Зображення сторінки
PDF
ePub

variation in the order of proceeding. But for the present our inquiry is addressed to that kind of organization which we designate a scale, and of which the intimate structure and manner of growth have obtained little notice from naturalists, although in their nature they are well worthy of inquiry. The illustrious Cuvier had directed his attention to the subject, and with some degree of success, as was certain to be the case with whatever he took in hand. But it is apparent that his observations are not in all cases to be relied on; and from what appears in the translation of his “Animal Kingdom," there seems reason for believing that they were not made over a wide extent of the families of fishes that had passed under his notice. But the interest of this subject is such as to deserve a more intimate examination, and so much the rather, as in its pursuit we may find it to assist us in ascertaining something of the affinity of what might appear distantly removed species; and also it may point out some adequate explanation of the fact, that some sorts of fishes, even in families well supplied with this clothing, have them altogether concealed within a smooth and soft skin, and others nearly related to them are altogether destitute of them. In this latter case we may remark, that in many instances the internal and external layer of the skin are separated by a thickened layer of what may be termed a retè mucosum, or soft cellular network; in which case the surface of the body remains moist for a longer period when it is exposed to the air; and this may be the cause why some fishes are capable of living long when out of their native element. Experiment has shown that there are fishes which speedily become lighter in weight when exposed to air, while others diminish but little. There is reason also for believing that if some fishes, when out of the water—of which, as an instance, some of the blennies may be mentioned — be frequently wetted over the surface of the body, to the exclusion of the gills, the skin alone will perform a function that shall long sustain the life. It would even appear that such is the case also with some fishes which cannot properly be said to be without scales, although generally so regarded as the common eel, in which they lie embedded beneath the surface, but are visible on close inquiry, and of which a satisfactory magnified representation is given in the Honourable Mrs. Ward's “Microscope Teachings,” Pl. V., Figs. 8, 9; but nothing similar has been discovered in a fish so closely resembling it as the common conger. It is in remarkable contrast with this that the power, one of the Gadoid fishes, becomes deprived of its scales very readily; while the ling, which belongs to the same family, is not furnished with scales, so far as we are able to ascertain ; and the megrim, or scaldfish among the flat fishes (Pleuronectido), is rarely seen with a scale on it, while others retain them firmly, and again others of the same family are not possessed of any. The anchovy loses its scales so readily, that Belon, who was an observant naturalist, believed it to be without them. The herring also, and pilchard, will lose them without much violence, but the shads, which are of the same family, retain them rather firmly. But whether held loosely or firmly, in every case the scales of a fish are clothed to their free borders where there are such, with the common skin, of which a duplication is carried over that border to the lower surface, so far as it remains free; by which it is arranged that each scale becomes enclosed within a case or cell, of which the anterior border is overlapped by the next preceding one, and the firinness with which it is held depends chiefly on the extent to which it is thus overlapped. The cuticle which covers the more exposed and looser portion of the scale is sometimes exceedingly thin, and therefore feeble; but it is the portion of the surface which furnishes the colour, and from which, in some degree, the substance of the scale obtains its nourishment and growth, although this latter is chiefly from a lower and less coloured portion of the case or cell. In some instances the scale itself, and also its covering skin, are so transparent, especially in individuals of early growth, that an exudation and deposit beneath it becomes conspicuous through its substance, of which we shall produce some examples. But the growth or expansion of a scale, as the fish advances in size, is the more material property of its nature; and this, on close examination, will be found to have its source of nourishment and increase from about the middle of its surface, and from which it spreads itself on all sides, although in a greater degree to its free edge and its root, which latter lies forward in the direction of its body. It is at about this middle portion of the scale, but varying a little in situation in different species, that we discover an organized disk, into which, as we can perceive in the larger scales of fishes, there are vessels inserted, and those not always of a very minute size ; but those that pass to each surface of a scale may be distinguished from each other, as well in arrangement as capacity; and it is from this circumstance we draw the conclusion that the scale itself consists of a double layer, like that of the nail of the human finger and toe; although, in the generality of instances, the structure is too fine to allow of our obtaining a positive proof of the fact.

The examples which will be selected for description will illustrate these remarks; but although there is a general resemblance in the marking of the scales of each species, the outline of their shape is found to vary much according

to the part of the body from which they are taken. Those which clothe the back, and especially nearest the head, are the smallest, and they are often oblong and crooked; and those along the line of the belly, especially in the pilchard and sprat, are of singular shape. Our examples, therefore, as offering the best illustrations of the growth and character, are taken from the side, where the outline is the most regular and the structure most easily discerned. In general, the scales of the lateral line are nearly alike in each family, but with a subordinate variation that renders them in some degree peculiar to the individual species, as we shall notice in the common sea-bream, Pagellas Centrodontus. The special purpose of the structure which so generally marks the scales of the lateral line is to secrete and pour out a peculiar fluid, that is intended to lubricate the general surface, so as to enable the fish to pass the more smoothly through the water, and perhaps also to secure it from any ill effect of the fluid in which it swims; and it is observed that this mucous does not speedily diffuse itself amidst the surrounding water. Those fishes which have few or no marks of this structure in a lateral line, or which seem to require an additional supply of this lubricating fluid, are also provided with additional pores about the head and mouth for the same purpose.

Of the scale of the perch, Perca fluviatilis, a representation is to be found in the work“ Microscope Teachings, Pl. V., and also of the sole, Solea vulgaris. The scales of the perch are of good size in proportion to that of the fish, for the most part wider than long, with a disk rather obscurely marked, more than three-fourths of the length towards the free edge, and from which pass backward toward the covered portion or roof five or six lobes, of which those toward the border are the widest; those in the middle narrow, with their narrowest portions meeting close together at the disk, and all of them showing separating marks of growth. At the free edge are short fine points, from each of which passes back to near the disk a line, of which each is marked with a cross line in succession, as if each had advanced by successive starts of progress. This disk, therefore, appears to form the centre of all its growth, as well in regard to length as breadth, as we shall see it does in every other instance, and conspicuously in that of the Scianae (S. Aquila). The scales of this latter fish are large, so that in a full-grown example one of them would cover a florin; but in their shape, or on different parts of the body, they vary more than in most of our native fishes. Many of them taken from one fish appear as if they had been injured, and had recovered the injury as by some natural process of vitality. The disk, which is slightly nearer the free edge than to the root, is large, and covered with apparent orifices, which do not penetrate through the scale; and toward the free edge there are numerous lines or ridges, which come from the disk, each of them being joined to the next by a cross line in regular order. This series of lines ends abruptly at each border of the scale, where it is narrower than in the middle, and from this ending begins a series of fine lines, which encircle the disk, from which portion of these lines the perforations are continued in the same manner as on the disk itself. From the disk again there passes toward its root a large number of diverging lines, of which some are divided in their progress, and each of them is separated from the next by a narrow channel; but such of these diverging lines as are nearest the lines of the border which encircle the disk, end as they approach to them. On all the scales of this fish that came under observation there were apparent marks of the suspension of growth and its renewal; and, in one instance, the disk appeared to extend over the whole breadth of the scale. A large number of fine circular lines, of which about fifty were counted, pass round the disk, and are also marked on or across the diverging lines, or sections, that are carried to the covered root. When fresh from the fish these scales were observed to be smeared or lined with a silvery pigment. The description now given at some length of the scales of the Sciena will apply in its principal character to those of a large number of fishes of somewhat similar shape, with the addition that in several of them, as the river perch, the raised lines or minute ridges which pass from the disk to the free edge, are carried still further forward, so as to form a serrated border, which constitutes a rough surface over the otherwise smooth skin. Such is minutely the case in the comber, Serranus cabrilla; the common sea-bream, Pagellus centrodontus; surmullet, Mullus surmuletus ; piper, Triyla piper ; conspicuously in the top-knot, Rhombus hirtus, where the border of the scale is turned outward; but this spicular arrangement is so minute in some species, as the shad, Clupeidlæ, that its existence might be questioned. The object of it seems to be to afford an additional bond of security to the scale, for the skin is found to pass over these projecting points.

The disk which is thus the centre of extension in growth is not always exactly at the middle of the scale, as it is in the old wife, Cantharus griseus, where it reaches across its extent; and the scad, Caranx truchurus, top-knot, saury, Scomberesiox saurus, and many others ; especially including the whole of the cod-fish tribe, the hake; but in many cases, as the perch and sea-bream, the disk is much nearer the free edge, in which case the growth towards the more concealed

an

root must proceed to a larger extent, and perhaps more rapidly than toward the free border. The raised lines or sections which proceed toward this covered portion of the scale, and to which reference has already been made, are sometimes carried out in a crenated form, as in the sea-bream, and conspicuously in the perch, as also in the Labridæ or wrasses, where the common wrass, Labrus balanus shows a large number of them, together with extensive disk; but in some instances those on the side are more lengthened out, and in others, as the pike, Esox lucius, and piper, Trigla piper, where the number is only three, the middle segment extends beyond the others. The atherine, Atherina presbyter, can scarcely be said to have more than one, but the scale of this fish is marked with a structure that forms a special character in another genus, the Clupeidæ, the herring, and pilchard, to which, therefore, that fish may seem to bear some distant affinity. It consists in some well marked hard lines, which, behind the disk, pass from one border to the other, but apparently with particular reference to a middle longitudinal depression. These somewhat waved crossing lines are so formed in their intimate structure, that when the scales are in partial decomposition, the separated portions cease to adhere to each other, except where they are brought together at the longitudinal depression which proceeds from the disk.

The scales of the lateral line are those only which require further general remark, and their organization is for the particular purpose of being the channels through which a lubricating fluid shall be conveyed from a secreting order of vessels beneath them to the outer surface. This channel usually lies along the middle of the scale that bears it, with its opening directed toward the tail of the fish; at near its extremity this is often divided into branches, as in the case in the Sciana; but even in the same fish all these branched channels are not exactly alike, and in the sca-bream they have some curious arrangements of direction ; their direction being on the upper border of the lateral line, but covered by the scale next before each one, yet through which it may be seen, while the orifice points to the lower border, where it is bent a little, by which means the thin edge of the coverring scale slightly covers the order of the tube. It is thus that the lower side of the tube is above the apparent lateral line, although close to it.

That the conclusions here advanced on the intimate structure of the scales of fishes, and their mode of growth, as well as the affinity of their structure, have not been obtained from very limited observation, will appear from the following list of species that have been subjected to examination; which with

« НазадПродовжити »