158. A variety of appendages are attached to these rings, such as jointed legs, or in place of them stiff bristles, oars fringed with silken threads, wings either firm or membranous, antennæ, movable pieces which perform the office of jaws, &c. But however diversified this solid apparatus may be, it is universally the case that the rings, to which every segment of the body may be referred as to a type, combine to form but a single internal cavity, in which all the organs are enclosed, the nervous system, as well as the organs of vegetative life, (63.)

159. The muscles which move all these parts have this peculiarity, that they are all enclosed within the more solid framework, and not external to it, as in the vertebrates; and also that the muscular bundles, which are very considerable in number, have the form of ribbons, or fleshy strips, with parallel fibres of remarkable whiteness. Figure 27 represents the

Fig. 27.

disposition of the muscles of the caterpillar which destroys the willow, (Cossus ligniperda.) The right side represents the superficial layer of muscles, and the left side the deepseated layer.

160. The Vertebrata, like the articulated animals, have solid parts at the surface, as the hairs and horns of mammals, the coat of mail of the armadillo, the feathers and claws of birds, the bucklers and scales of reptiles and fishes, &c. But they have besides this, along the interior of the whole body, a solid framework not found in the invertebrates, well known as the SKELETON.

161. The skeleton is composed of a series of separate Jones, called vertebræ, united to each other by ligaments.

a

Each vertebra has a solid centre with four branches, two of which ascend and form an arch above, and two descend, forming an arch below the body of the vertebra. The upper arches form a continuous cavity (a) along the region of the trunk, which encloses the spinal marrow, and in the head receives the brain, (61.) The lower arches (b) form another cavity, similar to the superior one, which contains the organs of nutrition and reproduction; their branches generally meet below, and when dis. joined, the deficiency is supplied by fleshy walls. Every part of the skeleton may be reduced to this fundamental type the vertebra, as will be shown, when treating specially of the vertebrate animals; so that between the pieces composing the head, the trunk, or the tail, we have only differences in the degree of development of the body of the vertebra, or of its branches, and not in reality different plans of organization.

Fig. 28.

162. The muscles which move this solid framework of .he vertebrata are disposed around the vertebræ, as is

well exemplified among the fishes, where there is a band of muscles for each vertebra. In proportion as limbs

are developed, this intimate relation between the muscles

and the vertebræ diminish

es. The muscles are un

equally distributed and are concentrated

about the

limbs, where the greatest amount of muscular force is required. For this reason, the largest masses of flesh in the higher vertebrates are found about the shoulders and hips; while in fishes they are concenrated about the base of the

Fig. 30.

tail, which is the part principally employed in locomotion.

TION.

SECTION II.

OF LOCOMOTION.

163. One of the most curious and important applications of this apparatus of bones and muscles is for LOCOMOBy this is understood the movement which an animal makes in passing from place to place, in the pursuit of pleasure, sustenance, or safety, in distinction from those motions which are performed equally well while stationary, such as the acts of respiration, mastication, &c.

164. The means which nature has brought into action to effect locomotion under all the various circumstances in which animals are placed, are very diversified; and the study of their adaptation to the necessities of animals is highly interesting in a mechanical, as well as in a zoological point of view. Two general plans may be noticed, under which these varieties may be arranged. Either the whole body is

equally concerned in effecting locomotion, or only some of its parts are employed for the purpose.

165. The jelly-fishes (Medusa) swim by contracting their umbrella-shaped bodies upon the water below, and its resistance urges them forwards. Other animals are provided with a sac or siphon, which they may fill with water and suddenly force out, producing a jet, which is resisted by the surrounding water, and the animal is thus propelled. The Bîche-le-mar, (Holothuria,) the cuttle-fishes, the Salpæ, &c., move in this way.

Fig. 31.

166. Others contract small portions of the body in succession, which being thereby rendered firmer, serve as points of resistance, against which the animal may strive, in urging the body onwards. The earth-worm, whose body is composed of a series of rings united by muscles, and shutting more or less into each other, has only to close up the rings at one or more points, to form a sort of fulcrum, against which the rest of the body exerts itself in extending forwards.

167. Some have, at the extremities of the body, a cup or some other organ for maintaining a firm hold, each extremity acting in turn as a fixed point. Thus the Leech has a cup or sucker at its tail, by which it fixes itself; the body is then

elongated by the contraction of the muscular fibres which encircle the animal; the mouth is next fixed by a similar sucker and by the contraction of muscles running lengthwise the body is shortened, and the tail, losing its hold, is brought forwards to repeat the same process. Most of the bivalve mollusks, such as the clams,

Fig. 32.

move from place to place, in a similar way. A fleshy organ, called the foot, is thrust forward, and its extremity fixed in the mud, or to some firm object, when it contracts, and thus draws along the body and the shell enclosing it. Snails, and many similar animals, have the fleshy under surface of their body composed of an infinitude of very short muscles, which, by successive contractions, so minute, indeed, as scarcely to be detected, enable them to glide along smoothly and silently, without any apparent muscular effort.

168. In the majority of animals, however, locomotion is effected by means of organs specially designed for the purpose. The most simple are the minute, hair-like cilia, which fringe the body of most of the microscopic infusory animalcules, and which, by their incessant vibrations, cause rapid movements. The sea-urchins and star-fishes have little thread-like tubes issuing from every side of the body, furnished with a sucker at the end. By attaching these to some fixed object, they are enabled to draw or roll themselves along; but their progress is always slow. Insects are distinguished for the number and great perfection of their organs of motion. They have at least three pairs of legs, and usually wings also. But those that have numerous feet, like the centipedes, are not distinguished for agility. The Crustacea generally have at least five pairs of legs, which are used for both swimming and crawling. The Worms are much less active; some of them have only short bristles at their sides. species 'use their fringe-like gills for paddles. (Fig. 33.) 169. Among the Vertebrata, we find the greatest diversity in the organs of locomotion and the modes of their applica tion, as well as the greatest perfection, in whatever element

Some of the marine

Fig. 33.