Cartilaginous fish

“ Homologies of Bones of Fishes.—But this homology does not extend to the details of structure. The bones of the arm of the specialized fish are not by any means identical with the humerus, coracoid, clavicle, radius, ulna, and carpus of the higher vertebrates. The vertebrate arm is not derived from the pectoral fin, but both from a cartilaginous shoulder-girdle with undifferentiated pectoral elements bearing fin-rays, in its details unlike an arm and unlike the pectoral fin of the specialized fish. ”

“ The rhinè, which is one of the cartilaginous class, is very digestible and light; but those of the largest size are the most nutritious; and, as a general rule, all the cartilaginous fish are [Pg 562] apt to create flatulence, and are fleshy, and difficult of digestion, and if they are eaten in any quantity, they are bad for the eyes. The cuttle-fish, when boiled, is tender, palatable, and digestible, and also good for the stomach; but the juice which comes from it has the property of making the blood thin, and is apt to cause secretions by hæmorrhoids. ”

“ In the cartilaginous fishes, the skeleton is cartilaginous and transparent; but in some species, the skin has dense osseous particles or plates on the skin, as in the Rays; and in others, the head and body are protected by large osseous scutcheons, as in the Sturgeon. There is also an intermediate group of fishes, termed the fibro-cartilaginous, in which the skeleton contains lime, but in a much less proportion than in the true osseous fishes. In some genera, certain portions of the skeleton, as the bodies of the vertebra, are cartilaginous, while the spinous processes, ribs, &c. ”

“ There are but few natural orders of fishes—and they divide themselves for the purposes of our enquiry chiefly into the Acanthopterygii, or those that possess bony skeletons, with prickly, spinous processes on the dorsal fins, such as the perch, the mullet, and the gurnard; the Malacopterygii, or soft-finned fishes, including the carp, the salmon, &c.; and the Chondropterygii, with cartilaginous spines and bones, embracing such fish as the sturgeon, shark, and skate. ”

“ The vertebræ are undeveloped, the notochord being surrounded by its membranes, without bony or cartilaginous segments. The gills have the form of fixed sacs, six to fourteen in number, on each side, arranged in a cartilaginous structure known as "branchial basket" (fig. 289a) , the elements of which are not clearly homologous with the gill-arches of the true fishes. Fish-like eyes are developed on the sides of the head. There is a median nostril associated with a pituitary pouch, which pierces the skull floor. An ear-capsule is developed. ”

“ The various characters employed for classification have been examined [20] throughout the whole class, and their relative importance has been duly weighed and understood. Though Linnæus had formed a category of “Amphibia nantes” for fishes with a cartilaginous skeleton, which should coincide with Cuvier’s “Poissons Cartilagineux,” he had failed to understand the very nature of cartilage, apparently comprising by this term any skeletal framework of less firmity than ordinary bone. Hence he considered Lophius, Cyclopterus, Syngnathus to be cartilaginous fishes. ”

“ In the more specialised Fishes, on the contrary, the need of an axial support for the body, which, while retaining the necessary flexibility, must possess greater strength, has resulted in the development of a series of solid cartilaginous, calcified or bony, discoidal joints or segments, the centra, which surround and more or less completely replace the notochord, and, while supporting, form also a bond of connexion between the dorsal and ventral arches. ”

“ In this connection, we must bear in mind that the two lowest groups of existing fishes are entirely without osseous skeletons, and hence, however abundant, would leave no permanent record in the deposits in which remains of fishes are usually preserved. It is safe to infer, from the knowledge which we now possess of the simpler forms of life, that even more of the early fishes were cartilaginous, or so destitute of hard parts as to leave no enduring traces of their existence. ”

“ So far as is yet known, all the fish of the earliest fossiliferous system belonged to the placoid or “broad plated” order,—a great division of fishes, represented in the existing seas by the Sharks and Rays,—animals that to an internal skeleton of cartilage unite a dermal covering of points, plates, or spines of enamelled bone, and have their gills fixed. The dermal or cuticular bones of this order vary greatly in form, according to the species or family: in some cases they even vary, according to their place, on the same individual. ”

“ In no other organisms is the evidence of design in the adaptation of means to the ends contemplated, more apparent. In the number of species and variety of forms they exceed all other vertebrate animals.
The skeleton of a fish is usually divided into four parts: the head, respiratory organs, vertebral column and limbs.
The head is very suggestive, and of itself presents a profitable study. It is not hung or poised on a neck, but attached immediately to the body. In most species it is large, making a large mouth possible, but pointed, to lessen the resistance met in passing through the water. ”

“ They are covered with a tough skin, which either supports a set of tuberculed plates as in the thornback, or a thick crop of small rounded bony points or plates, as in the shagreen of the sharks. The head consists of a single cartilaginous box. The spinal column is likewise formed of cartilage, built up in the higher genera of partially ossified vertebræ. The tail is heterocercal or unequally lobed, inasmuch as the spinal column, instead of ending off abruptly as it does in the herring, trout, and all our commoner fishes, passes on to the extreme point of the upper half of the tail. ”

“ The fin rays are very long stems of cartilage, set so closely together as to form a solid sheet of cartilage extending from the thoracic skeleton out to the extremities of the fins, which taper out to nothing. The thoracic skeleton gives shape to the body of the ray, particularly the back, and it must be left in place, with the skin of the back attached to it. Cut through the fin rays where they join the body, and this will enable you to skin down each side of the fish until you get so near the outer edge there is no longer any flesh. ”

“ Body entirely plated; peduncle of tail keeled; the three dorsal spines conspicuously serrated on their edges; anterior fin a little in advance of the base of the pectoral; insertion of ventrals in advance of the second dorsal spine—their own spines serrated on both edges; posterior margin of caudal somewhat hollowed. The colour of the freshly-caught fish is greyish-olive along the dorsal line; but on the sides, particularly in the male, it shades away into an iridescence, like that seen on mother-o’pearl, again changing to pure silvery-white on the abdomen. ”

“ The larva of sharks and rays, and also of Dipnoans and Crossopterygians, are provided with bushy external gills, which disappear in the process of development. In most soft-rayed fishes the embryonic fringe which precedes the [Pg 143] development of the vertical fins persists for a considerable time. In many young fishes, especially the Chætodontidæ and their allies (butterfly-fishes) , the young fish has the head armed with broad plates formed by the backward extension of certain membrane-bones. ”

“ Among the typical fishes the skeleton becomes ossified and reaches a very high degree of complexity. Very great variations in the forms and relations of the different parts of the skeleton are found among the bony fishes, or teleostei. The high degree of specialization of these parts gives to the study of the bones great importance in the systematic arrangement of these fishes. In fact the true affinities of forms is better shown by the bones than by any other system of organs. In a general way the skeleton of the fish is homologous with that of man. ”

“ A multiplicity of vertebral joints is useless to the tropical fish, but to the eel strength and suppleness are everything. No armature of fin or scale or bone is so desirable as its power of escaping through the smallest opening. With the elongation of the body and its increase in flexibility there is a tendency toward the loss of the paired fins, the ventrals going first, and afterwards the pectorals. This tendency may be seen in many groups. Among recent fishes, the blennies, the eel-pouts, and the sea-snails furnish illustrative examples. ”

“ Fishes are readily kept in captivity in properly constructed aquaria. Unless injured in capture or transportation, there are few species outside the deep seas which cannot adapt themselves to life in a well-constructed aquarium.
Reproduction of Lost Parts.—Fishes have little power to reproduce lost parts. Only the tips of fleshy structures are thus restored after injury. Sometimes a fish in which the tail has been bitten off will survive the injury. The wound will heal, leaving the animal with a truncate body, fin-rays sometimes arising from the scars.
[Pg 151] ”

“ Agassiz found that the study of fossil fishes exhibits a remarkable parallelism between the development of the individual and that of the class in geologic time. During part of the embryonic life of fishes, and even in some adult forms, the dorsal cord exists as a simple gelatinous cylinder, surrounded by a fibrous sheath, in which, after a time, there is found a cartilaginous and then an osseous deposit, which goes to form the vertebrae, the ossification taking place first in the apophyses. ”

“ The term opisthure is suggested by Ryder for the exserted tip of the vertebral column, which in some larvæ (Lepisosteus) and in some adult fishes (Fistularia, Chimæra) projects beyond the caudal fin. The urosome, or posterior part of the body, must be regarded as a product of evolution and specialization, its function being largely that of locomotion. In the theoretically primitive fish there is no urosome, the alimentary canal, as in the worm, beginning at one end of the body and terminating at the other.
Fig. 70.—Gephyrocercal tail of Mola mola (Linnæus) . (After Ryder.) ”

“ All of the fins are supported by a skeleton of rays, either horny or spinous in character.
There are but two chambers to the heart, and the occurrence of a swim bladder, an organ for regulating or recognition of the depth of the water in which the fish is, is very frequent. Externally fish have a defensive armor of scales or bony plates embedded in the skin. They breathe throughout life by means of gills, which are delicate folds or filaments connected with openings (gill slits) in the sides of the throat. The fishes live in water, and reproduce their species by eggs. ”

“ No other fish shows finer proportions in the shape of its body. Every “line” of its build is designed and eminently adapted for rapid progression through the water; the muscles massed along the vertebral column are enormously developed, especially on the back and the sides of the tail, and impart to the body a certain rigidity which interferes with abruptly sideward motions of the fish. ”

“ If a vertebrate fish should die a natural death, which of itself must be a rare occurrence, the carcass would soon be devoured whole or bit by bit by other creatures near by. Possibly the lower jaw, or the teeth, spines, etc., in the case of sharks, or a bone or two of the skeleton, might be buried unbroken, but a whole vertebrate fish entombed in a modern deposit is surely a unique occurrence. ”

“ The cartilaginous tribe, which have gristles instead of bones, unite the principal of both the other classes in their conformation: like the cetaceous, they have organs of hearing, and lungs: like the spinous, they have gills, and a heart without a partition. From the structure of their gills, these animals are enabled to live a longer time out of water than other fishes. The cartilaginous Shark, or Ray, lives some hours after it is taken; while the spinous Herring, or Mackarel, expires a few minutes after it is brought on shore. Some of this class bring forth their young alive ”

“ Similar impressions may be obtained from the bodies of delicate invertebrate animals 10 but they are seldom preserved because of the softness of the tissues. The smaller fishes provide much better material for the production of fossils according to this method. While the fish is being flattened by the weight of surrounding sediments, scales, fins, and soft bones retain their positions and provide the necessary resistance to leave an impression of the body form when the flesh is gone. ”

“ When they rise to the surface of the water they are either dead or in a helpless and dying condition. The ligaments and tissues by which the bones and muscles were held together whilst the fish lived under the immense pressure of great depths have then become loosened and torn by the expansion of the internal gases; and it is only with difficulty that the specimens can be taken entire out of the water, and preserved afterwards. Every specimen found has been more or less mutilated ”

“ The period of change of form varies widely in the different groups of fishes, from a few weeks' to longer than a year's duration; and the extent of the changes that the larva undergoes are often surprisingly broad, investing every organ and tissue of the body, the immature fish passing through a series of form stages which differ one from the other in a way strongly contrasting with the mode of growth of amniotes; since the chick, reptile, or mammal emerges from its embryonic membranes in nearly its adult form. ”