96 ANIMAL BIOLOGY 



This welter of Arthropod forms, as already suggested, is built 

 on the plan of a chain of segments; two or more of the anterior 

 segments constituting the head, with the mouth, and the posterior 

 one containing the anus. In the simplest Arthropods there is 

 relatively little differentiation between either the segments or 

 the characteristic pair of jointed appendages that each bears; but 

 proceeding to more complex forms, one finds a progressive union 

 and specialization of segments in certain regions of the body and 

 a shifting and transformation of their appendages and internal 

 organs for one function or another. Indeed it would seem that 

 all the possible changes are rung on the pervading segmental 

 chain: a fact to be illustrated later when we study the Crayfish. 

 (Figs. 62, 63.) 



Another conspicuous feature of the phylum is the presence of 

 a hard, unyielding external armor, or exoskeleton, with flexible 

 joints moved by attached muscles. This skeleton hampers the 

 increase in size of the inhabitant, so periodically it is shed — the 

 animal moults. Seizing the opportunity, so to speak, the animal 

 rapidly increases in size at the expense of material stored for this 

 purpose, and also secretes a new skeleton. Of course, a ' soft-shelled ' 

 Crab is one which has recently moulted and has been taken at a 

 disadvantage before the newly secreted skeleton has had time to 

 harden. 



The life history of many of the Arthropods is so surprisingly 

 complex, involving such radical form changes, that it is termed a 

 metamorphosis. Thus the embryo of certain Crustacea may be 

 hatched as an unsegmented larva, then after moulting assume a 

 segmented larval form, and so on until the adult state is attained. 

 In other Crustacea one or more of these stages may be briefly 

 summarized, as it were, in the egg before hatching. Finally, 

 animals like the Crayfish hatch with essentially the adult form. 

 And this series of metamorphic stages in the development of the 

 higher Crustacea is of considerable theoretical interest, because 

 they are very similar to the larval or adult forms of certain other 

 Crustacea that are regarded as more primitive in organization. 

 Thus it would seem that individual development in the higher 

 Crustacea briefly and very broadly and incompletely summarizes 

 — recapitulates — the ancestral, or evolutionary, history of the 

 race. 



However, metamorphosis is called to our attention more promi- 



