Fishes are the most primitive vertebrate, i.e. backboned, creatures known. All reptiles, birds, and animals have gradually evolved from fish-like ancestors by a series of age-long processes, the stages of which are recorded in fossilized remains that are found in various rock strata throughout the world. A fish lives exclusively in water. It has no lungs, but extracts oxygen from the water as it passes over the surface of its gills. Instead of limbs, it has fins, with which it balances itself and propels itself through the water. Its skin is either bare, e.g. the cat fish, or is covered with scales, e.g. the herring, or with bony plates, e.g. the sturgeon. The skin of certain sharks is studded with minute teeth and produces, when cured, the well-known shagreen leather. In nearly all cases the skin of fishes is liberally supplied with small glands which constantly produce a lubricating mucus. This mucus greatly reduces friction between the fish and the water through which it moves.

The body of a fish is adapted to move swiftly and smoothly through the water; it is shaped more or less like a torpedo, but this form is greatly modified in different species. Certain species of fish living at the bottom of the sea, for example skates and rays, have become flattened, as though by a pressure applied vertically downwards. Others, for example plaice, flounder, sole, appear to have been flattened sideways. In the various members of the eel family, the body is greatly elongated.

The body of a fish is generally coloured and marked in such a way that it becomes practically invisible when seen from above or below, the under-surface being silvery white, and the upper surface generally olive or blackish-green. Sometimes, as in the mackerel, the upper surface is mottled, resembling rippled water.

Most small fish in ponds and streams reflect their surroundings so well, and are coloured and marked in such a way, that they are almost invisible to the large fish, for example pike, that prey upon them. Generally, they reveal their presence by the flash of light reflected from above by their scales, as they turn suddenly to snap at a morsel of food. In the same way, many predatory fish, e.g. the angler fish, resemble their surroundings so closely that the fish for which they are lying in wait swim within easy reach of them without perceiving their danger. Many fishes, particularly in tropical waters, are remarkable for their bright and gorgeous colouring. It is impossible to preserve these colours in their natural brightness after the fish have been taken from the water, but amongst the brightly coloured corals, and anemones and seaweeds, in the crystal clear water of their natural environment, they flit like gorgeous tropical birds in a tropical forest.

Distribution. Fishes are found in practically every ocean, lake and river in the world, with a few notable exceptions, such as the Dead Sea, in which the concentration of salt is too high. They appear to exist at all depths of water, and have been found in the sea as deep down as 2,720 fathoms. Fish living at this depth generally possess enormous mouths, long, attenuated, soft bodies, and are equipped with highly developed phosphorescent organs.

The distribution of a particular species appears to depend upon the salinity of the water, the temperature of the water, the kind and quantity of food available and the prevailing intensity of sunlight. It is possible to divide fish into four well-defined groups, according to the salinity of the water in which they are found: (1) Marine fish: those that live always in the sea, for example herring, haddock, shark. (2) Fresh-water fish: those that live always in fresh water, for example carp, trout, pike. (3) Many fish live in brackish water, and appear to be able to accommodate themselves easily to considerable changes in salinity, e.g. sticklebacks, gobies, grey mullets and blennies. Such species naturally are widely distributed; thus, a particular kind of grey mullet (Mugil capito) is found without any appreciable difference in form on nearly every coast of the Atlantic Ocean. (4) The fourth group of fish are migratory. Some species, for example salmon and shad, live and develop in salt water, but ascend rivers to spawn, i.e. to lay their eggs, in fresh water. Others, such as eels and certain pleuronectids, for example the flounder, live and develop in fresh water, and descend rivers to the sea to spawn. Many fresh water fish, e.g. trout, forsake the large streams in the spring and ascend small brooks, where the young can be reared in greater safety.

Of these different groups or species, the marine fishes are industrially by far the most important, for at least two-thirds of all the fish in the world live in the sea, and the capture of these sea-fish in enormous quantities constitutes the fishing industry, with which we are concerned.

The different species of marine fishes can be divided into three well-marked groups, according to their habits and habitats.

(1) There are the true deep-sea fishes that live at the bottom of the sea, for example cod, haddock, plaice, sole. These are called “demersal” fish. Fish, like birds, inhabit a medium that is continuous throughout the world. A glance at the map of the world will show that the three great oceans—Atlantic, Indian and Pacific—are united in the southern hemisphere. In Tertiary times, it is practically certain that the Pacific and the Atlantic oceans were also united at Darien, and that the Mediterranean was united with the Red Sea. Apart, therefore, from differences in local conditions, for example of temperature and food supply, there is practically no obstacle to the world-wide distribution of any particular species of fish. At the bottom of the sea, the temperature, the food supply, and the general conditions of life are singularly uniform all over the world, consequently there are no barriers at all to the dispersion of demersal fish, and we find various species widely distributed in all seas. Demersal fish, on the whole, are more primitive in type than those that live nearer the surface. They have well-developed senses of touch and smell by means of which they hunt for their food. They differ markedly in structure and shape from surface or shallow-water fish, their bodies being designed to resist the greater pressure of deep water. The body is generally lean and is enclosed by a wall of muscular fibre. Shallow-water fish, if introduced into deep water, would be crushed inward by the pressure. Similarly, the deep-living, demersal fish are unable to accommodate themselves to shallow water and, if placed in it, soon become unhealthy. A cod floats helplessly on its side when placed in shallow water, owing to the dilatation of its swimming bladder. If the bladder is pricked it collapses, and the fish is able to regain an upright position. This is done when cod and other similar demersal fish are kept alive in sea-water tanks on board ship, to be delivered to the markets alive. In Denmark, fish are delivered alive to the shops. When fishes from great depths are brought to the surface, their bodies break into pieces owing to the reduced external pressure, the scales start from their skin and the eyes from their sockets.

There are two distinct types of demersal fish: the “round” and the “flat.” The body of a round fish is more or less circular in cross-section, for example cod, while that of a flat fish is flattened, for example sole, ray.

The most important edible demersal fish can be classified as follows—

(a) The Gadidae—related to the cod.

(c) The RaÜdac.

(2) The various species of fish that inhabit the surface waters of the sea are called “pelagic.” They include the herring, mackerel, tunny, flying fish, sword fish, and many sharks, also various marine mammals, such as whales, grampuses, porpoises, dolphins. Amongst pelagic fish are included some of the smallest (plankton) as well as some of the largest (whales) of all living creatures. Pelagic fish pass their whole life swimming at or near the surface. They enter the shallow water offshore only for prey or, in some cases, periodically to spawn. The majority spawn in the open sea, far from land. Unlike demersal fishes, the distribution of the different species of pelagic fishes depends very much upon local conditions of light, water temperature, and the character and quantity of food available. They do not hunt their food individually to the same extent as demersal fishes, but generally filter it from the water as it passes through their gill-openings. Although not so widely dispersed as demersal fish, they are, in favourable circumstances, dispersed over large areas by swimming and by ocean currents.

All pelagic fish are “round.” With the exception of the mackerel, the important edible pelagic fishes belong to the herring family, and are known as the Clupeidae. They include—

(3) The shallow-water of the seashore is inhabited by certain animals (shellfish) not found elsewhere, including various mollusca, e.g. mussel, cockle, oyster and periwinkle, and crustacea, e.g. lobster, crab, prawn, shrimp. In addition to these, there are various species of immature offshore fish, e.g. plaice and dabs. The inhabitants of this shallow, coastal water are called “littoral” fish. The distribution of such littoral fish depends not only upon the water temperature and the amount of light, but also upon the character of the shore—whether it is rocky, or soft and sandy—and more especially upon the animal and vegetable products of the adjacent land, e.g. plants, seaweed, worms. Littoral fish do not swim very far, but become scattered inadvertently over considerable distances by currents and other mechanical means.

Certain kinds of shellfish, for example oysters, mussels, cockles, live in the sand or attached to the stones or seaweed on the seashore, generally between high and low watermarks. They obtain their food from the water as it streams over their gills. They require adequate room for growth and development, and constant irrigation by water containing sufficient floating food. When mussel beds or oyster beds become overcrowded, the fish are ill-nourished, their health is impaired and their growth is arrested. It has been shown that, if they are transferred to new beds, their condition rapidly improves and ultimately they increase considerably in size. All edible shellfish need systematic care and attention. Their cultivation by man affords the simplest instance of an attempt at a systematic aquiculture.

Food. The surface water of the sea abounds in minute forms of vegetable and animal life. This vast floating population of microscopic organisms is called the “plankton.” Just as man and all land animals depend ultimately for their food supply upon grass and other green-leaved plants which, under the influence of sunlight, are able to transform the inorganic constituents of the atmosphere and the soil into organic foodstuffs—albumen, fat, carbohydrates—so the minute unicellular marine plants of the plankton are able, under the influence of sunlight, to convert the inorganic constituents of their environment into fat, albumen and carbohydrate. Upon these minute organisms, therefore, directly or indirectly, all marine life depends.

In addition to these minute plants, the plankton contains nearly all forms of marine life at some stage or other of their life history. Fish are only found in it as eggs, or larvae. Crustacea of all kinds are present, and form one of its most important constituents. Crabs and lobsters spend their larval, free-swimming career among the plankton, until they reach the adult stage and settle down to the bottom. Various minute crustacea, known as “Copepoda” (lit., oar-footed) spend the whole of their lives drifting about in the surface water. They occur in incredibly large numbers, and are the most abundant of all forms of marine life. These copepoda form the main source of the food of pelagic fish, such as the herring, mackerel and sprat.

The larvae of the edible molluscs, oyster, mussel, cockle, develop in the warm surface water until they settle to the bottom and begin their adult life.

There are also many larval forms of marine worms and jellyfish, and many kinds of microscopic, unicellular organisms, some of which are vegetable and others are clearly animal. The chief animal forms belong either to the Infusoria, the Foraminifera or the Radiolaria. The shells of the two latter forms accumulate at the bottom of the sea, producing the deposits known as the Globigerina and Radiolarian oozes. In this way, chalk deposits were formed in primitive times.

The most important vegetable planktonic organisms are the Diatoms. Their accumulated shells form important deep-sea deposits.

The numerous varieties of planktonic life can thus be divided into two groups: those minute animal and vegetable organisms that pass the whole of their existence at the surface of the sea—the true constituents of plankton all the year round—and the eggs and larvae of many species of fish that are found among the plankton only at certain times of the year—notably in spring and summer.

The quantity of organic food substances such as albumen, fat and carbohydrate, that is contained in the plankton produced annually by a given area of the sea, has been compared with the quantity of such substances produced by a similar area of land in crops such as pasture, hay, lupine and peas. In this way, it has been estimated that the productivity of the sea is about 20 per cent less than that of average land.

Unlike that of the land, the productivity of the sea is greater in colder latitudes than in the tropics. This somewhat unexpected fact is attributable to the action of denitrifying bacteria which, flourishing more readily in warm, tropical waters, effectively reduce the amount of available nitrogen compounds in the water. In colder waters, denitrifying bacteria are less active, and nitrates and nitrites are available in larger quantities for the nourishment of the plankton.

All the great fisheries of the world are prosecuted in cold or temperate seas; as examples of this we have the Banks of Newfoundland, the cod fisheries of Norway, and the great trawling grounds of the North Sea and the North Atlantic.

All fish, during the larval stage of their development, feed first upon the contents of the yolk sac which, when they are hatched, is attached to their ventral surface. When the yolk is absorbed, the larvae feed upon the microscopic plankton that abound in the water on every side. The surface water, with its warm temperature, high plankton content and sunlight, forms an ideal nursery for the very young fish of all species. Demersal fish, as they complete the larval stage of their development and descend into deeper water, have to rely for their food either upon the various species of young shellfish and crustacea that drop from the surface water as they develop, or hunt for their food amongst the small fish, mollusca, crustacea, worms and seaweeds of the sea-bottom. Plaice feed chiefly upon cockles and other mollusca, which in their turn feed upon diatoms. The cod is almost omnivorous, greedily devouring small fish, crustacea, worms or mollusca; its favourite food, however, is shrimps and prawns. These, in their turn, feed upon smaller invertebrates, for example small jellyfish and larval molluscs, and these upon microscopic plankton.

Pelagic fish, herrings and mackerel, feed almost entirely upon the larger plankton, mainly copepoda (small, shrimp-like crustacea). These may be present in the surface water in enormous quantities at certain times. In many cases, shoals of herring or mackerel probably follow special swarms of copepoda. Mackerel also feed upon young fish, hermit crabs, and prawns.

With a few notable exceptions, the various species of demersal fish feed upon smaller fish. Thus—

The hake, normally a deep-water fish, ventures inshore in pursuit of herrings, pilchards, mackerel.

The ling, turbot, brill, dog fish live entirely upon small fish. The dog fish swarms on certain fishing grounds and is often a serious pest to the drift-net fishermen, destroying their nets as well as the fish that are attached to them.

The whiting, like the cod, feeds upon small fish, and upon crustacea and mollusca.

The food of the haddock consists of mollusca, crustacea and marine worms, etc.

The sole lives on small crustacea, for example shrimps, and marine worms.

Skates and rays feed upon mollusca and crustacea.

Most shellfish live in shallow water and feed upon the plankton.

The methods by which fish obtain their food differ greatly according to the species of the fish. Pelagic fish, e.g. herring and mackerel, sprat and pilchard, obtain their food almost automatically as they swim open-mouthed through the water in which it abounds. These direct plankton-feeders possess comb-like structures—the gill-rakers—attached behind the gill openings, and as the food-bearing water streams through the mouth and gill openings of the fish, these structures strain the food from it. The fish licks the plankton from its gill-rakers with its tongue and swallows it.

Many pelagic fish, e.g. carp, trout, salmon, look for their food while swimming through the well-lighted surface water.

Demersal fish—flat fish, cod, haddock, etc.—seek their food by scent and touch. The cod possesses a barbel attached to its chin, by means of which it feels for its food.

The Angler or Devil fish is a curious creature, from three to four feet long, and appearing to consist almost entirely of head. It has a large mouth, and teeth that are hinged so as to admit food, but prevent it from escaping. The devil fish has a long feeler on the top of its head, terminating in a tassel which, moved by the water, attracts the attention of small fish and lures them to their fate. This tassel is a sensory organ and, when it is touched by the small fish, the angler fish snaps upwards with unerring aim at a point immediately in advance of the tassel.

The dog fish seeks its food exclusively by scent. If its sense of smell be destroyed, it ceases to feed spontaneously.

The sole also seeks its food by smell. It is quite unable to recognize a worm by sight or touch, even when hung just above its head, but feels aimlessly over the ground seeking it by smell.

Reproduction. Fish are male and female and, with few exceptions, reproduce their kind by laying eggs. The number of eggs laid by an individual female fish during a single spawning varies greatly, according to the species. The average number of eggs spawned by a single female fish in the course of one season, is—

The eggs of the cod, whiting, haddock, fluke, plaice, etc., are relatively small, varying from ¹/₆ of an inch in the case of a halibut, to ¹/₂₅ of an inch in a flounder. The eggs are discharged into the water by the female. This process takes place gradually, and generally occupies many weeks. A few of the eggs come to maturity at a time, and are extruded. They are fertilized in the water by the spermatazoa of the male, which are discharged into the water at the same time as the eggs. The fish, both male and female, are closely crowded together on the spawning grounds, so that the fertilization of the eggs is fairly complete. With few exceptions, the eggs of most species are buoyant and float to the surface, where they drift in the warm surface water until, happily, they hatch. Unhappily, however, a very large proportion of them never reach maturity, for, either as eggs, embryos or larvae, or post larval young fishes, they soon fall a prey to marauding fish. It is estimated that, of the thirty-two thousand eggs laid annually by each female herring, not more than two reach maturity.

The spawning grounds of the herring are not definitely known. Research is being carried out at present with a view to solving this question. Haddock are to be caught in various likely parts of the sea, marked with the place of capture, and their interiors examined for herring spawn.

Certain demersal fish, notably shark, dog fish and skate, deposit a few large, demersal eggs—about a dozen in the year—in a carefully selected spot. The incubation period of these eggs is unusually long, being from six months to over a year, according to the species and the temperature of the water.

Parental care is exhibited by very few fishes in this part of the world, although many foreign fish build nests and care for their young, often carrying them in their mouths. Certain kinds of dog fish and angel fish keep their young inside their oviducts until they are completely formed. The only notable example of a fish common to British waters that exercises parental care is the stickleback. Spawn is deposited by a number of different females in a nest constructed of stones and weed, and is guarded by a male until all the eggs are hatched.

The eggs of the crustacea, for example the lobster, are found attached in large numbers to the swimmerets—feathery processes that are situated underneath the tail. When in this condition, the lobster is known as “berried,” and, if captured, should be returned to the sea. The eggs are sticky and are laid while the lobster lies on her back, and so become attached to the hairs of these feathery processes. Berried crabs, prawns and shrimps may also be observed on the seashore in the spring and early summer.

The mollusca, e.g. mussels, periwinkles, oysters, deposit their eggs in the sea-water. The eggs float to the surface, hatch out, and drift about with the other constituents of the plankton. The fully developed larvae fall to the sea bottom and become attached to seaweed and stones.

The period of incubation of fish eggs varies according to the species of fish, and for the same species is prolonged by a low temperature. Plaice eggs, fertilized in January, hatched in eighteen days; others, fertilized in April, were hatched in nine days.

All fish, on emerging from the egg, enter upon a larval stage in which they resemble each other very closely (see Fig. 1). (Thus, the larvae of plaice are quite symmetrical, like those of the cod or other round fish.) The newly hatched larvae drift helpless in the water for two or three weeks, during which time they subsist upon the contents of the yolk sac, which they carry attached to their ventral surface. When this is exhausted, they feed upon the microscopic plankton which abound in the surrounding water.

The characteristic forms of the different species of flat fish are gradually assumed by the young fish during the period of their larval development. The appearance of a newly-hatched young plaice exhibits little change during the first week or so, other than that due to the gradual disappearance of the yolk sac. The young fish grows very slowly, and, twenty-one days after hatching, is only 3/8 of an inch in length. For thirty days the development of the young fish is entirely symmetrical. During the succeeding fifteen days, the shape and appearance of the fish become profoundly modified. The left eye gradually moves upwards and forwards, until it attains its final position above and in front of the right eye. At the same time, the fish gradually acquires a new swimming position, finally swimming on what is really its left side. This left side becomes colourless. With these changes in form and habit, there proceeds a transformation in the diet of the fish. At twenty-one days it feeds upon the young stages of various crustacea. Gradually it acquires a taste for copepoda and the larvae of mollusca and crustacea. After its metamorphosis is complete, it feeds upon various worms, small shrimps and small, bottom-living crustacea. The adult plaice feeds upon mollusca of the cockle and mussel families.

The Migration of Fishes. Fishes, like birds, migrate over great distances at certain seasons of the year. In most cases, this migration occurs just before spawning, and is evidently connected directly with the spawning instinct. True marine fishes, such as the herring, haddock, plaice, cod, associate in vast numbers at spawning time, choosing a locality in which the temperature and food supply will be favourable to the development of the young larvae. Generally, the spawning ground is in deep water. The eggs are buoyant, and drift up to the warm surface water and hatch out amongst the plankton. The herring differs from most other pelagic fish in laying its eggs in relatively shallow water, over a rocky bottom covered with seaweed. The eggs are denser than sea-water and are covered with an adhesive substance, so that they sink to the bottom and become attached to the stones and seaweed.

It is at the time of this annual migration to the spawning grounds that the fish are most profitably caught, for not only are they gathered together in large numbers, but, just before spawning, the fat content and general condition of the fish, and therefore its food value, reach a maximum. After spawning, the food value of the fish is at a minimum, and remains comparatively low until a few months before the next spawning.

The plaice migrates in the autumn from the feeding grounds in various parts of the North Sea to the spawning grounds near the Straits of Dover. Spawning takes place between December and March. In the spring and summer it returns northwards to the feeding grounds in the centre of the North Sea.

In the Irish Sea, there are two distinct annual migrations of plaice. The first occurs in summer (from June to September), the larger plaice moving from the warmer, shallow water inshore to the deeper, cooler waters offshore. In winter and spring, (from October to May), the mature plaice migrate from Morecambe and Liverpool bays to the spawning ground in deep water to the North-East of Douglas (Isle of Man).

In winter, also (from November to January), a large number of plaice gather in Red Wharf Bay, off the north coast of Anglesey, probably because it is sheltered from the prevailing south-east winds. In February they commence their spawning migration round the coast of Anglesey to Cardigan Bay.

Certain species of fish, instead of migrating from one part of the sea to another, migrate from the sea to rivers (anadromous), or from rivers to the sea (katadromous).

Thus, in the spring or autumn, according to species, the anadromous salmon and shad ascend rivers to spawn. The eggs are deposited on clean gravel in clean water, where they are likely to remain undisturbed. The salmon does not feed when in the river, and after spawning, becomes very thin and in poor condition.

The Alaskan salmon, from which the bulk of American canned salmon comes, exists in five species. It has a similar spawning habit to the British salmon, except that the same species always tends to use the same rivers. Once having spawned, the fish dies, so that the parents never see their offspring. The young larvae hatch out in the fresh water and make their way to the sea, where they pass the whole of their lives until they are mature, some years later, and then, in their turn, ascend the rivers to spawn.

Eels are normally fresh-water fish. After living for six or seven years in rivers and ponds and streams, they become mature and migrate to the sea to spawn. This spawning always takes place in deep water (over five hundred fathoms), the particular region chosen depending upon the species. Eels from the British Isles and North-West Europe spawn in deep Atlantic, some hundreds of miles west of Ireland. In the autumn, the mature eels move down the rivers to the sea. When approaching maturity, the yellowish coat of the eel changes to silver. These “silver” eels pass into the sea and are never seen again. It is probable that the eel only spawns once in its life and then dies. The spawn floats to the surface and hatches out into curious little transparent, leaf-shaped larvae. These larvae develop rapidly into elvers and commence the return journey to the shores and rivers. In the spring, the young eels ascend the rivers in enormous swarms. Many of them leave the rivers and travel over damp ground and grass to isolated pools and lakes. It is probable that the eels that are found in the Thames travelled overland from the Severn.

The Baltic flounder migrates in winter from rivers and estuaries to the open sea, and spawns in spring in deep water. It returns in the summer when the spawning is over. By observing the movements of marked fish, it has been shown that the fish move at an average rate of from three to four miles per day. During its seaward migration, the flounder takes no food, but uses the material stored up in its tissues for the development of its reproductive organs.

In addition to these spawning migrations, there are migrations that are prompted by a search for food, or for warmer or colder water.

In northern and temperate seas, the surface water grows warmer with the spring. This warming influence spreads northwards from the equator, producing what is known as the annual wave of sea temperature. A direct result of the rise of temperature and the increased sunshine is a rapid increase in the amount and quality of the plankton. It is not surprising, therefore, that fish migrate in the wake of this annual wave of sea temperature, attracted by the increased food supply, and possibly, also, by the warmer water.

The mackerel is a southern fish, and prefers the warm water of the Mediterranean and West African coast. In spring, as the wave of rising sea temperature travels northwards, it migrates to the English Channel and the North Sea. This migration is often directly associated with the presence, in large quantities at that season, of a particular kind of copepod in the surface water of the English Channel.

Phosphorescence. Many marine creatures, ranging from deep-sea fish living in the dark abysses of the ocean to various species of the minute plankton drifting in the surface water, possess phosphorescent organs, which emit light of low intensity similar to that of a glow-worm and firefly. In many cases the light appears to possess some important function, and highly specialized organs are developed. In such cases the light is only emitted in response to some stimulus—thus, the phosphorescence of the surface water of the sea, when disturbed by the blade of an oar, is due to the disturbance of myriads of minute planktonic organisms, equipped with phosphorescent organs, either protozoa or protophyta; many pelagic copepods are phosphorescent. In other cases, phosphorescence appears to be a more or less accidental by-product of some other process, and of little or no significance. The substance which produces the glow is contained in the slimy secretion produced by the epidermal glands of the fish, and, as phosphorescence can only occur in the presence of oxygen, it is evident that the light is produced by the slow oxidation of this substance. The colour of the light emitted by marine organisms is generally blue or light green, but red and lilac also have been observed. The distribution and colour of the light or lights produced by individual fish vary with the different species. In many cases it would appear that these points of light provide the means by which fish recognize each other in the dark depths of the ocean. Some fishes possess highly developed phosphorescent organs known as photophores, consisting essentially of a group of gland cells that secrete the phosphorescent fluid. These organs are generally distributed in rows along the sides and ventral surface of the fish. Some fishes possess more complex and highly developed organs containing, in addition to the gland cells, a system of blood vessels and nerves, a transparent, protecting membrane and reflector, an iris-like diaphragm and a lens. These more complex organs are generally larger and less numerous than the simpler ones. Possibly they are used to search for, or to attract, prey.

The phosphorescence of decaying fish and meat is due to the presence on the fish or meat of certain bacteria of putrefaction, which are themselves phosphorescent. When seen under the microscope, the individual bacteria appear as shining points of light.