This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1738). On examining the skeleton of a Flat-fish, we at once see that what we suppose to be the dorsal and ventral regions are in reality the two sides, which are thus strangely different in colour, and that the great peculiarity of their structure is the want of symmetry between the lateral halves of the body, arising from the anomalous circumstance that both the eyes are placed upon the same side of the head. Their cranium, indeed, is composed of the same bones as that of an ordinary fish; but the two lateral halves are not equally developed, and the result is such a distortion of the whole framework of the face, that both the orbits are transferred to the same side of the mesial line of the back.
Fig. 315. Osteology of the Flounder (Pleuronectes flesus).
(1739). The position of the pectoral and ventral fins slightly participates in this want of symmetry; but in other respects the skeleton (fig. 315) precisely corresponds with that of the generality of osseous fishes. The superior and inferior spinous processes of the vertebrae are amazingly developed, and the interspinous bones (74) of inordinate length; so that the vertical diameter of the body is disproportionately increased, and the animal is obliged to swim and rest upon one side. The dorsal fin (75) runs along the whole length of the back; the anal fin (a) reaches from the large spines that form the posterior boundary of the abdomen to the tail, which latter holds the same position as in other tribes: so that the reader will have little difficulty in comparing the different pieces of the skeleton of the Flounder (Pleuroneetes fliesus) with the corresponding bones of the Perch already described.
(1740). The skeletons of the cartilaginous fishes (Chondropterygii*) will require a distinct notice, inasmuch as they present very remarkable peculiarities of no inconsiderable interest. In the Sharks, Skates, and other genera belonging to this important division of the great class we are now considering, the interior of the bones remains permanently cartilaginous; but the skeleton is in some regions incrusted, as it were, with osseous granules. No centres of ossification from which radiating fibres of bony matter progressively extend themselves, as is the case in the osseous fishes, are ever developed; and consequently the skull, although it presents externally the same regions, eminences, and apertures that are usually met with, is never divided into separate bones, but is formed of a single mass of cartilage, in which no sutures or lines of division are ever distinguishable.
(1741). The face is likewise much more simple in its structure; for, instead of the numerous pieces composing the palato-temporal region of the Perch (§ 1714), two bones only are met with, one of which, the palatine, performs the office of an upper jaw and supports the teeth, while the other connects the lower jaw with the cranium. The lower jaw itself, moreover, consists of but one piece on each side, to which the teeth are attached.
(1742). From the peculiar conformation of the respiratory apparatus, which will be explained hereafter, there is no occasion for any opercular flap; this, therefore, is not present: nevertheless the hyoid and branchial arches resemble pretty much those of osseous fishes; only the latter are situated further backwards, being placed quite behind the skull, under the commencement of the spine.
(1743). The bones of the shoulder are represented by a strong cartilaginous zone, which in Sharks is quite unconnected with the vertebral column, but in the Skate (Raia) it is fixed to two large lateral apophyses derived from the spine (fig. 316.) The zone, representing the scapulary apparatus, consists of a single piece, which surrounds the body, and on each side supports the bones of the fore-arm. The enormously-developed pectoral fin is composed of the carpus, amazingly augmented in size, and of the no less remarkable hand, which in the Skate is made up of an immense number of fingers or rays, and forms by itself nearly half the circumference of the body.
(1744). The pelvis, or cartilaginous framework that supports the hinder extremities, i. e. the ventral fins, is a single transverse piece of cartilage quite detached from the rest of the skeleton: it expands on each side into a broad plate, to which the fin, the representative of the foot of higher animals, is appended; and likewise in the male it gives attachment to additional organs called claspers, the use of which will be explained in another place.
Fig. 316. Cartilaginous skeleton of the Skate.
(1745). The anterior portion of the spine in the Skate is not as yet divided into distinct pieces; and even in the posterior part, the number of vertebral arches is twice as great as that of the separate bodies of the vertebrae.
(1746). In all the Chondro-pterygii the ribs are mere rudiments, and in some cases can scarcely be said to exist at all.
(1747). The Sturgeons (Stvri-onidae) form a kind of connecting link between the osseous and cartilaginous fishes; and in them a large swimming-bladder exists, from which is obtained the valuable material called isinglass: but in the Sharks and Rays this organ is not found; consequently, especially in the tribe last mentioned, it is only by means of the vigorous flappings of their enormous hands that these ground-fishes are able to raise themselves from the bottom. The disposition and relative importance of different parts of the muscular system is therefore necessarily changed to meet these altered circumstances: the muscles of the trunk, which in osseous fishes formed the great agents in locomotion, become now of secondary importance; while those of the pectoral fins, so feebly developed in the Perch, are massive and powerful in proportion to the unwieldy size of the anterior extremities. Another peculiarity in the skeleton of the Chondropterygii is observable in the construction of the caudal fin, which, even in the Sturgeon and the Shark, notwithstanding the importance which this organ still maintains in those genera as an instrument of locomotion, begins to differ very remarkably from the tail of an osseous fish.