order that we may become acquainted with the few technical terms which will, as a matter of convenience, be used in the descriptions of species.

Taking first the external features, we note that the body is generally covered with scales, sometimes very large and distinct, but often so small and closely set that they are not visible without careful examination; indeed they are often so small, and so thoroughly embedded in the slimy skin as not to be discovered without the aid of a microscope. When the scales have unbroken edges and overlap one another they are said to be cycloid, but when the projecting edges are toothed or serrated, giving a roughness to the touch, they are described as ctenoid. Sometimes the scales are modified into bony plates or little isolated bony granules, and in either case they are practically identical in structure with teeth, consisting as they do of dentine, capped with a little harder substance resembling enamel.

We often observe a row of scales, of a different nature from those covering the body generally, running along each side of a fish from near the eye to the end of the tail; and these constitute what is called the lateral line. If we examine these scales closely, we observe that each one is pierced by a hole that communicates with a little sac beneath containing a gelatinous material, and in which a nerve tendril terminates. The presence of the nerve filament evidently denotes that the scales in question, with the little sacs beneath them, are organs connected with sensation, and it is also believed that they have something to do with the secretion of the slimy mucus that covers the scales of the body.

The mouth of a fish is generally situated on the extreme front of the head, but occasionally, as in the sharks and rays, quite on the under side. If it contains a tongue at all, this organ is of small size and simple structure; thus it is highly probable that the sense of taste is very feeble in these animals, and this is just what one might expect when one remembers that fishes never retain their food in the mouth for any length of time, but simply bolt it without any attempt at mastication.

The arrangement and nature of the teeth are very variable. Often they are developed on the membrane of the mouth only, in which case they are generally renewed as fast as they are worn down, but sometimes they are persistent structures more or less embedded in the bone of the jaws. In some cases teeth are altogether wanting, but in others they are situated not only on the jaws, but also on the tongue, the roof of the mouth, and even on the bony arches that support the gills.

A glance at the fins of a typical fish will suffice to show that they may be divided into two groups—the paired fins, representing the two pairs of limbs in most of the higher animals, and the median fins occupying the middle line of the body. The former comprise the two pectoral fins that correspond with our arms, and are attached to the bones of the pectoral or shoulder girdle; and the pelvic fins, corresponding with the lower extremities. The pectorals, too, are present in nearly all fishes, while the pelvic pair are rather more frequently absent than the pectorals.

The medial fins comprise the dorsal, the ventral, and the caudal or tail-fin, and are not to be regarded as limbs, but rather mere outgrowths of the skin. They are not directly connected with any part of the main bony framework of the body, though they are generally jointed with a series of bones (interspinal bones) that run between processes of the vertebral column. The dorsal and ventral fins are often divided into two or more parts, and the tail fin is commonly distinctly forked.

Although the seven fins above mentioned differ considerably in general form, some being fanlike, while others form fringe-like expansions, yet they generally agree in that they consist of bony or cartilaginous rays, between which is a soft membrane. The rays, however, vary much in character, being sometimes developed into very hard and sharp spines, and sometimes quite soft and flexible. The fins also differ in function, as will be seen when we observe the movements of a fish as it swims. It will then be noticed that the caudal fin, which is spread in the vertical plane and moved sharply from side to side by the powerful muscles of the tail, is the chief propelling organ, while the others are concerned principally in maintaining the balance of the body. This latter point becomes much more evident when we observe the movements of a fish in which one or more of the fins have been injured or lost, as we shall see presently.

It is true that the pectoral fins are sometimes used to propel, but forward movement is brought about almost entirely by the caudal fin, which acts much in the same way as the blade of the propelling ‘screw’ of a steam-vessel, the pectorals being used at the same time for steering. Occasionally, too, the latter are both spread out at right angles to the body when the fish desires to stop suddenly, and are even employed at times in swimming backwards.

When a fish wants to turn to one side, it will be seen to give the tail a sharp motion to the opposite side. The pectoral of the latter side is also brought into play, while the other is kept close against the body.

If the pectoral or pelvic fin of one side is injured, the body of the fish will incline to the opposite side; and if all the paired fins are functionless the fish swims with its head inclined downwards. Observations of fishes in which the dorsal or ventral fins are injured will also show that these organs are necessary to maintain a steady motion in the water.

In addition to the above facts, it may be mentioned here that the paired fins are often modified into long finger-like processes that serve as organs of touch, and even as means by which the fish can creep along the bottom. This is notably the case with gurnards and a few of the other fishes that spend their time almost exclusively on the bed of the sea.

Fishes are essentially gill-breathers, the gills being generally fringe-like organs, supported on bony arches (the gill arches), numbering four on each side, the cavity containing them being covered by a gill-cover (operculum) that opens behind. Water is taken in at the mouth, whence it passes into the gill-chamber; and after passing between and around the gills, it escapes under the opercula. The gills themselves are richly supplied with bloodvessels that are distributed close to the surface, and an exchange of gases takes place through their exceedingly thin walls, carbonic acid gas passing from the blood to the surrounding water, and oxygen, held in solution in the water, passing from the water to the blood.

When fishes are in foul water, containing but little oxygen in solution, they rise to the surface in order to make up the deficiency by taking oxygen direct from the air. This, however, is an unnatural proceeding with the majority of fishes; but there are some that are provided with accessory breathing organs specially adapted to the extraction of oxygen direct from the air, and these are so dependent on the supply from this source that they are suffocated if prevented from reaching the surface.

In other fishes, such as the sharks and rays, the gills are of an entirely different character from those described above, for they are pouch-like and five in number on each side, each pouch communicating with the pharynx as well as with the exterior by a slit-like opening.

Before leaving the external characters of fishes we must say a word or two about their forms and colours. As regards the former, it is well known that fishes are well adapted for rapid progression through water, but there are many exceptions to this rule. These exceptions, however, apply principally to those species that have no need to swim rapidly, and a study of their habits will show that their form is just as perfectly adapted to their mode of life. They are often species that live on the bottom, or hide in the crevices and holes of rocks, and examples will be given in our future descriptions.

Variations in colour are even more interesting, especially as they are so commonly connected with the nature of the surroundings and the protection of the animals. In nearly all cases the colour is darker on the upper surface than on the lower, thus making it appear that the influence of light has something to do with the formation of the pigments of the skin, and experiment proves that this is, at least to a certain extent, the case; for when fishes have been kept for some time in an aquarium into which light is admitted through the bottom only, pigment spots have formed in the skin on the lower surface.

Fishes that swim at the surface are generally tinted on the dorsal side with some shade that closely resembles the colour of the water as viewed from above, and are white and silvery below. Such colouring is of course highly protective, for they are not readily distinguished by the sea birds and other enemies that would pounce on them from above, and are almost invisible against the sky to eyes below. This form of protective resemblance is beautifully illustrated in the mackerel, which is barred on the back with black and green, closely imitating the ripples on the surface of the deep green sea, while the under side is of a silvery whiteness that is hardly visible from below with the bright sky as a background.

The flat fish afford other interesting examples, for these live on the bottom, and are coloured above so as to resemble the bed on which they live; the tints being those of mud, sand, or gravel.

But what are we to say of the gaudy colours of the gurnards, rock fishes, &c.? These are certainly not protective in all cases, for we sometimes find brightly coloured species conspicuous among duller surroundings. Such instances, however, are comparatively rare, the gaudy species living principally among the variously coloured rocks, weeds, and corals; and when they do occur it is probable that they serve principally as a means by which the brightly coloured sex—usually the male—attracts its mate. We say ‘usually the male,’ but why so? Because the female requires the protection of a more sombre colour in order that she may with safety deposit her spawn for the perpetuation of her species. Again, the male referred to needs the assistance of his gaudy coat only during the breeding season, hence we find that he assumes the bright colours as a wedding garment, to be cast off when the breeding season is over.

This leads us to the subject of changeability of colours in the same individual. That such changes do occur is well known, and it is still more remarkable that they are produced in rapid succession, apparently at the will of the fish concerned; for its tints will vary as it moves from place to place so as to always harmonise with the surroundings, and also in response to other conditions. The mechanism by which such variations are produced has also been studied and explained:—The colouring matter is held in little vesicles beneath the skin, and these vesicles are capable of being compressed by muscles quite under the control of the fish. When they are globular in form the contained pigment appears dark, but when they are flattened by muscular compression, the pigment is spread over a much larger area, and thus greatly reduced in depth of tint.

As with all vertebrates, the central axis of the internal skeleton of a fish consists of the backbone and the skull. The structure of the latter is so complicated, and its description so full of technicalities, that we deem it advisable to pass it over in a work like this where the scope is so large in proportion to the space available; and this we do with reluctance, because the detailed study of the skull is of real importance to those who would thoroughly understand the principles of classification.

The backbone consists of a variable number of cylindrical vertebræ, united end to end to form a continuous column, both the anterior and posterior faces of each being concave. On the dorsal surface of each vertebra there is a V-shaped arch, surmounted by a spine, the former serving to protect the spinal cord, and the latter giving attachment to the muscles of the back. Some of the vertebræ are also provided with processes for the attachment of the ribs, and those of the tail possess an arch and a spine on the ventral as well as on the dorsal side.

It has already been shown that the pectoral fins are jointed to a girdle. This girdle corresponds with the shoulder-blade of higher animals, and gives direct attachment to the rays of the fin, which may be regarded as the equivalent of the fingers, and thus there is no part of the limb corresponding with the arm. The pelvic fins also are frequently jointed to a pelvic girdle or hip, but this is a very rudimental structure, or is even entirely absent in some species.

The rays of the caudal fin articulate with the extremity of the backbone, but this portion of the fish’s anatomy undergoes such remarkable changes that we must devote a few words to it. It is probably well known to our readers that the tails of fishes exhibit three distinct forms. The first of these is a simple fringe formed by the union of unaltered dorsal and ventral fins; the second is the unsymmetrical or unequally lobed tail so characteristic of sharks, dogfishes, and rays; and the third is the broad symmetrical tail fin, often distinctly forked or bi-lobed, such as we meet with in the majority of our bony fishes. These three kinds are known respectively as the diphycercal, heterocercal, and the homocercal tails.

Now, it is an interesting fact that the most ancient fishes of our globe possessed tails of the first type; and that these gradually gave place to the heterocercal form; while the higher fishes of the present day nearly all possess the homocercal tail. Thus as time advanced the heterocercal tail was gradually evolved from the diphycercal, and the homocercal from the heterocercal.

Further, if we watch the development of one of the highest fishes of the present day from its embryo, we find that similar changes take place in the individual. At first its tail is a simple fringe round the extremity of the backbone, the latter being straight, or nearly so, to the end, so that the embryo fish, as yet still in the egg, reflects a characteristic of its very early ancestors. Then the end of the vertebral column turns upward, and strong fin-rays are developed on its ventral side, so that the tail becomes a heterocercal one like that of the less remote ancestors of a later geological period. Next, the upward-bending portion of the vertebral column is slowly absorbed, till nothing of it remains except a small upturned bony spine, while, at the same time, the ventral lobe expands on the upper side until the tail fin is once more of a symmetrical form.

Following these interesting changes, it becomes evident that the symmetry of the tail fin of the bony fishes is really a false one, the whole of it having been formed from the ventral lobe of a heterocercal tail; and although the backbone seems to terminate abruptly exactly opposite the middle of the fin, it still contains the remnant of the raised extremity of the backbone that ran to the tip of the dorsal lobe when the tail was of the heterocercal type.

The flesh or muscle of fishes is usually white, but it often assumes a pink colour in the case of those fishes that feed largely on crustaceans. This is due to the presence of a substance in the horny or calcareous skins of the crustaceans that is turned red by the action of the digestive fluids—the same substance that is turned red when the crustaceans are boiled. This is notably the case with the salmon; but the red pigment thus derived originally from the crustaceans frequently shows itself more in the skin of the fish than in the flesh, as observed in the common red gurnard.

Most fishes possess a membranous bag containing air, situated just below the backbone, and known as the air-bladder; but this organ does not exist in sharks and rays and in some of the heavier bony fishes that live on the bottom. The air-bladder is capable of being compressed by the action of certain muscles, and its principal use seems to be the adjustment of the specific gravity of the fish to that of the surrounding water; but it is interesting to note that the development of this air-bladder is precisely the same as that of the lungs of air-breathing animals, and that in some fishes which live in foul muddy waters it is really a functional lung by means of which the fishes can breathe direct from the atmosphere.

We can find space to refer only to one other internal structure of the fish, namely, the roe of the female. This usually consists of a very large number of eggs of small size, sometimes numbering many thousands, and even millions, in a single individual. So numerous, indeed, are the eggs, that were it not for the multitudes of carnivorous animals that devour both eggs and fry, the sea and fresh-water lakes and rivers would soon become so thickly populated that the fish would die in millions for lack of food and air.

In some cases, however, the eggs are much larger and fewer in number, but these are generally protected from the ravages of predaceous species by a hard covering, as we shall observe in the sharks and rays.

Finally, a word or two must be said about the distribution of fishes. We have already referred briefly to species that live principally at the surface, and others that make the bottom their home: but some of the former go to the bottom for food or to deposit their spawn, while some of the latter occasionally rise to the surface and swim in shoals. We have noticed, too, that the paired fins of bottom fishes are sometimes modified into feelers, or into fingerlike processes adapted for creeping. Similar organs, employed undoubtedly as organs of touch, and called barbels or barbules, are often developed on the chins or jaws of these fishes.

Although we have to deal principally with the species that belong more or less to the shore—the littoral fishes—we should like to refer briefly to one or two interesting features of those that live at great depths. It will be readily understood that much light is lost as the rays penetrate into deep water, so that the bottoms of deep seas must be more or less darkened. To allow for this loss, we find that the species living at moderate depths are provided with larger eyes to enable them to see their prey and their mates; but at still greater depths, where the sun’s light cannot penetrate, the fishes are either blind, or are possessed of luminous organs which enable them to see their way. Again, as the sea is so thinly populated at such great depths, the carnivorous species do not find abundant food always at hand, hence they are often provided with such mouths and stomachs as will allow them to make the best of favourable opportunities, some being capable of swallowing a fish quite as large as themselves.

We often find fishes roughly classified into fresh-water and salt-water species, and although such a division is at times convenient, it must be remembered that some of the former migrate into brackish and even into salt water, while some of the latter ascend estuaries and rivers either for the purpose of obtaining suitable food, or for the deposition of their eggs.

The fishes that frequent our coasts may be classified into two main groups, those with cartilaginous skeletons (Elasmobranchii), and the bony fishes (Teleostomi). Both these are divided into family groups, and we shall deal more or less briefly with all the important families that include common British marine fishes, but giving more attention to those species that are truly littoral in habit—species that may be found in the rock pools or under stones at low tide, and which may be obtained by the amateur angler working from rocks, piers, &c.

The cartilaginous fishes include the Sharks, Dogfishes, and Rays. They have pouchlike gills, five or more on each side, each one opening to the exterior by a separate slit. The skin generally contains bony elements that are toothlike in structure and often in form; the mouth is usually on the under side of the head, and the tail is nearly always of the heterocercal kind. They are all carnivorous creatures, and often exceedingly voracious; and are represented in our seas by the Rays and Dogfishes.

Rays or Skates (family Raiidæ), of which there are six or seven British species, are readily known by their broad flattened rhomboidal bodies, with the mouth on the under side of the head, a longitudinal fold on each side of the tail, and pectoral fins extending quite or nearly to the front point of the head.

Two of these fishes are very common in our markets, one being the Thornback Skate (Raia clavata), distinguished by the clawlike spines down the middle of the back as well as on other parts of the body; and the Common Skate (R. vulgaris), a very voracious species, from two to four feet long, with a very sharp muzzle.

All the members of this family are bottom fish, without air-bladders; and their eggs, which are large and detached, are enclosed in horn capsules which are so commonly washed up on the beach that they are well known to frequenters of the sea-side, who call them Skates’ Barrows or Shepherds’ Purses. These cases are oblong in form, with a process at each corner, and the material of which they are composed looks very much like that of some of the coarser sea weeds after they have been dried in the sun. As a rule only the empty cases are cast ashore by the waves, open at the end where the little skate made its escape; but occasionally we meet with the complete egg, and the case, while still wet, is sometimes sufficiently transparent to show the form of the embryo within.

Dogfishes are also fairly well known to sea-side ramblers, for not only are some species used as food in many places, but they are also frequently to be seen cast aside with the refuse from the fishermen’s nets. The common Spiny Dogfish (Acanthias vulgaris), belonging to the family Spinacidæ, frequents all parts of our coasts. It reaches a length of three or four feet, and is of a slate-blue colour above and very pale yellow below. The pectoral fins are very large, the ventral fin absent, and there is a very sharp spine in front of each dorsal. The creature is ovo-viviparous; that is, the eggs are hatched while still within the body of the parent.

Another family (Scylliidæ) contains two British species without spines, and is also characterised by having the first dorsal fin far behind. They are the Larger Spotted Dogfish (Scyllium canicula) also known as the Nurse Dog and the Bull Huss; and the Lesser Spotted Dogfish (S. catulus), called also the Huss and the Rough Hound. The egg capsules of both these are occasionally washed on the beach, and those of the latter species may be known by the yellowish colour and the long tendrils by which they are anchored to sea weeds.

In addition to these we may briefly refer to two of the Blue Sharks (family Carchariidæ) that frequent our shores, distinguished by their long and prominent muzzle, and the crescent-shaped mouth. They may be regarded as higher in the scale of fish life, as compared with the sharks and rays previously named, because the vertebræ are more or less hardened by the deposit of calcareous matter, and, therefore, make a nearer approach to the character of true bone. The species referred to are the Common Blue Shark (Carcharius glaucus), and the Smooth Hound (Mustelus lævis). The former often exceeds twelve feet in length, and is commonly seen off our south and west coasts during the summer months. It is a nocturnal marauder, and is said to sleep at the surface by day with its tail exposed above the water. The Smooth Hound is a bottom feeder, subsisting on molluscs and crustaceans, the shells of which are easily crushed by its flat and blunt teeth. It is a small shark, measuring only three or four feet in length, and brings forth its young alive.

The next division (Teleostomi) contains all the bony fishes, which may be distinguished generally from the cartilaginous group by the following features:—The skeleton is more or less hardened by the deposit of calcareous matter, and the tail is generally not of the heterocercal type. The paired fins are fan-like, and the pectoral girdle is attached to the hinder part of the skull. These fishes generally have an air-bladder, and the gills lie close together in a cavity covered by an operculum. The eggs, too, are generally very small and numerous, and massed together.

Of these we will take first the family Salmonidæ, of which the Salmon (Salmo salar), and the Smelt (Osmerus eperlanus) are well-known examples. Several species of the family are remarkable for their periodical migrations from fresh to salt water or vice versa, and we cannot do better than briefly relate the interesting life-history of the salmon as a striking instance of these peculiar wanderings. This fish quits the sea at the close of the summer, and ascends the rivers for the purpose of depositing its spawn, the colder water of the rivers being necessary for the development of the young. Its upward journey is beset with many difficulties, for it has to shoot the various rapids and leap the cascades, the latter often demanding the most prodigious efforts on the part of the fish, which frequently leaps several feet out of the water, and even then has sometimes to renew its attempts over and over again before it finally succeeds. Indeed, the difficulties to be overcome are so numerous that the fish often reaches the goal in such an exhausted condition that it would hardly be recognised as the salmon by those who have only seen it in the prime condition in which it is captured during its return to the sea in the following spring or summer. The male, at this period called the kipper, is of a dull red colour, irregularly blotched with yellow and light brown, and its skin is covered with a slimy secretion. Its body is lean, and the head, now large and out of all proportion, is rendered still more unsightly by the protrusion of the lower jaw, which at this season, when the males are particularly pugnacious, becomes a formidable weapon of offence. The condition of the female, now called the baggit, is equally poor, and the skin has changed its bright silvery colour for dark and dingy shades.

The female digs a nest in the form of a deep trench by wriggling her body in the gravel of the bed of the stream, and there deposits her eggs, many thousands in number, small quantities at a time. As each batch is deposited the eggs are fecundated by the kipper, and then covered over lightly with gravel by the baggit; and this work having been accomplished, both male and female rest and feed, with the result that their condition is rapidly improved.

After about eighteen weeks the eggs begin to hatch, and the fry wriggle out of the nest and seek shelter under stones in the immediate neighbourhood. They are now peculiar little creatures, as much like tadpoles as fishes, with big heads and narrow bodies, and a bag of albuminous yolk-matter attached to the ventral side. The young subsist on this store of food for from twelve to twenty days, during the whole of which time they remain under shelter, having, of course, no need to expose themselves to the numerous enemies with which they are surrounded, and they then leave their hiding-place in search of food, being now about an inch in length. They feed on aquatic and other insects, which are now becoming plentiful on the approach of the warm weather; and, growing rapidly, reach a length of four inches in a month or two. They are now called parr, and are distinguished by the dark bars that cross their bodies transversely—a feature that persists for a year or more from this time.

Towards the end of May the parr migrate seawards, accompanied by the adult salmon, but as their enemies include the voracious fishes, wading birds, and even the adults of their own species, it is probable that only a small proportion of the original number ever enter salt water.

In the sea they feed on crustaceans, molluscs, and small fishes, the young still growing rapidly, and attaining a weight of about five pounds in the following autumn, when both young (now called grilse) and old again ascend the rivers to spend the colder half of the year; the former will have reached a weight of ten pounds or more on their return to the sea in the following year.

The Smelt may be seen in thousands in our estuaries during the spring, for at that time they come up to spawn in the brackish water. In the summer they swim about in shoals along the coast, and are caught largely in nets for the market. In some parts they are taken in large shallow circular nets suspended on a line. This is lowered into the water, and hauled up when the fish are seen swimming above it. Many amateurs secure numbers of smelt by means of rod and line, fishing from piers, jetties, &c. They bite freely at almost any kind of bait, and will snap at an almost bare hook, with the tiniest fragment of the bait at its point.

The Herring family (Clupeidæ) contains some well-known food-fishes to which we need only casually refer. They are mostly littoral species, none inhabiting deep water, and none straying into the open ocean. Their bodies are covered with silvery scales, and are laterally compressed, so much so on the ventral side that there is a moderately sharp ridge along the middle line. The principal fishes of the family are the Herring (Clupea harengus), the Sprat (C. sprattus), and the Pilchard (C. pilchardus).

These fishes are particularly interesting on account of their gregarious habits and the enormous size of the shoals they form, a single shoal often containing millions of individuals; and they are often captured in such quantities that large numbers are sold to farmers as manure to enrich the soil. The shoals are followed closely by many larger carnivorous species that devour them in great numbers, as well as by flocks of sea birds that prey on them, and yet their numbers are not appreciably reduced by such ravages. They spawn in shallow waters near the coast, and feed principally on the crustaceans and worms of the littoral zone.

Sprats were once considered to be the young of the Herring, but it is now universally acknowledged that they are a distinct species, and quite a number of characteristics have been given as a means of distinguishing between the two. The young of the herring are, however, used largely as food, for that miscellaneous mixture of fry and small species known as Whitebait consists largely of these and the young of the sprat.

Herrings are captured principally off the north and east coasts, but the pilchards, which are often confused with them, and even at times sold under the same name, are caught chiefly off the coast of Cornwall.

Although the Eels (Anguillidæ) are so readily distinguished by their general form and appearance, yet it may be advisable to call attention to one or two of the leading characters that would possibly be overlooked by an ordinary observer, and in doing this we ask the reader to note that our remarks apply to the true eels only, and not to the sand eels and other fish that may be confused with them.

The elongated bodies of the Anguillidæ are covered with a slimy skin that is apparently scaleless, but an examination with the microscope will show that there are small scales embedded in it. The dorsal and ventral fins extend to the tail, and the pelvics are absent; the gill-slits, which are very narrow, are at the base of the pectorals.

It might well be expected that eels would be possessed of some form of accessory breathing apparatus, seeing that they can live so long out of water, but this is not the case. They have, however, a pouch-like gill-cavity which can be inflated and filled with water by the fish, thus keeping the gills moist and functional. In most other fishes the gill-chamber is not capable of holding water, and thus the gills soon become dry and sticky, so that they adhere together and fail to absorb the necessary oxygen when the fish is out of water.

Thus the Eel (Anguilla vulgaris), in the remarkable migrations for which it is noted, is capable of travelling over dry land for considerable distances in search of suitable homes.

If an eel be taken out of the water, these gill-pouches will be seen to swell out almost immediately, and remain filled with water as long as the fish is kept on land; but when it is returned to its natural element, it will at once discharge the water that kept its gills moist, and which has become foul with the products of respiration, and, with a few vigorous gulps, renew the supply.

Eels spend their breeding season, which extends from July to September, in salt or brackish waters; and early in the following summer, the young, which are now called elvers, and measure from three to five inches in length, ascend the rivers, travelling enormous distances and overcoming obstacles that we might well expect to be insurmountable. Thus they perform two migrations annually, though it is thought by some observers that the adult never returns to the sea, but dies soon after it has deposited its spawn.

The family of Flat-fishes (Pleuronectidæ) present many interesting points of structure and habit in which they stand alone, the variations in structure as compared with other fishes being due, of course, to the habits which they have acquired.

One of the first features that strike the observer on looking at a flat-fish is the unsymmetrical form of the body. It is very much compressed, and the fish having acquired the habit of lying on the bed of the sea, sometimes on the left and sometimes on the right side, the lower surface has become flattened more, and is of an almost pure white colour, while the upper convex side is more or less coloured with pigment produced by exposure to light. The dorsal and ventral fins are both very long; and, as is usual with bottom fishes, the swimming or air bladder is absent.

Young flat-fish are at first perfectly symmetrical in form, with one eye on each side of the head, and they swim freely in the water with their bodies in a vertical plane; but they very soon acquire the habit of swimming on one side, and the eye of that side slowly passes round to the other side of the skull, rotating in its orbit as it moves, till at last both are on the uppermost surface. This, of course, is accompanied by a considerable distortion of the bones of the skull, which is very evident in the skeleton of the adult. The young fish then takes to the bottom, with the result that its under-surface is flattened, while the upper becomes strongly pigmented.

These fish spend almost the whole of their time on the bottom, only occasionally rising for short intervals, when they swim by undulatory movements of their bodies and fins; their food consists of crustaceans, worms, and other small marine animals.

They furnish very interesting illustrations of protective colouring, the upper surface always closely resembling the ground on which they rest and feed; and thus they are not only protected from their own enemies, but are enabled to lie unseen by the animals that form their prey. Those which live on sandy shores are finely spotted with colours that closely imitate the sand, while those that lie on mud are of dark and dingy hues. Others, again, are irregularly marked with spots of various sizes and colours that resemble a gravelly bottom; and most species are still further protected by their habit of throwing sand or mud on the top of their bodies by means of their dorsal and ventral fins.

Small flat-fishes, especially young Plaice and Flounders, live so close to the shore that they are often left behind in rock pools and sandy hollows by the receding tide, and it is very interesting to observe the habits of these in their natural conditions. It will generally be noticed that it is most difficult to detect them while they are at rest; and when disturbed, they usually swim but a short distance, settling down very abruptly, and immediately throwing a little sand over their bodies by a few vibrations of their fins.

Another peculiarity of some of the flat-fishes is their indifference to the nature of the water in which they live. Flounders may not only be caught in the estuaries of our rivers, but they even ascend to, and apparently live perpetually in, perfectly fresh water. In many instances they may be seen miles from the sea, and even flourishing in little fresh-water streams only a few feet in width. Thus they may be found in numbers in the upper waters of the small rivers of the Isle of Wight and of many streams of the mainland.

The principal British flat-fishes are the Plaice (Pleuronectes platessa) and Flounder (P. flexus) above mentioned, and also the Sole (Solea vulgaris), the Lemon Sole (S. aurantiaca), the Turbot (Rhombus maximus), and the Halibut (Hippoglossus vulgaris); and as all these are well-known food-fishes it is hardly necessary to describe them.