Fish are divided into two sub-classes in one of which are placed all fish with cartilaginous skeletons and in the other those with skeletons of bone. The sharks, dogfish, skates and rays are cartilaginous fish, while the class of bony fish, which are the prevalent type of to-day, includes the herring, haddock, salmon, cod, roach and perch. 72
Fishes show many structural features which enable them to make a sucoess of life in water, and one of the most important of these is the shape of the body. An actively swimming fish, such as the herring, has a streamlined shape which allows the fish to move through the water with the least frictional resistance. Such a fish is oval in vertical section and has no neck, for the head passes insensibly into the body so that the torpedo-like outline of the body is unbroken.
Fishes with bony skeletons, such as herring and trout, have a covering of thin scales which overlap so that the free ends point backwards. A delicate, slimy skin overlies the scales. Such fish are thus covered by a thin armour of bone, which protects the body and yet does not hamper movement. In most fish the free edge of the scale is rounded, but the perch, a fresh-water fish, has scales with a serrated, comb-like edge. In many fish the scales are marked with concentric rings, which represent periods of growth in much the same way as do therings seen when a tree trunk is cut through. The rings often enable the age of the fish to be calculated. The scales of sharks and other cartilaginous fish are more strongly made, and form a tough spiny covering for the body, which is made into the leather known as shagreen. Each scale consists of a base of bone-like substance which is sunk into the skin and from which a point of dentine projects. The point is tipped with enamel, and the whole structure is known as a placoid scale. Examination of the tooth of a mammal shows that it has essentially the same structure as a placoid scale. The scales are continued over the edges of the jaws, where they are large and function as teeth.
The characteristic feature which distinguishes fish from other vertebrates is the possession of fins, which are flat outgrowths of the body having an internal skeleton. In bony fish the proximal part of the fin is supported by many rods of bone, and the distal part consists of many horny fibres. In cartilaginous fish small plates of cartilage take the place of the bony rods. A fish bears fins of two types, namely median unpaired fins and paired fins. A typical arrangement of these fins can be seen in the goldfish. This fish has a long median dorsal fin running down the middle of the back, a median ventral fin on the ventral side behind the anus, and a bilobed caudal fin radiating from the tail. There are two sets of paired fins, the first pair of which lie towards the ventral side of the body behind the gill flap, and are known as the pectoral fins. The second pair, or pelvic fins, occur behind the pectoral fins about half-way along the body. There is, however, considerable variation among fishes from this typical arrangement, both in the number andof the fins. The perch has two median dorsal fins, which is the commonest number, while the cod has three. In the cod the pelvic fins are found beneath the pectorals, and in the whiting actually occur in front of the pectorals. The flying fish has broad and wing-like pectoral fins, which by acting as planes enable the fish to glide through the air for distances of 100 to 200 yds.
The rapid swimming of such fish as the herring or goldfish is carried out by the side-to-side movements of the tail and caudal fins. In such fish the muscular part of the body, to which 3A the name tail is given, lies behind the anus, and may be nearly half the total length of the body. The tail is first gently curved to one side and then straightened suddenly, so that the resistance offered by the water causes the fish to be moved forward. When the tail is curved, the head is pointed slightly in the opposite direction. This ensures that the fish shall swim in a straight line and not in a series of curves. The caudal fin is folded up as much as possible during the curving movement of the tail, and so the tendency for the fish to be dragged backwards by this movement is reduced. When the tail is straightened, however, the fin is fully expanded in order to give the maximum pushing effect.
The pectoral and pelvic fins can be moved independently, and are used for steering and balancing. When the tail is still the paired fins are used as oars for slow forward movement, but during rapid movement they are folded down closely to the body. The median fins act in the same way as the keel of a ship, helping to stabilize the fish and keep it on a straight course. When at rest the fish may fan its pectoral fins slowly backwards and forwards in order to create a current of water for breathing purposes.
The eel has become adapted to a mode of life quite different from that of the swift-swimming type of fish. Its long cylindrical body is suited for burrowing in the mud at the bottom of a river or creeping in and out of the crevices between stones. It is further aided in this by the slippery nature of its skin, caused by the copious secretion of slime. The eel has no pelvic fins, but there is a continuous fin running along the upper and lower sides of the body, formed by the joining of the dorsal, caudal and anal fins. The eel swims by wave-like movements of its whole body. The fresh-water eel has minute scales embedded in its skin, but the marine conger eel has no scales.
The Skate and Plaice
In those fish which lie in wait for their prey on the sea bottom the body is flattened and not suitable for rapid movement. The skate and ray are flattened from above downwards and lie on the ventral surface, but the flattening occurs from side to side in the plaice and others which lie on one side of the body. The plaice, sole, halibut and dab lie on the left side, while the turbot lies on the right. The flat fish of the plaice and turbot type begin life as small fish shaped like the herring, and which swim in the usual position. Gradually the small flat fish changes its habits, for it now seeks its food on the bottom and begins to lie on its side. The eye on the under side, which would be useless in such a position, slowly passes over to the exposed side of the head, which thus acquires a distorted appearance. The fins and gill cover of the under side, however, remain in the normal position.
Both the skate and plaice can swim, but the fins form the main propelling organ and the tail is not used. In both types of fiat fish fins are continuous along the edges of the body, and swimming is carried out by wave-like movements which pass along the fins. Although the appearance and movement of the fins in the skate and plaice are similar, the fins are not comparable. In the skate they represent the much extended pectorals, while in the plaice they are the much lengthened dorsal and anal fins. The straight and narrow tail of the skate is useless for swimming, and in the sting-rays has become a weapon which may be 6 ft. long and carry a serrated spine 6 in. long.
On either side of the head of a bony fish is a semicircular flap called the gill cover or operculum, beneath which lie the five gill slits in the wall of the pharynx. The slits are separated by four narrow partitions, the gill arches, which are supported internally by small rods of bone. The gill filaments are delicate pink thread-like structures which project into the space beneath the operculum from the sides of the gill arches. Each arch bears a longitudinal row of filaments on each side, so that the filaments appear to be arranged in double rows. On the surface of each filament there are many plate-like projections which are richly supplied with a network of blood capillaries, so that gaseous exchange can easily take place between the blood within the filaments and the water which flows over them. The fish breathes by taking in water through its mouth and passing it through the gill slits and then out beneath the operculum. Cartilaginous fish can be distinguished from bony fish by the fact that they have no operculum, and consequently the five or more gill slits are visible at the surface on either side behind the head. The gill filaments are not free at their ends as in bony fish, but are attached along their whole length to the gill arches. Cartilaginous fish also possess a small hole, called the spiracle, behind each eye. This hole is a vestigial gill slit. At rest these fish breathe in through the spiracle instead of through the mouth, and the entrance to the spiracle can be closed by a small flap-like valve. In the skates and rays the spiracle lies in the dorsal surface of the body and the gill slits and mouth lie on the ventral side. It is owing to this arrangement that the gills do not become blocked with sand when the fish is lying on the sea bottom.
The eel has narrow gill openings protected by an operculum lying close to the pectoral fins. Within each gill slit is a cavity in which the gill filaments lie. The eel can live for quite a long time out of water because it can retain enough water in these pockets to keep the filaments covered. As a consequence the eel can wriggle considerable distances across land from rivers toor other streams.
The Feeding of Fish
The mouth of a shark or other cartilaginous fish is a curved opening on the under side of the body, some distance behind the tip of the snout. It is connected with the olfactory organs by a pair of grooves. In bony fish the mouth is terminal, and as in all fish has a very wide gape when fully opened. Thehabits of fish are very varied, and only a short reference can be made to them here. Many fish on smaller fish and have sharp-pointed teeth which are not used for chewing, but help to prevent the escape of the slippery prey. Shellfish form the food of the cod, plaice and skate, while some rays have teeth in the form of a pavement of flat grinders for crushing up this type of food. Mullet have no teeth and on the sea floor, swallowing mud and worms and the decaying remains of plants and animals which have sunk to the bottom. The adult herring is not very particular as to diet, and will even eat young herrings, but feeds mainly on small crustaceans and other minute animals and plants.
Where the gill slits open into the pharynx of a bony fish there are small spines along the edge of the gill arches. These are called gill rakers, and serve to protect the delicate gills from food particles which might pass out through the gill slits. Gill rakers are well developed in such fish as the turbot, which feeds on hard-shelled molluscs. In the herring the gill rakers are long, densely packed bristles forming a sieve which prevents food passing into the gill slits.
The Colour of Fish
Fish which inhabit the surface waters of river or sea are silvery-white on their under surface and sides, with a darker covering above. This light colouring of the under side tends to make the fish invisible to enemies swimming beneath it, for the surface layer of the water with the light falling on it from above has a shining, mirror-like appearance, and the fish is difficult to see against such a background. The colour of the upper parts of the fish gives it protection from sea birds flying above the surface, since the fish harmonizes with the dark appearance of the water. Many flat fish, such as the flounder, turbot and plaice, are able to change in colour until they match the sea floor on which they are lying, so that they are extremely hard to detect. A plaice lying on a sandy background has a brownish appearance, but if placed on a background of small stones it assumes a grey, motded appearance, and becomes almost invisible in a surprisingly short time. The colour changes are due to the alteration of pigment cells in the skin, and this is caused in some way by the light falling on the eye, for a blind fish does not change in colour.
The Sense Organs
The eyes of a bony fish are large and have no eyelids, but are covered by transparent skin. Together they have an almost complete range of vision. In sharks and dogfish ther are two thick folds of skin which act as eyelids.
The olfactory organs are a pair of hollow chambers in the snout, each of which, in bony fish, opens by a pair of holes on the dorsal surface. The anterior of these holes is provided with a valve. In cartilaginous fish these chambers open on the under side of the snout, and are connected with the mouth by grooves. We find it difficult to understand smelling under water, but by smelling we mean the power to distinguish between different substances in solution, and this is the use to which the olfactory organs of a fish are put. Our own olfactory organs are only affected by substances in solution, I.e. a substance can only be smelt after its particles have dissolved in the mucus of the sensitive layer of the nose. A shark which has its eyes covered can smell out its food from a distance of several yards. Some fish have olfactory pits on the head or even all over the body.
The ears of a fish are not visible from the outside, and are used in helping the fish to maintain its balance, but whether fish can hear or not is doubtful.
The Lateral Line
Along the sides of a fish there is a well-marked line, termed the lateral line, running from behind the head to the tail and marking the position of a canal running beneath the skin. This lateral line canal contains sensory cells and opens to the exterior by pores. It probably acts as a sense organ which enables the fish to perceive vibrations in the water, and to distinguish between differences of pressure. In cartilaginous fish the same function is served by a number of unbranched canals arranged inon the head and front part of the body. The canals are filled with jelly and end in swellings which are connected with nerve endings.
The Swim Bladder
Many bony fish have an organ known as the swim bladder lying in the abdominal cavity beneath the backbone. It is a long narrow sac which in many fishes opens into the dorsal side of the oesophagus by a short tube. The swim bladder contains gas and its size can be adjusted to some extent, so that the specific gravity of the fish can be altered. The specific gravity of the fish must rise or fall owing to the increase or decrease of the pressure of the water as the fish descends or ascends, and it can be made approximately the same as that of the water by an increase or decrease of the amount of gas present in the swim bladder.
The swim bladder also seems capable of acting as a kind of extra breathing organ in some fish, for it has been found that if a perch is placed in stagnant water the oxygen in the swim bladder is absorbed and replaced by nitrogen and carbon dioxide.
A few fishes bring forth their young alive, but the majority shed the eggs and spermatozoa in the water, where fertilization takes place. The eggs of most of our food fishes are about the same density as the sea water, and drift about at all depths between the bottom and the surface. At the breeding season the fish travel to the spawning grounds, but after spawning they disperse, for most fish show no parental care for their young. The helpless young are the easy prey ofenemies ; in fact the parents themselves, should they meet their offspring, are liable to eat them. To meet this heavy mortality the eggs are produced in tremendous numbers, so that a few young fish escape being eaten, although they are not necessarily cleverer or stronger than their fellows. A single cod, for instance, lays 5 to 6 million eggs, while a herring produces as many as 30 to 60 million eggs.
The eggs of the salmon and herring do not drift about in the water as is the case with most fish, but are laid on the bottom. Salmon make their way up rivers to gravelly shallows, where the female lays the eggs in a hollow made by flapping with her tail. The male sheds the spermatozoa over the eggs and covers them with gravel. The eggs of the herring are laid on the sea floor where it is stony, and, because they are sticky, become clumped together in the spaces between the stones.
The stickleback furnishes an example of a fish which takes care of the eggs until they hatch, and then looks after the young until they are old enough to fend for themselves. The male builds a nest out of pieces of water weed which are stuck together by mucus secreted by the kidneys. The nest is barrel-shaped with an opening at both ends, and the male causes one or more females to enter and lay their eggs in it. After fertilizing the eggs the male closes up the nest and guards it until the young fish hatch out. Then he protects them, in particular from the females, and catches food for them until they can do this for themselves. The horny black capsules known as mermaids ’ purses are frequently found on the shore, and are the empty egg cases of skates and dogfish. At the corners of the egg case of the dogfish are long curling tendrils which become attached to seaweed and so prevent the case from being washed up by the tide. The egg case of the skate is broader and bears a spine at each corner. It is laid in deep water and partly buried in the sand so that two spines stick out. The developing fish draws water for respiration into the case through the spines.
The Migration of the Eel
Many fish travel great distances to the breeding place, which is so placed that the young will drift with the current in the direction of suitable feeding grounds. The case of the eel is particularly interesting, for it was not until 1922 that its migration and breeding habits were properly understood. It is now known that when the fresh-water eels swim down the rivers to the sea in autumn it is to commence a journey across the Atlantic Ocean. After several months they reach the breeding place, which is deep below the surface in the part of the Atlantic known as the Sargasso Sea. The eels die after spawning, and the eggs develop into small-shaped transparent creatures which swim slowly across the Atlantic and reach Europe in about two and a half years. Formerly they were considered to be a new kind of fish, and were given the name of Leptocephalus. When they hatch, the young eels are only ½ in. long, but by the time they have reached the coasts of Europe they have grown to a length of about 3 in. Now they lose their leaf-like shape and gradually become cylindrical and more eel-like. The elvers, as they are called at this stage, leave the sea and ascend the rivers in great numbers, and after feeding and growing for a period varying from five to twenty years they reach their full size. Then they begin the long journey across the Atlantic to the breeding grounds.