. Text book of zoology. Zoology. Arthropoda. 185 a certain thinness, so ttat movement can take place at these points. All Arthropods moult* periodically, at least, as long as growth continues; the cuticle loosens from the underlying tissue, breaks at some point, and is cast ofE as a whole {, the animal creeps out of it) after the epidermis has secreted a new cuticle. This is thin and soft at first, but becomes thick and hard later. Such periodic ecdyses are indispensable for growth, for the stiff, un- yielding cuticle allows only of very slight increase in the size of the body. The growth
. Text book of zoology. Zoology. Arthropoda. 185 a certain thinness, so ttat movement can take place at these points. All Arthropods moult* periodically, at least, as long as growth continues; the cuticle loosens from the underlying tissue, breaks at some point, and is cast ofE as a whole {, the animal creeps out of it) after the epidermis has secreted a new cuticle. This is thin and soft at first, but becomes thick and hard later. Such periodic ecdyses are indispensable for growth, for the stiff, un- yielding cuticle allows only of very slight increase in the size of the body. The growth of the animal would therefore cease, if the sur- rounding case were not now and again thrown off and replaced by a new and roomier one. Upon the body, there are larger or smaller tracts of setae, evagina- tions of the cuticle, each contaiaing a process of the soft epidermis; the cuticle at the base of the hair is thinner, so that it can move about. The cuticle consists of chit in, an organic substance^ of a horny appearance, chemically however, quite different from horn. Lime salts, principally carbonate of lime, are often deposited in the chitin, especially in the Crustacea. The skin is never ciliate among the Arthropoda, nor indeed is any other organ; in fact ciliated cells are entirely absent. The muscular system is closely connected with the skin; the formation of a segmented exoskeleton, however, necessitates important deviations, from the Annelid type. Instead of a continuous musculature beneath the skin, there is usually a large number of separate muscles passing from one segment to another, and attached by their extremities to the inner side of the skin : by their contraction the segments of the body, as also the joints of the appendages, move upon one another. The muscles are often connected by the so-called tendons, which, in the Arthropods, always consists of invaginations of the cuticle, sur- rounded of course by a corresponding invagination of the epidermis. mf Pig
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