STRUCTURE OF INSECTS AND ARACHNIDA.

105

life, which was interrupted at a very early period. In some tribes, however, the general form is the same from the first, and the wings are the only parts deficient; these gradually

make their appearance, and the insect is then complete. Such is the case with the Grasshopper and Cricket; and a change of this kind is termed an incomplete metamorphosis.

98. The animals of the class ARACHNIDA, which includes the spiders, scorpions, and mites, are, like Insects, articulated, breathing air, and possessing legs, but the number of these legs is never less than eight; there is an entire absence of wings, and the head is united with the thorax, so that the body seems to be formed of two principal divisions,-the cephalo-thorax (as it is termed), and the abdomen. In fig. 46 we have a representation of the arrangement of the parts con

106

STRUCTURE OF ARACHNIDA AND CRUSTACEA.

tained in these cavities. At ct is seen the cephalo-thorax opened from below, and giving attachment to the legs; at m is shown the place of the mandibles or jaws; at p is seen one

of the palpi, which are appendages to the mouth; pa is the foremost leg; t, the large nervous mass, from which the legs are supplied; a, the collection of ganglia supplying the abdomen; a b, the abdomen; p a, the respiratory chambers; ss, the stigmata or openings into these; 7, the leaf-like folds within them (§ 323); m a, the muscles of the abdomen; a n, the termination of the intestine; f, the spinnerets; o, the ovaries; and or, the opening of the oviduct.

99. The class of CRUSTACEA, of which the Crab, Lobster, and Cray-fish are the best-known forms, differs from both the preceding, in being adapted to breathe by means of gills, and thus to reside in or near water, instead of inhabiting the air. Moreover, the body is inclosed in a hard covering, which generally contains a good deal of carbonate of lime, and which is thrown off at regular intervals. This covering also incloses the members, which are never less than ten in number, and are frequently more numerous. There is great variety of form among the animals of this group, which is altogether one of great interest.-In the Crab tribe, the head, thorax, and abdomen are all drawn together, as it were, into one mass; and the general arrangement of the organs it contains is exhibited in the succeeding figure, which shows them nearly as they are found to lie, when the upper part of the shell, or carapace, is removed. At t there is left a portion of

fo

STRUCTURE OF CRUSTACEA.

107

the membrane which lines the carapace and covers in the viscera. On the central line, at c, is seen the heart, which in the Crustacea is large and powerful in its action; from it there passes forwards the artery a o, which supplies the

eyes

and the front of the body; whilst the artery a a passes to the lower and hinder parts; at b are seen the gills of the left side in their natural position; whilst at b' are seen those of the right side, turned back to show their under-surface, and to disclose the lower portion of the shell, fl. At e is seen the stomach, situated close behind the mouth; and at m are pointed out its powerful muscles, by the action of which the food is ground down. The bulky ovary is seen on either side of the stomach; and the space between this and the edge of the shell is occupied by the very large liver, fo.

100. In most of the Crustacea, however, the body is more prolonged. In some, as the Lobster, there is an indication of a division of the body into three parts, representing the head, thorax, and abdomen of insects; whilst in others, as the Sand

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STRUCTURE AND METAMORPHOSIS OF CRUSTACEA.

hopper, the rings or segments are almost as similar to each other as they are in the centipede tribe. There is no class in which we find the same parts exhibiting so great a variety of forms, and rendered subservient to so many uses. Thus in the Crab and Lobster the members of the first pair are not used for walking, but form the claws or arms by which the food is seized; in the Cray-fish, these members may be used either as legs or claws; whilst in the Sand-hopper, they closely resemble the other legs. And the jaws of the higher Crustacea, of which there are several pairs, are really metamorphosed legs; as may be seen by comparing them with the corresponding appendages of the Limulus or king-crab, the first joints of which act as jaws, whilst the remaining portions of these members serve either as legs for locomotion, or as claws for prehension.

A

101. Most of the Crustacea, like insects, come forth from

the egg in a state very different from their adult form; and afterwards undergo a series of changes, which are in some instances so remarkable as to approach the complete metamorphosis of insects, and which end in the production of the complete form. An early form of the common crab, at a time when it is of the minute size indicated on the scroll, is shown in fig. 48. The immature Crustacea of different tribes bear much more resemblance to each other, than do the forms into which they are ultimately to be developed; and the differences they afterwards present are chiefly due to a variety in the amount Fig. 48.-ZOEA, OR LARVAL of growth which the different parts

FORM OF THE CRAB.

undergo.

102. It is one of the most remarkable results of modern zoological research, that in immediate connexion with the class of Crustacea, if not as actual members of it, we have to place a group of animals which were for some time associated with the Mollusca; their bodies being inclosed in shells, which do not fit closely around them, nor give more than a general protection to their members. This group is

STRUCTURE OF CIRRHIPEDA.

109

the Barnacle tribe, forming the class CIRRHIPEDA, or tendrilfooted animals. They agree with the lower Mollusca, in being fixed to one spot during all but the earliest period of their lives; the shell being sometimes attached by a long membranous or leathery tube, as that of the Barnacle (fig. 49); and sometimes being itself fixed on the surface of

a rock, or on another shell, as is that of the Balanus or acorn-shell. In both cases, the form and structure of the animal are essentially the same. When taken from the shell (in which it lies doubled up, as it were) and spread out, its articulated nature is evidenced by its division into segments, and by the regularity of the arrangement of their tendril-like appendages. These are not formed like legs, since they could be made no use of, the animal being incapable of moving from place to place; but they serve to produce currents in the surrounding water, by which food is brought to the mouth, and the blood is submitted to the influence of a fresh supply of air. The nervous system of this group is formed precisely upon the plan of that of the Articulata generally (§ 94): and if any doubt could have remained as to its true place in the series, it is removed by the knowledge of the fact, that the animals composing it bear a strong resemblance in their early condition to some of the lower Crustacea, possessing eyes and legs, and swimming freely about; and that they attain their adult form by passing