The chromatophores and iridocytes are chiefly disposed in two layers in the skin, one outside the scales and the other on the inner surface of the scales, between the latter and the underlying muscles; and although the two kinds of coloration elements may be present in both layers, their relative abundance varies in different Fishes, and in different parts of the surface of the same Fish. Where chromatophores are most abundant, usually on the back, the reflecting tissue is relatively scanty, and vice versâ. On the sides and belly of a Fish the place of the inner layer of the dorsal surface may be taken by the "argenteum." This layer is devoid of chromatophores, and consists of reflecting tissue in which the iridocytes form a continuous stratum, either in the form of granules, or as a close network of rod-like bodies or of polygonal plates in contact with one another, instead of being less numerous and more scattered as on the back. When iridescence is produced, it is due to the iridocytes of the outer layer of the skin; the dead whiteness and silvery lustre, on the other hand, have their origin in the different ways in which incident light is reflected from the inner layer or argenteum.

To the relative abundance of chromatophores, the kind of pigment they contain, and the manner in which they are distributed and blended, combined with the different reflecting properties, or the iridescence, of the iridocytes, are due the extraordinary wealth and variety of colour in Fishes.

The part played by the different coloration elements in the production of the characteristic colours of different Fishes may be illustrated by two examples.^[130]

In the common Whiting (Gadus merlangus) the back of the Fish is a dark bluish-grey; the sides have a beautiful iridescence and silvery glitter, while the belly is very nearly a dead white. Briefly, these appearances are due to the fact that chromatophores (black and deep yellow) are most abundant on the back, less numerous on the sides, and wanting altogether on the belly; while the iridescence and silvery appearance of the sides are due to the iridescence of the iridocytes external to the scales, combined with the non-iridescent but highly reflective property of a layer of iridocytes internal to the scales; and the dead white of the belly to the different reflecting power of the argenteum, and the absence of chromatophores in that region.

In the Mackerel (Scomber scombrus) the distribution of coloration elements is different, inasmuch as they are mainly situated in the deeper part of the skin, internal to the deciduous scales. The back is marked by the well-known alternating wavy bands of black and green; the sides gleam with the most brilliant iridescence, changing from silver to yellow or red gold, according to the angle at which the Fish is viewed. The black bands of the back are produced by the crowding together of black chromatophores and the diminished number of yellow; the green bands by an equal blending of yellow and black. Over the dorsal surface and sides of the Fish, where the coloured bands extend, there is also a reflecting layer external to the chromatophores, and to this layer is due the silvery reflection and iridescence. On the belly the disappearance of the chromatophores and the greater thickness and opacity of the argenteum account for the lighter colour and the diminished iridescence and silvery glitter of this part of the skin.

Many Fishes are known to have the power of changing their colours, and in some the change is rapid. Such changes are due to incident light reflected from surrounding surfaces, acting through the visual organs and the nervous system on the differently coloured chromatophores. The latter are capable of contraction and expansion. Expansion of any particular kind of chromatophores is accompanied by a diffusion of their pigment—black, red, orange, or other colour as the case may be—and, according to the number and distribution of the chromatophores affected, the prevailing tint or tints of the whole body will be intensified, or only spots, bands, patches, or flushes of colour will be produced. Conversely, when chromatophores contract, they may shrink up to mere dots and bring about a diminution in the vividness of their respective colours, or even an alteration of colour, seeing that yellow chromatophores become orange when contracted, while orange or red appear brown or black. Colour changes of this kind may be artificially brought about. Experiments with Sticklebacks (Gastrosteus)^[131], kept in glass dishes with a bottom of black or white tiles, have shown that the Fishes over the white tiles became partially bleached, while others with a background of black tiles retained their original coloration. Bleached Fishes exposed to the white tiles for a relatively short period (three to ten days) tend to regain their original colour when subsequently removed to a background of black tiles, but prolonged exposure to the former conditions (five to six weeks) seems to render the acquired light colour more or less permanent. The interior of a Minnow-can is sometimes painted white, so that the bait may assume a lighter colour, and thus become more conspicuous in the deeper and darker water where Perch and Pike abound. Hence the colour of a Fish may vary with its surroundings; and, as will shortly be shown, the capacity for producing such changes under natural conditions is of the greatest importance to Fishes in various ways.

Change of coloration may take place through the development of new chromatophores under the influence of new conditions, and is then comparatively slow. Artificial illumination of the unpigmented white side of a Flounder (Pleuronectes flesus), by means of a mirror, induces the formation of chromatophores, and produces a coloration more or less closely resembling the upper pigmented side.^[132] A similar change sometimes occurs as a natural variation, and is then usually associated with structural deformity in other respects.

Coloration also varies with age, sex, ill-health, and even with the emotions. Young or immature Fishes are often marked by bands, bars, or blotches of colour (e.g. the Parr-marks of young Salmonidae), which, as they disappear when the Fish approaches the adult state, are perhaps residual traces of ancestral coloration; although, no doubt, change of habits and surroundings are sometimes responsible for such colour changes as are observable during growth. Conspicuous coloration is one of the most frequent of secondary sexual characters, the colours of the male being brighter than those of the female, particularly during the breeding season. A diminution of colour has been noticed in the artificially-induced pigmentation of the lower side of a Flounder when the Fish was suffering from partial suffocation owing to the temporary failure in the supply of fresh water, the normal colour returning when the deficiency had been remedied. A similar pallor was caused by fright when the same Fish was disturbed.^[133] A nocturnal colour-change has been recorded in the Tasmanian Trumpeter (Latris hecateia).^[134] In addition to the olive-green longitudinal bands which are always apparent, there are visible at night five broad, transversely-arranged, blackish bands. As illustrating the importance of vision in colour-changes, it may be mentioned that in a specimen of this Fish, living in a tank, which had been blinded, probably by a rat or a cat, the dark bands were permanently retained.

Changes of coloration sometimes take place, which either have no discernible relation to age, condition, or surroundings, or are brought about by domestication; and in individuals of the same species there is often a wide range of colour-variation, which is sometimes, but not always, associated with particular localities. In some fresh-water Fishes a yellow colour may replace the original tint (xanthochroism). The usually dull greenish Tench (Tinca vulgaris) occasionally becomes a bright orange-yellow. Another Cyprinoid, the common Gold Fish (Cyprinus auratus), in its wild state in China is also a dull brown or green, but, when domesticated, assumes in the first year of its life a black colour (melanism), then a silvery hue, and finally the vivid ruddy golden colour of the adult; occasionally, but rarely, the Fish is an albino.

The value of a particular coloration in Fishes, either as an aid to concealment and protection from enemies, or by enabling them to secure their prey, may now be illustrated by a few examples.

As previously shown, the colours of Fishes may be artificially varied according to their surroundings. Changes of a similar kind occur naturally, and when they tend to assimilate the tints of the Fish to the prevalent hues of its surroundings, and consequently aid concealment, we have examples of what has been termed variable protective resemblance. Individuals of the same species vary in colour according to the opacity of the water they live in, becoming darker in muddy or peaty water, and brighter and lighter in shallower or clearer water. Trout caught in a stream with a gravelly or sandy bottom are lighter in colour than those obtained from a muddy stream, and it is well known that the same Fish changes colour as it passes from the one background to the other.^[135] In a lake in County Monaghan, Ireland, the Trout are darker on that side which is bounded by a bog, but are of the beautiful and sprightly variety generally inhabiting rapid and sandy streams on the opposite side where the bottom is gravelly; and narrow as the lake is, the two kinds of Trout appear to confine themselves to their respective areas.^[136] Trout obtained from a stream near Ivy Bridge have become much lighter since the pollution of the water by white china clay.^[137] As an illustration of the necessity of vision to such colour-changes, it may be mentioned that blind Fishes cannot vary their tint in this protective fashion. A blind Turbot living upon a light sandy bottom differed from its fellows in being much darker and more conspicuous. Dark Trout have been observed among their light-coloured brethren in a chalk stream in Hampshire, but the former were invariably blind, probably, as their larger size indicated, through age.^[138]

Of other forms of protective resemblance, reference may be made to the bottom-feeding Flat-Fishes (Pleuronectidae), many of which have the upper surface of the body coloured with various shades of brown, speckled with black or light specks or blotches, in harmony with the prevailing tints of the sandy banks which usually form their feeding-ground. When disturbed these Fishes court concealment, and render themselves still less conspicuous by partially burying themselves in the sand. Many of the Skates and Rays, which have a white ventral surface, have the back mottled and coloured in accordance with the colour of the sea-bottom, but in this case it is possible that the advantage lies in enabling the Fish to secure passing prey by concealing its own whereabouts.

Striking examples of protective coloration occur among the Pipe-Fishes and Sea-Horses (Syngnathidae), which usually frequent groves of Zostera, Fucoids, and other sea-weeds. A British species of Pipe-Fish (Siphonostoma typhle),^[139] which lives among the blades of the sea-grass, Zostera, is olive-green in colour, and is a typical example of protective resemblance both in colour and in the slender elongated shape of the body. Similar protective resemblances are noticeable among the Sea-Horses, the coloration varying with the general hue of their environment of sea-weed; but the climax is certainly reached by the singular Australian species, Phyllopteryx eques (Fig. 388).^[140] In this Fish the skin is produced into numerous long, flattened, branched filaments, which are prolonged from the extremities of spine-like outgrowths of the dermal skeleton, and marked by alternate bands of brown and orange,^[141] thus resembling both in shape and colour the fronds of the surrounding fucoids and other marine algae amongst which the Fish lives.

Many of the Fishes frequenting the coral reefs of the East Indian and Pacific areas, especially those belonging to the Teleostean families Chaetodontidae and Pomacentridae, have a most brilliant and vivid coloration, frequently marked by bands or stripes of different tint. So far from rendering these Fishes unduly conspicuous, there can be little doubt that, by harmonising with the striking and varied colours of the anemone-like coral polypes, their coloration is distinctly protective; and it is interesting to note that similar colour-patterns have been independently reproduced in both families.^[142] Even the reef-frequenting Flat-Fishes (Pleuronectidae) have the usually sombre upper surface ornamented by vivid colours and striking patterns.

Pelagic Fishes, like the Herring, Mackerel, Flying-Fish (Exocoetus), and many others, often have the belly and sides silvery or white, and the back dark green, black, or steely blue. Seen from below against the light sky, or viewed from above against the background of the dark water, these Fishes would seem to be practically invisible to their predatory foes, whether Fishes or Birds, or at all events not easily detected.

Coloration may not only be protective, but also aggressive, by helping to conceal the proximity of an animal from its prey; add to this some device for deceiving and attracting the prey, and we have an example of "alluring" coloration.^[143]

As an example of coloration which is both aggressive and alluring, the Angler-Fish or Fishing-Frog (Lophius piscatorius) of our own coasts may be quoted. Naturally sluggish and inactive in its habits, and often using its muscular pectoral fins for crawling about the sea-bottom, the Angler-Fish usually hides itself in the sand or amongst sea-weeds, which it closely resembles in general colour. Curious branched tag-like processes of soft skin fringe the sides of the head and body, and in appearance and colour resemble the smaller fronds of the surrounding sea-weed. So far the coloration is simply aggressive, and helps to conceal the Fish from its prey, but in addition the animal is provided with a special device for luring its prey within the reach of its capacious and Frog-like mouth. The first three spines of the dorsal fin are detached from one another and greatly elongated, and moreover extend along the middle of the dorsal surface of the head. The first, which is the longest, terminates in lobes or lappets of skin, and can be freely moved in every direction by the muscles inserted into its base. By the agitation of this lure or bait smaller Fishes, probably mistaking the disturbance for the presence of a wriggling worm, are tempted to their fate, and soon find themselves engulfed in the enormous mouth of the artful angler.^[144] In some allied forms (e.g. Ceratias bispinosus and Oneirodes eschrichtii)^[145] which live in the abyssal darkness of the deep sea, use is made of the attraction which light has to aquatic animals, and the fishing-rod spine terminates in a phosphorescent organ, which is probably used for enticing smaller Fishes within the reach of the jaws of these singularly modified Angler-Fishes.^[146]

It is by no means improbable that examples of "warning" coloration occur amongst Fishes. The brilliant colours of some of the Trigger-Fishes (Balistes, Monacanthus), Coffer-Fishes (Ostracion), and Globe-Fishes (Tetrodon) are perhaps of this nature. They are often associated with the presence of strong spines, defensive and often erectile, either in connexion with the dorsal fin or on the general surface of the body, and may therefore serve the purpose of a danger signal to such predatory foes of these Fishes as might otherwise be tempted to attack them—to the mutual advantage of the Fishes themselves and their would-be enemies. The British Weever-Fish (Trachinus) may perhaps offer another example of warning coloration.^[147] The Fish is armed with poisonous spines on its opercula, and, in addition, has a conspicuous black dorsal fin. When the body of the Fish is buried in the sand, only its projecting dorsal fin remains to indicate its whereabouts to predatory Gurnards, which might otherwise mistake the Weever for harmless Fishes of similar size and habits. The existence of "recognition" colours or markings peculiar to the species, to enable individuals of the same species to recognise one another and to keep together in shoals, has not yet been proved. It is probable that the relatively limited range of vision, even in the clearest water, would render coloration unsuitable for this purpose. Recognition sounds are likely to be far more effective, and there is evidence of their production by a special vocal mechanism in many Fishes.^[148]

The examples given above show how natural selection may lead to the evolution of distinctive forms of coloration which are advantageous to the Fish either for concealment, aggression, or protection, and in conclusion it may be pointed out that by the same cause colour may be eliminated or its development checked if in any way harmful to the animal; and further, that if a particular coloration becomes useless to the Fish by reason of a change in its habits or environment, natural selection ceasing to act where its intervention is no longer necessary to maintain the coloration, the latter will sooner or later tend to disappear.

The absence of pigment is sometimes protective. The surface-swimming larvae of many Teleosts have no chromatophores, and therefore no obvious pigmentary colours. Their bodies are so translucent that they can be seen through, and hence are visible only with difficulty. The transparency of the body may even be increased by the absence of the red haemoglobin of the blood, as is the case with the pelagic Leptocephalus-larvae of the Eel.^[149] The iridocytes of the reflecting tissue may also disappear under the influence of changed surroundings. The larvae of various species of Onus (Gadidae) are silvery in hue during their pelagic career, owing to the presence of iridocytes in the skin, but on becoming mature they change to a dull dark colour, and live under stones or in holes and crevices in the rocks. During the change of habit the reflecting tissue (argenteum) is lost, and the needful chromatophores are acquired.^[150]

Instances of the loss of pigmentary colours, owing to the cessation of the controlling influence of natural selection, are to be found in the absence of chromatophores on the white under surface of the Flat-Fishes, where such colours are useless but not necessarily harmful, and in the colourless, cave-inhabiting Fishes, of which the Blind-Fish (Amblyopsis) of North America may be taken as an example.

Poison Glands of Fishes.

A few Teleosts are provided with weapons of offence or defence in the shape of poison-glands, probably derived from the epidermis, and associated with spines on the gill-covers, or in connexion with the dorsal fin, or with both.

The two British species of "Weever" (Trachinus draco and T. vipera) are both provided with poison-organs in connexion with a spine on the operculum and with the five or six spiny rays of the anterior dorsal fin.^[151] The first of these spines is a structure projecting backwards from the hinder margin of the opercular bone of the gill-cover, and is traversed along both its upper and lower margins, from base to point, by a deep groove. Except at its protruding naked point the spine is ensheathed in an extension of the external skin. Along each of the grooves there extends a solid pear-shaped mass of gland-cells, the broad base of which coincides with the base of the spine, while the gradually tapering, narrower portion is continued as far as the sharp point. The glands enclose no cavity, and there is no duct, so that whatever poisonous fluid their cells secrete is probably set free by the rupture of the cells and discharged into the grooves, along which it passes to the point of the spine, somewhat after the fashion of a hypodermic syringe. The origin of the gland-cells from an in-pushing of the epidermis is indicated by the continuity of the two structures near the point of the spine. Both in structure and in their relation to poison-glands each of the spines of the dorsal fin is almost precisely similar to the opercular spine. There is no evidence as to how the poison is ejected into a wound, and it can only be conjectured that it may be caused by the pressure exerted on the gland when the spine is forcibly thrust for some distance into the flesh. Certain it is that these structures are capable of inflicting painful and troublesome wounds when the Fish is incautiously handled and the skin accidentally punctured, and no doubt they can be used with great effect as offensive organs.

A similar poison apparatus exists in certain species of Batrachidae, such as Thalassophryne reticulata,^[152] which is by no means uncommon at Panama. This apparatus is formed by a spinous outgrowth from the opercular bone and by the first two dorsal spines. Instead, however, of having two grooves, the opercular spine resembles the fang of a venomous snake, and is perforated by a complete canal which is only open at the base and point of the spine. A poison-sac at the base of the spine discharges its contents into the canal. The nature of the glands which secrete the poison has yet to be discovered, but it is probable either that there are glands in connexion with the poison-sac, or that the latter is lined by a glandular epithelium. The structure of the dorsal spines is similar. In some species of the Scorpaenoid genus Synancia^[153] (e.g. S. verrucosa, from the Indian Ocean), the terminal portions of the dorsal spines are deeply grooved on each side, and at the origin of each groove there is a pear-shaped bag containing a milky poison. The bag is prolonged into a duct which, after traversing the groove, opens at the extremity of the spine.

Many Siluridae are armed with powerful and often serrated dorsal and pectoral spines which are certainly capable of inflicting dangerous wounds, and not a few of them possess a sac-like organ with an external opening in the axilla of the pectoral fin. It is possible that the sac secretes a poison for anointing the spine, but at present there is no evidence that such is the case, or that the sac produces any poisonous secretion at all.^[154]

Among the Elasmobranchs the Eagle-Rays (Aëtobatis),^[155] and the Sting-Rays (Trygon), have barbed or serrated spines on the tail, which inflict wounds far more severe than those caused by mere mechanical laceration; but, except the mucus secreted by the gland cells of the skin, which may possess venomous properties, no special poison-forming glands in connexion with the spines are at present known.

Phosphorescent Organs.^[156]

In common with many other animals of similar habitat, phosphorescent organs (photophores) are highly characteristic structures in many deep-sea Teleosts belonging to widely different families (e.g. Stomiatidae, Scopelidae, Halosauridae, and Anomalopidae). These organs probably had their origin in local aggregations of the gland cells of the epidermis, which had acquired the power of secreting a luminous slime. Luminous organs vary greatly in number and in their mode of distribution in the skin. Usually they are found on the sides and ventral surface of the body and head, very rarely on the dorsal surface, and they often present the appearance of brightly glistening jewels set in the skin. A very frequent method of arrangement is in one or two longitudinal lines along the lateral and ventral surfaces, sometimes extending continuously from the head to the end of the tail (Fig. 371, A, and Fig. 379), but occasionally interrupted and limited to portions of the body and tail; and in a few a distinctly metameric disposition is obvious. On the other hand, the very numerous and simple organs of Opostomias are disposed in many transverse bands along the sides of the Fish. In addition to these organs, which are usually numerous, and whose arrangement is linear, specially large and often structurally complex luminous organs are present on different parts of the head and body. In Opostomias micripnus there is a phosphorescent organ on a median barbel depending from the chin. Sternoptyx diaphana has one on the lower jaw. The presence of one or two organs beneath the eyes (Fig. 96) is characteristic of several species (e.g. Opostomias micripnus, Astronesthes niger, Pachystomias microdon, Scopelus benoitii, Malacosteus indicus). Opostomias micripnus has a luminous organ on the isolated and elongated first fin ray of the pectoral fin, while in certain deep-sea Angler-Fishes (e.g. Ceratias) there is one on the anterior cephalic fin-ray of the dorsal fin. The Scopelid Ipnops murrayi^[157] (Fig. 371, C) has a singular organ, probably luminous, beneath the transparent superficial bones of each side of the roof of the skull. Another member of the same family (Scopelus benoitii) is interesting in having a phosphorescent organ in the middle of the back, which is directed backwards. An American genus of Batrachidae (Porichthys) has about 350 photophores in relation with the lateral sense-organs of each side of the head and body.^[158] The existence of luminous organs has also been noticed in the Haddock (Gadidae).^[159] A primitive form of photophore, distributed in considerable numbers on the head and trunk, either in lines or diffused over the surface, exists in eleven species of Selachii (Spinacidae), of which some are known to be luminous.^[160]

Diversity of structure is equally marked. The essential part of each luminous organ is always a collection of gland cells, usually disposed so as to form the lining of a series of radially arranged gland-tubules in the deeper part of the organ, which also contains ganglion cells, and is supplied with nerves from contiguous spinal or cranial nerves. The simplest form of phosphorescent organ consists of little more than these essential elements. In the more complex organs an investing pigment-sheath, reflecting and lens-like structures, and an iris diaphragm, either singly or in combination, may be added. Fig. 97 represents one of the simplest types of phosphorescent organ, which, in groups of 50 to 100, are arranged in transverse bands on the sides of Opostomias micripnus, and appear as small white spots on the otherwise black skin of this Fish.

Each organ has the shape of a biconvex lens, sunk to about half its thickness in the skin. The inner half is formed of radially-arranged gland tubes filled with small granular cells, and converging towards the centre of the organ. Into the connective-tissue walls of the tubes extend blood-vessels and nerves. External to the gland tubes there is a layer of long slender cells arranged perpendicularly to the surface, and more externally still a layer of ganglion cells. There is evidence that these organs multiply by division. Such simple phosphorescent organs as these differ little from the groups of epidermic gland cells, which probably formed the evolutionary starting-point in the development of these singular structures.

A much more complex type of luminous organ is to be found in the suborbital organs of Pachystomias microdon, of which there are two on each side, appearing as conspicuous white masses, one in front of the other, and situated just below the eye. The more anterior of the two organs is somewhat pouch-shaped in section, its walls consisting of several concentric layers (Fig. 98). Externally there is a layer of black pigment, within which is a stratum of irregular gland tubes. More internally still there is a thick layer of light-reflecting spicules, probably derived from an inverted and modified dermal scale. The axial part of the organ is occupied by a number of radial-disposed structures, probably similar to the gland tubes of the simple organs of Opostomias, and continuous with a lens-like structure which, as it were, closes the expanded mouth of the pouch. The superficial skin which forms the margin of the aperture partially projects over the outer surface of the lens-like body, somewhat after the fashion of an iris-diaphragm. The organ is supplied by a branch of the fifth cranial nerve. Between such simple and complex organs as those above described there are various other types which are more or less intermediate in character.

A particular type of phosphorescent organ is not necessarily restricted to the same species; both the simplest and one or more of the more complex types may be represented in the same Fish. Thus, Opostomias micripnus, which frequents depths of over 2000 fathoms, has not only the simple organs described above, but also others differing from the former in having an external pigmentary sheath, which are scattered all over the body at intervals of 1 to 3 mm. There are also larger and still more complex organs which are disposed in two parallel rows along each side of the body; and finally, the same species has special luminous organs on a median chin-barbel, and also on an elongated fin-ray pertaining to the pectoral fin.

The light emitted by phosphorescent organs is probably of use to deep-sea Fishes in enabling them to seek and detect their prey in the sunless depths which they frequent. The position of the organs on the sides and ventral surface of the body, and the frequent presence of special luminous organs in the vicinity of the mouth, render them admirably adapted to light up the water in front of and beneath the Fish, while the existence of optical accessories for intensifying the luminous beams, and for regulating their distribution, combined with an abundant nervous supply, suggests that the emission of light is under the control of the Fish, and may be varied as the occasion requires. That these organs may also be defensive, in some instances at all events, seems not improbable. A flash-light from the dorsal luminous organ or "stern-chaser" of Scopelus benoitii would probably dazzle and frighten an enemy in hot pursuit of the Scopelus. The use of phosphorescent organs as baits or lures for enticing prey has already been alluded to. There is some evidence that the colour of the emitted light differs in different Fishes; and as there is considerable variety in the precise disposition of the organs, it seems probable that in deep-sea Fishes recognition lights may take the place of the recognition colours and sounds of those whose lot is cast in a sunnier habitat.

CHAPTER VII

THE SKIN AND SCALES

The skin of the Cyclostomata and Fishes consists (1) of the epidermis, formed of several layers of epidermic cells, which are constantly being recruited by the division of the cells of the basal layer; and (2) of a stratum of connective tissue with intermingled unstriped muscle-fibres, blood-vessels and nerves, which constitutes the deeper layer or dermis. From the epidermis are formed the various unicellular or multicellular glands with which the skin is provided; and from one or both of the skin layers originate the different calcareous structures which constitute the hard exoskeleton.

In the Cyclostomata the epidermis is particularly rich in goblet-shaped, mucus-secreting, gland-cells. The Myxinoids also possess numerous pockets of so-called "thread-cells." In each of these cells the protoplasm secretes a long spirally-coiled thread, and under the influence of appropriate stimuli the thread is shot out and unwound to a great length. The threads and the mucus are so abundant that one of these animals will convert a bucket of water into a thick mass of jelly. No scales or other hard exoskeletal structures are present in any of the Cyclostomata.

In Fishes mucus-glands are also abundant in the epidermis, and to their activity is due the slimy mucus which lubricates the surface of the body. They are specially numerous in the Dipnoi (e.g. Protopterus), where, in addition, there are many simple multicellular glands which secrete the "cocoon" or capsule in which the Fish is enclosed during the dry season. From the epidermis are derived the poison-glands of some Teleosts, and also the "glandula pterygopodia" in relation with the claspers of the male Elasmobranchs. The glandular structures in connexion with the phosphorescent organs of the deep-sea Fishes will no doubt be traced to the same source.

In the great majority of Fishes the skin becomes the seat of calcareous deposit, and gives rise to such diverse exoskeletal structures as the varied forms of spines and scales with which the surface of a Fish is invested.^[161] These structures, probably the most ancient form of Vertebrate skeleton owing its existence to the presence of lime salts in the tissues of the body, present highly characteristic modifications in the different groups.

Exoskeletal structures are of two kinds: (1) those which owe their formation to the secretory activity of cells belonging both to the epidermis and the dermis, and (2) those which are derived solely from the dermis. To the first belong the dermal denticles or so-called placoid scales of most Elasmobranchs, and to the second the scales which form the skin-skeleton of living and extinct Teleostomi and Dipnoi. With the exception of enamel, which is always formed by the cells of the epidermis, the hard exoskeletal tissues owe their existence to the secretion of certain cells of the dermis (scleroblasts),^[162] the inclusion of which in a growing calcifying tissue is the cause of whatever cellular structure the tissue may present. It will shortly be apparent that the dermic scleroblasts are by no means uniform in their products, and that in different Fishes they give rise to widely different hard tissues.

The dermal denticles or "shagreen" of the ordinary Sharks and Dog-Fishes (Elasmobranchii) probably represent the most primitive form of exoskeleton. In the development of a dermal denticle a papilla of the dermis grows up into the overlying epidermis, pushing before it the basal layer of epidermic cells, which forms an investment to the papilla and constitutes the so-called "enamel organ" (Fig. 100). The papilla itself subsequently becomes converted into dentine, leaving, however, a central pulp-cavity, while the apex of the papilla is invested by a cap of enamel formed by the enamel organ. Ultimately the base of the papilla widens out into a more or less rhomboidal basal plate formed of bone. In this way there is formed a pointed, enamel-tipped spine of dentine which protrudes through the epidermis, and projects backwards on the surface of the body, but is firmly fixed in the skin by the basal plate with which it is continuous. The centre of the under surface of the basal plate is perforated for the entrance of the blood-vessels which pass to the cellular pulp in the axis of the spine. In the adult Fish the denticles form a fairly close-set covering to the whole body, including the head and even the surfaces of the fins, and are larger on the dorsal than on the ventral surface (Fig. 99). In the Rays (Raia) they are more sparsely scattered, and in different parts of the body may form spines of considerable size for offensive or defensive purposes. The spines vary greatly in shape in different members of the group, sometimes being acutely pointed, and sometimes flattened or depressed, and often they are furnished with smaller accessory spines developed at their bases or from the surface of the basal plate. An arrangement of the denticles in oblique transverse rows is observable in some genera (e.g. Scyllium). In the Saw-Fishes (e.g. Pristis) the denticles which fringe the lateral margins of the long flattened rostrum are not only enormously enlarged, but are implanted in sockets and form the teeth of the saw (Fig. 262). In the Holocephali the smooth skin is almost entirely devoid of exoskeletal structures, but dermal denticles are present on the frontal and anterior claspers, and in the young there may be a double row of small denticles along the back.

In the remaining groups of Fishes, the Teleostomi and the Dipnoi, the spine of the primitive dermal denticle is either evanescent or entirely wanting, while the equivalent of the basal plate remains to form the unit of a scaly armature. Evidence of this may be found in the presence of transitory evanescent spines, provided with an enamel-cap, secreted by the basal epidermis, on the developing rhomboidal scales, as in the young Lepidosteus^[163] (Fig. 101); while the entrance of blood-vessels into the scales through perforations on their inner surfaces, as in Polypterus and Lepidosteus, obviously recalls the perforated base of a dermal denticle (Fig. 99). The epidermis now ceases to take any part in the formation of the scales, and hence enamel no longer enters into their structure. A more regular and definite arrangement of the scales is noticeable, and whether distinct, or articulating with one another, or overlapping like the slates on the roof of a house, they are usually disposed in a series of successive oblique transverse rows. In some of these Fishes the embryonic epidermic covering of the scales becomes lost, and their outer surfaces are naked. More frequently, as in the generality of Teleosts, and in the Dipnoi, the reverse is the case, and the scales are more or less completely invested both by the dermis and the epidermis. As regards their shape, size, and minute structure there is much variation. In some Teleostomi the primitive rhomboidal shape of the dermal denticle is retained; in others a rounded or cycloid scale supplants the earlier rhombic type. Within the limits of the same group (e.g. Crossopterygii) there are examples of the independent evolution of a cycloid from a pre-existing rhombic squamation; and with the introduction of the cycloid type an overlapping or imbricated disposition of the scales always takes the place of the marginal articulation of the rhombic type.