Chap. XII.] THE CRAYFISH. 235 



matically in F. Each of these processes meets the expanded 

 head or capital of an endosternite (c. en. st.). So that the head 

 of each endosternite is connected with the anterior horizontal 

 process of one endopleurite and the posterior horizontal process 

 of another endopleurite, as indicated in F., which, it must be 

 remembered, only represents the upper surface of the endo- 

 phragmal system, and that quite diagrammatically. On com- 

 paring these diagrams with the real thing prepared by boiling 

 with dilute caustic potash, the essential structure will, it is 

 hoped, be understood. The minute details are beyond our 

 scope. 



The Appendages. There are twenty pairs of appendages 

 which are serially-homologous or homodynamous. They are 



1 pair of eyestalks or ophthalmites. 1 



1 antennules. 



1 antennae. ^ Cephalic. 



1 mandibles. 



2 ,, maxillae. 



3 maxillipeds. -\ 



1 chelate forceps or pincers. f 



2 chelate legs. f Thoracic. 

 2 ., simple legs. / 



6 ,, swimmerets, of which the pos- ) 



terior are much enlarged, f AbdommaL 



By comparison and careful study a typical or ideal appendage 

 may be conceived from which all the others may be derived, 

 according to Huxley's law, by suppression, by coalescence, or by 

 metamorphosis. 



The student should carefully remove the appendages from 

 one side, and lay them in order on a sheet of white paper. In 

 removing the legs, forceps, and maxillipeds, care must be taken 

 not to tear away the gill from its attachment to the base of the 

 limb. The first maxilliped and the maxillae are exceedingly thin 



1 The ophthalmite is, by some morphologists, not regarded as a homologue of 

 the other appendages. 



