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.
(1727). Every branchial arch consists of several pieces (57,58, 59, 60, 61), so joined together by ligaments that the whole is perfectly flexible; and their edges are studded with little osseous plates, generally armed with teeth, and so disposed as to prevent food taken into the mouth from being forced out through the branchial fissures with the issuing streams of water; so that, in reality, these pieces fulfil in their way the same office as the epiglottis of Mammalia.
The last parts found to enter into the composition of this portion of a fish's skeleton are called, from their position, the pharyngeal bones. They are placed immediately behind the branchial apparatus, and form a second set of masticatory organs, generally even more efficient than the jaws themselves, being for the most part provided with very strong teeth.
(1729). In the Perch there are eight of these bones, situated just at the entrance to the oesophagus, - two inferior (56), and six above (62): their office and efficiency as organs of mastication must be obvious to the most superficial observer.
(1730). Upon reviewing the general disposition of the skeleton in one of the osseous fishes, it is at once apparent that the great instrument of locomotion is the tail, which by extensive and vigorous lateral movements sculls the body rapidly along through the yielding element in which these creatures live. In the construction of the caudal extremity of the skeleton, every precaution has evidently been taken to convert this part of the body into a broad and expanded oar, possessed of the utmost possible flexibility in the lateral direction. No pelvis, therefore, trammels the movements of the spine, neither do any transverse processes limit the extent of flexion from side to side; while, on the contrary, the extraordinary development of the spinous processes, both above and below, and more especially the vertical caudal fin, give an extent of surface proportionate to the wants of the animal.
(1731). The dorsal and anal fins, situated upon the mesial plane, steady, and perhaps, in some measure, direct the movements of the body; while the arms and legs, or, rather, the pectoral and ventral fins, which are in this case of secondary importance as locomotive instruments, exhibit a very rudimentary condition, and are but feeble agents in progression.
(1732). The posterior extremities, or ventral fins, are even less efficient than the pectoral in this respect, and their position is found to vary remarkably in different orders. In the Perch, these organs are, as we have seen, attached to the bony framework of the shoulders. In the Carp tribe (Cyprinidae) they are removed far back towards the commencement of the tail, and the bones supporting them are merely imbedded in the muscles of the abdomen. In the Cod (Gadidae) the legs are absolutely in front of the arms, being suspended under the throat; and in the Anguilliform fishes, the Eel for instance, the ventral extremities are altogether wanting.
(1733). Such being the imperfect development of the usual locomotive organs, we are quite prepared to expect a corresponding modification in the disposition and efficiency of different parts of the muscular system. When we compare the muscles of a fish with those of any of the higher Vertebrata, the contrast is indeed very striking.
Fig. 314. Myology of the Perch. (After Cuvier).
(1734). Delicate muscles (fig. 314) are provided for the erection or depression of the different rays sustaining the dorsal and ventral fins, and thus the fins themselves are expanded or folded up at pleasure. Similar fasciculi spread out or approximate the rays of the tail, increasing or contracting at will the extent of surface presented by that organ. The muscles of the pectoral and ventral limbs are small in proportion to the feebleness of these extremities; the muscles of the trunk alone constitute the great bulk of the body, and form the efficient agents in progression.
(1735). These great lateral masses commence at the back of the head, where they take an extensive attachment to the largely-developed cranium: from this point backwards they fill up the entire space intervening between the skin and the vertebral column, with both of which they are intimately connected, reaching even to the origin of the tail fin. The whole force of these powerful muscles is evidently exerted in bending the spine from side to side, and in effecting those vigorous lateral movements of the tail whereby the fish is propelled through its liquid element. We need, therefore, feel little surprise at the strength with which this part of the body of fishes is not unfrequently endowed, or at the velocity of their movement - at seeing how easily their speed outstrips our fleetest ships - how the Flying-fish (Exocetus), urged on by fear, darts like an arrow to a distance through the air - or how the Salmon, in obedience to an imperious instinct, defies even the thundering cataract to stop its course towards the locality where it is instructed by Nature to deposit its eggs.
(1736). There are sundry tribes of Fishes which, being destined to remain at the bottom of the sea, present certain peculiarities of structure, whereby they are not only distinguished from all others of the class, but form most remarkable exceptions to the general law in accordance with which the Vertebrata are organized.
(1737). The animals presenting this anomalous configuration are the Pleuronectidae, or Flat-fishes, as they are generally termed, which when at rest lie quietly upon the ground, where, from the colour of the upper part of their bodies, they are scarcely distinguishable. To an ordinary observer the Pleuronectidae would seem to have their bodies flattened and spread out horizontally, so that, while resting upon their broad and expanded bellies, their eyes, situated upon the back of the head, are thus disposed for the purpose of watching what passes in the water above them; and this, the vulgarly-received opinion, is considerably strengthened by the fact that what is usually called the belly is white and colourless, while the back is darkly coloured and sometimes even richly variegated, so as to harmonize with the prevailing tints of the sea-bottom. Hence the appearance of these fishes is deceptive, and few imagine that, in applying the terms back and belly to the upper and under surfaces of a Plaice or a Turbot, they are adopting a phraseology quite inadmissible in an anatomical point of view.