100 STRUCTURE OF FISHES. a Fish or as a Reptile, so complete is the mixture of characters which it presents. 88. The class of FISHES is distinguished from all other Vertebrata, by the adaptation of the animals composing it to breathe by means of water in their adult state, so as to be capable of living in that element only. Like Reptiles, they are oviparous and cold-blooded; and in these characters they differ completely from the Whales and other Mammals, which are, like them, inhabitants of the great deep, but which are warm-blooded, viviparous, and air-breathing animals. There is a simple external character, by which the members of the two classes may be at once distinguished. The animals of the Whale tribe are, like fishes, chiefly propelled through the water by means of a flattened tail; but in the former the tail is flattened horizontally, so that its downward stroke may serve to bring the animal to the surface to breathe; whilst in Fishes it is flattened vertically, that its strokes from side to side may simply propel the fish through the water. A flattening or compression of the body is seen more or less in almost all fishes, and is intimately connected with the nature of their motion through the element they inhabit; as it serves the double purpose of diminishing the resistance which is offered to their progress, and of increasing the extent of the oar-like surface, by the lateral stroke of which the body is propelled forwards (Chap. XII.). This stroke is given by a series of muscles of great power, which pass from the prolonged extensions of one vertebra to those of another, and altogether make up the principal part of the bulk of the animal. The fins which represent the limbs are not so much used in propelling the Fish, as in changing its direction either laterally or vertically. Thus in the lowest group of the Vertebrated series, the act of motion is chiefly performed by the vertebral column itself, instead of being committed to the limbs, as in Mammals, Birds, and most Reptiles. The larger number of Fishes swim with great activity; and their lives may be said to be passed in seeking their subsistence and in flying from their enemies. 89. Fishes are for the most part very voracious, and their food consists in great part of the members of their own class. In seeking it, they appear to be chiefly guided by the sight; for their eyes are usually large and highly developed, while STRUCTURE OF FISHES. 101 the other organs of sense are formed upon a very inferior type. They swallow it without much division in the mouth; but it seems to undergo rapid digestion. The blood of some Fish, whose muscular activity is peculiarly great, is rich in red corpuscles, and of a temperature not much lower than that of Mammals; but, generally speaking, it contains much less solid matter than that of the warm-blooded Vertebrata, and its temperature follows that of the surrounding medium. 90. Although Fishes breathe by gills instead of by lungs, these gills are connected with the mouth, so that the water which passes over them is received into it, in the same manner as the air is in the higher Vertebrata. This is a character which distinguishes the position of the gills of fishes from that of the corresponding organs of any of the inferior tribes. They are lodged in a cavity on each side of the throat; and this cavity opens outwardly, either by one large valve-like aperture on either side, or by several; through these apertures the streams of water which have been taken in by the mouth, and forced over the gills by the action of its muscles, make their exit. 91. All Fishes are oviparous; and the number of eggs which they produce is generally prodigious. It is very seldom that after the eggs have been deposited and fertilized, the parents take any further concern in regard to them; though there are a few instances in which a kind of nest is made, and others in which the egg is retained and hatched within the body, so that the young comes forth alive. This last is the case with the Sharks and Rays, which, notwithstanding that their skeleton is cartilaginous, are higher than Fishes generally in several other parts of their organization. 92. All the animals which are destitute of a vertebral column are called Invertebrata; and this division into the Vertebrated and Invertebrated groups was formerly regarded as the first step in the classification of the animal kingdom. But it was pointed out by Cuvier, that in the Invertebrated division are comprehended three groups, of which the members differ as much from one another as they do from Vertebrated animals; and that each of these ought, therefore, to rank with the first, as a primary division. This is evident to those who are but slightly acquainted with the structure of the animals already named (§ 69) as characteristic speci 102 GENERAL STRUCTURE OF ARTICULATA. mens of these divisions; and it will become more apparent as we proceed. 93. In the second division, that of ARTICULATA, or Articulated (jointed) animals, we find a conformation very different from that which has been just described. The exterior of the body is still perfectly symmetrical, as in the Vertebrata; and the interior is even more symmetrical; for the organs that represent the heart and lungs are equally disposed on the two sides of the central line of the body. But the skeleton, instead of being internal, is external; and is composed of a series of pieces jointed together, which form a casing that includes the whole body. In general, these pieces are very similar to each other; so that the whole body appears like the repetition of a number of similar parts, as we see in the Centipede (fig. 42). The limbs are usually very numerous, where they exist at all; and they have a jointed covering, like that of the body. But in the lower tribes of this group, such as Leeches and Worms, the limbs or members are but slightly developed, or are altogether absent; and in the highest, which approach most nearly to the Vertebrata in their general organization, the number of members is much reduced,—although it is never less than six. The hard matter of which the external skeleton is composed, undergoes little or no change when it is once fully formed; and, in order to accommodate it to the increasing size of the animal, this covering is thrown off and renewed at intervals during the period of growth. Fig. 42. CENTIPEDE. 94. The nervous system consists of a series of separate ganglia, which are arranged in a cord or chain along the central line of the body. There is usually a pair of large ganglia in the head, bearing a resemblance (in their peculiar connexion with the eyes) to the ganglionic centres of the optic nerves in Vertebrata; and there is commonly one for each segment or division of the body, from which the nerves pass to supply its muscles, as they do from the spinal cord of Vertebrata. The cord which connects these ganglia is double, GENERAL STRUCTURE OF ARTICULATA. 103 and the ganglia themselves are composed of two halves, which have little connexion with each other. The chain thus formed (fig. 43) passes along the under-side of the trunk of the animal (as seen at g, fig. 44), not on what seems its back; and by the presence of this double chain of ganglia an Articulated animal may be distinguished, even when, in its general structure, it should seem to belong to the group of Mollusca (§ 102). 95. The general arrangement of the organs in the Articulata is shown in the accompanying figure of a Crayfish. The mouth, situated on a projecting head, opens into s, the stomach, from which passes backwards the intestinal tube, i, i, to terminate at the opposite extremity of the body. The upper part of the tube is surrounded by the liver, f, which is here very large. In the head are seen the ganglia, c; and along the under-side of the body is seen the chain of ganglia, Fig. 43.-NERVOUS SYSTEM OF g. Fig. 44.-DIAGRAM SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN THE ARTICULATA. The blood is nearly colourless, and is usually impelled through the body not by a single organ or heart, but by a succession of contractile cavities, one for each segment, which open into one other longitudinally, forming what is known as the dorsal vessel; in the Cray-fish and its allies, however, one part of this, h, is specially enlarged, so as in great degree to serve as a heart for the system generally. The respiratory organs are not connected with the mouth; and are not usually 104 STRUCTURE OF INSECTS. restricted to one part of the body, but are diffused either on its outside or through its substance. 96. The organs of sense, in this group, are less numerous than in Vertebrata, and are inferior in perfection; those of sight are the most developed, and are formed upon a very peculiar plan (§ 573); but all organs of special sense appear wanting in the lowest tribes. Yet we find that the muscular power is very great; for the animals of this group, taken as a whole, can move faster in proportion to their size, and possess greater strength, than those of any other. We observe, too, that with little or no intelligence, they are prompted to the most remarkable actions by instinct alone. They seem to act like machines, doing as they are prompted, without choice, or knowledge of the end to be gained; and consequently the different individuals of the same species have not that difference of capacity and of disposition, which we see in animals whose endowments are higher. 97. In the highest division of the Articulated series, we easily recognise, as forms quite distinct from each other, the Insects, the Spiders, the Crustaceous animals (crabs, lobsters, &c.), and the Centipedes. The class of INSECTS is distinguished, for the most part, by the presence of wings; but to this there are exceptions. It includes those of the higher Articulata, which breathe air by means of air-tubes distributed through the body (§ 320), which have no more than six legs, and whose body, in its perfect form at least, manifests a division into three distinct parts-the head, thorax, and abdomen (fig. 45). To the thorax alone are attached the six legs, as well as the wings; and its cavity is principally occupied by the muscles that move them: the abdomen contains the organs of digestion and reproduction, as in vertebrated animals. In the greater part of this class, the young animal comes forth from the egg in a condition very different from that which it is ultimately to possess; and it undergoes a complete metamorphosis, the larva which the egg produces bearing a close resemblance in form to the lower Articulata, and only attaining the condition of the imago or perfect insect by passing again into a state of inactivity, during which the store of nutriment which it has acquired is applied to the development of new organs. This pupa or chrysalis condition may be considered as a sort of postponed completion of the embryonic |