DEVELOPMENT OF THE ORGANISM
and similarly provided with joints or hinges. Each phylum is divided into classes, the members of which agree in their fundamental plan. Thus the Arthropoda (if we exclude certain smaller groups) are divided into three classes (1) the Insecta, in which there is a pair of feelers in front of the mouth and in which the body consists of three parts, the head, the thorax, and the abdomen; (2) the Arachnida (the scorpions, spiders, and mites), in which there are no feelers, but only a little pair of pincers in front of the mouth, and in which none of the legs are changed to jaws; and (3) the Crustacea, in which there are two pairs of feelers in front of the mouth and several jaws. These three broad classes are divided into orders, the orders into tribes, the tribes into families, the families into genera, and the genera into species. The Malacostraca form an order of Crustacea in which the body is divided into just twenty segments, and this order is classified into two main divisions, the Macrura, or the longtails (the shrimps, prawns, and lobsters), in which the abdomen is long, stretched out, and the Brachyura, or the shorttails (the crabs), in which the abdomen is reduced to a useless vestige and carried under the rest of the body. But there is an intermediate tribe, the Anomura, which includes the hermit crabs. A member of this tribe has an abdomen of moderate length, and many of them have the extraordinary habit of thrusting it into an empty snail shell, in consequence of which habit it has become curved and asymmetrical. Now no one who has studied animals of this type doubts that they are modifications of the type having a straight, symmetrical abdomen. If we trace the course of the development of these twisted-bodied crabs we find that when they are young they swim in the sea like little shrimps and that during this stage of their lives the abdomen is perfectly straight; so that the life history confirms the con-
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