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.
(1663). To whatever portion of the animal world we turn our attention, we find the lowest and least-perfectly organized tribes to be inhabitants of the water. To dwell upon the land necessarily demands no inconsiderable share of strength and activity - limbs sufficiently strong to support the weight of the body, muscles possessed of great power and energy of action, acute and vigilant organs of sense, and, moreover, intelligence and cunning proportioned to the dangers or necessities connected with a terrestrial existence.
(1664). The inhabitant of the waters, on the contrary, although less highly gifted, may be fully competent to enjoy the position it is destined to occupy. Being constantly buoyed up on all sides by a dense element, it is easily supported at any required altitude without much muscular effort; but feeble limbs are needed to guide its path through the water, and slight impulses suffice to impel it forward. Thus, therefore, in Pishes we are prepared to expect, a priori, that, as far as strength and compactness of structure are concerned, they will be found inferior to other Vertebrata.
(1665). We are likewise justified in anticipating that, in intelligence, and in the relative perfection of their senses, Fishes should be less highly endowed than the other vertebrate classes. Plunged in the immeasurable depths of the ocean, whereunto no sound can ever penetrate - dwellers in the realms of eternal silence, where even the roar of the storm is lost, vivid and distinct perceptions of sound can be little needed. Surrounded by a turbid element, through which the rays of light with difficulty make their way, the sphere of vision must necessarily be extremely limited. Immersed in a fluid but little adapted to distribute odorous particles, a refined sense of smell would be a useless provision. Taste, if it exists at all, must be blunted to the utmost, from the circumstances under which fishes seize and swallow prey; and even the sense of touch, in animals encased in scales and deprived of prehensile limbs, can only be exercised in a vague and imperfect manner.
(1666). With such inferiority in their powers of communication with the external world, and with faculties so circumscribed, we might justly infer that, as relates to their intellectual powers, Fishes hold a position equally debased and degraded. Destitute of the means of social intercourse, deprived of all sympathy even with individuals of their own species, friendless and mateless, the fish is denied even the privileges of sexual attachment; the female for the most part ejects her countless eggs into the sea, as heedless of the male that blindly fecundates them as she is careless of the progeny to which they give birth. Thus, to pursue and destroy their prey constitutes their chief enjoyment during life, and to be devoured at last is the great end of their existence.
(1667). We shall commence our account of the anatomy of Fishes by an examination of the internal skeleton which forms the framework of their bodies. The reader has already seen, in the Cephalopoda, the first appearance of an osseous system in the cartilaginous pieces described in the last chapter, and will necessarily expect that, between the rudi-mental condition which characterizes the cephalic ring of the Cuttle-fish, and the complete and perfect skeleton of the Fish, various gradations of development will occur as we advance progressively from lower to more elevated forms of the finny race. Nor in this will he be deceived. The lowest tribes of Fishes possess a skeleton but little superior in its organization to that of theCephalopod: in the Myxine and Lamprey the cranium is still cartilaginous; and even the spinal column, not yet divided into vertebrae, resembles a cartilaginous cord extending from the head to the tail. Even in the Sturgeon, the Skate, and the Shark, the skeleton is but very partially ossified; and thus we are gradually and almost imperceptibly conducted to the strong and bony framework of the typical Fishes.
(1668). But the most curious instance of gradation between the true Fishes and the Mollusca is met with in the Amphioxus. The Amphioxus is met with in all the European seas, but is more especially abundant in the Mediterranean. Its usual residence is upon banks of sand, where it finds both shelter and abundance of nourishment. Like the Ascidians, it seems to feed entirely upon infusorial organisms, either animal or vegetable, which abound in the localities that it frequents, and which it swallows, just as the Ascidians do, by the instrumentality of the vibratile cilia with which its mouth and branchial chamber are richly provided. Such is the activity of its movements, that, when dug up from its hiding-place in the sand, if left loose for a single instant, it buries itself again with astonishing rapidity, and thus almost instantaneously eludes the grasp of those who attempt its capture. Although decidedly a member of the vertebrate series of animals, the Amphioxus can hardly be said to possess a skeleton, so soft is the condition of those tissues which, from their arrangement, evidently represent this structure; still it is not difficult to point out the arches of the lower jaw (fig. 309, a) and of the branchial apparatus (d), as well as the structure and position of the spinal column.
(1669). One of the most interesting features in the anatomy of the Amphioxus is, that the canal which encloses the medulla spinalis presents anteriorly no cranial expansion, but the dorsal cord representing the spine extends quite from one extremity of the body to the other, projecting both behind and before considerably beyond the lateral muscles of the body, and extending anteriorly considerably further forward than the oral apparatus (a), or the anterior termination of the spinal cord.
(1670). The mouth (fig. 309) is surrounded by a cartilaginous ring, composed of several pieces, each of which gives off a prolongation to support the cirri that surround the oral orifice. The buccal cavity is lined with mucous membrane, and is densely ciliated, - the ciliary action forcing continuous currents of water towards the branchial chamber (d), or the branchial canal, as it is called by Muller, which, being continued backwards, terminates in the commencement of the alimentary canal (e).