DEVELOPMENT OF THE ORGANISM 



and similarly provided widi joints or hinges. Each phylum 

 is divided into classes, the members of which agree in their 

 fundamental plan. Thus the Arthropoda (if we exclude cer- 

 tain smaller groups) are divided into three classes (l) 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 long- 

 tails (the shrimps, prawns, and lobsters), in which the abdo- 

 men is long, stretched out, and the Brachyura, or the short- 

 tails (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 extraordi- 

 nary habit of thrusting it into an empty snail shell, in conse- 

 quence of which habit it has become curved and asymmetri- 

 cal. 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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