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this velocity of the fish is derived, aided by the strength and pliancy of the Back-bone: when in pursuit of its Prey, or avoiding an Enemy, all the smaller fins are laid close to its body; by the impulse of the tail alone, it then glides through the water with the swiftness of an arrow; the other fins are too minute and flexible, compared with the animal's weight, to impel it so quickly; their principal use is to adjust and moderate the movement, communicated by the energy of the tail; deprive a fish of these fins, and when put into water he darts downwards, upwards, and laterally, with all his usual quickness, but is unable to direct his course. The Ventral and Dorsal fins serve to keep the fish in a perpendicular posture, and possibly the first contributes to raise or sink them; the Pectoral fins assist and regulate progressive motion; by extending them, the progress is stopt when swimming rapidly forward, and by folding either, whilst the other continues to play, the turn to the left or right is accomplished*. It is supposed the swimming bladder
*The following, from Dr. PALEY'S Natural Theology, confirms the foregoing account.
"Fish have very nearly the same specific gravity with the Element in which they move. In the case of Fish, therefore, there is little or no weight to bear up: what is wanted, is only an impulse sufficient to carry the Body through a resisting medium, or to maintain the Posture, or to support or restore the balance of the body, which is always the most unsteady when there is no weight to sink it. For these Offices, the Fins are as large as necessary, though much smaller than Wings; their action is mechanical, their position, and the Muscles by which they are moved, are in the highest degree convenient. In most fish, beside the great fin the Tail, we
aids the fishes motion; and that they can dilate or contract this Organ, and by that means raise or sink.
find two pair of fins upon the Sides, one or two fins upon the Back, and one upon the Belly, or rather between the belly and the tail. The balancing use of these Organs is thus proved. Of the largeheaded fish, if the Pectoral fins (that is the pair which is close behind the Gills) be cut off, the Head falls prone to the bottom: if the right pectoral fin only be cut off, the fish leans to that side; if the Ventral fin on the same side be cut away, then it loses its Equilibrium entirely: if the dorsal and ventral fins be cut off, the fish reels to the right and left. When the fish dies, that is when the fins cease to play, the Belly turns upwards. The Use of the same parts for Motion is observed when put into Action, as follows. The pectoral and more particularly the Ventral fins, serve to raise and depress the fish: when the fish desires to have a retrograde motion, a stroke forward with the pectoral fin effectually produces it: if the fish desire to turn either way, a single blow with the Tail the opposite way sends it round at once: if the tail strike both ways, the motion produced by the double lash is progressive, and enables the fish to dart forwards with an astonishing velocity; the result is, not only, in some cases, the most rapid, but, in all cases, the most gentle, pliant, easy, animal motion, with which we are acquainted. However, when the Tail is cut off, the fish loses all motion, and gives itself up to where the Water impels it. The rest of the fins, therefore, so as respects Motion, seem to be merely subsidiary to this. In their Mechanical use, the Anal fin may be reckoned the Keel, the ventral fins, Out-riggers; the pectoral muscles, the Oars: observe, that it is not the resemblance of Imitation that is here meant, but the likeness which arises from applying similar mechanical means to the same purpose. We have seen that the Tail in the fish is the great instrument of Motion; now, in Cetaceous or warm-blooded Fish, which are obliged to rise every two or three minutes to the Surface to take breath, the Tail, unlike what it is in other fish, is horizontal; its stroke, conse. quently, perpendicular to the Horizon, which is the right direction for sending the Fish to the Top, or carrying it down to the Bottom."
themselves at pleasure, by increasing or diminishing the specific gravity of their body; and as their watery Element is of different degrees of weight, according to its depth, they thus possess a power of varying their specific gravity, of swimming easily at a great depth, and of poising themselves in any part of it: it is however insisted, that the bladder cannot be thus employed, as it has no muscles to enable the fish voluntarily to shrink or expand it. Many fish that live continually at the bottom of the water have an air bladder, such as the Eel and the Flounder; and many are entirely without any, that swim at ease in every depth, for instance the Gudgeon. Naturalists have assigned different uses to this bladder, and although it is possible, that some one, or more than a single purpose, foreign from their ideas, may be served by it, the Apparatus is too conspicuous to be formed in vain.
The Expedient, whatever it be, forms part, and perhaps the most curious part, of the Provision. Nothing similar to the Air bladder is found in Land Animals; and a life in the Water has no natural tendency to produce a bag of Air. Nothing can be. farther from an acquired Organization than this.
Fishes thus fitted by their internal structure, and outward shape, for their Element, seem equally well furnished with the means of happiness, as either Birds or Quadrupeds. Like them, they are furnished with an external Covering to defend them from the injurious turbulence of the Fluid which they inhabit; that slimy substance which is secreted from the pores of all fishes, not only defends their Bodies
from accidents, but to their progress through the
*The SCALES of Fishes are formed with surprizing Beauty and Regularity, and exhibit an endless variety in Figure and Contexture, not only in those taken from distinct sorts of Fish, but even in those of the same fish. The Scales from the Belly, the Back, the Sides, Head, and all the other parts, being very different from each other. These Scales are not imagined to be shed every year, nor during the whole Life of the Fish, but to have an Annual Addition of a new Scale growing over and extending every way beyond the Edges of the former in proportion to the Fish's Growth, something in the same manner as the Wood of Trees enlarges yearly by the increase of a new Circle next the Bark; and as the Age of a Tree may be known by the amount of the Ringlets its Trunk is made up of, so in Fishes the number of Plates composing their Scales denote to us their Age.
LEUWENHOECK supposed each Scale to consist of an Infinity of Scales, laid one over the other; or, more simply, of an infinity of Strata, of which those next to the body of the Fish are the largest. He took some Scales from an extraordinary large Carp (forty-two inches long, and thirty-three in the round, Rynland measure), which were as broad as a Dollar. These he macerated in warm water, to make them cut the casier; and then cutting obliquely through one of them, beginning with the first-formed and very little Scale in the Centre, he, by his Microscope, plainly distinguished forty Lamella or Scales, glued as it were over one another; whence he concluded that the Fish was Forty years
ments has been proportionably sparing. The Brain, the seat of sensation, is much smaller in fishes than in other animals. Their sense of Touch is, in all probability, far from being delicate, being obstructed by those strong teguments which surround them. The external Organs of Smell, and the nerves supplying them, are perceptible in the greater part of fishes; but as Air is the only medium we know for the distribution of Odours, it cannot be supposed that, residing in water, they can possess any capacity of being affected by them; if they have any perception of Smells, it must be that the Olfactory membrane in fishes serves them instead of a distinguishing palate, in the same manner as we distinguish by our Taste; by this use of the Organ, they may judge of substances, when Vapours, having tinctured the water, are sent to their nostrils, and doubtless produce some kind of sensation, as otherwise they would be provided with the instruments of a Sense which, from want of an opportunity, of using, they could not enjoy.
The sense of Taste in fishes must be very imperfect, if its delicacy arises from the softness of the Organ, the whole mouth of most fishes being covered with a hard bony substance, by which they are deprived of almost all power of discriminating different bodies by the Palate, insomuch, that the salt-water fishes have been known to swallow the Fishermen's plummet, instead of the bait. The greediness which Sea fish discover, and by which they are taken, is prodigious; the lines of the fishermen who go off to Sea are coarse and clumsy, their