LESSON XXVII 



THE CRAYFISH 



THE Starfish has furnished us with an example of an 

 animal in which an obvious radial symmetry is, as it were, 

 superposed upon an original bilateral symmetry : in which 

 also there is an extremely simple form of nervous system, 

 a unique type of locomotory apparatus, and no trace of 

 metameric segmentation. We have now to study, in the 

 crayfish, an animal formed upon quite the same general 

 plan of structure as Polygor'dius as to segmentation, arrange- 

 ment of organs, &c., but which reaches, in every respect, a 

 far higher grade of organisation. 



The Common British Fresh-water Crayfish is Astacus 

 fiuvialilis : allied species occur in Europe, Asia, and 

 America. The following description will apply almost 

 equally well to the Lobster, Homarus vulgaris. 



The body of the crayfish (Fig. 81) is divided into two 

 regions, an anterior, the cephalothorax, which is unjointed 

 and is covered by a cuirass-like structure, the carapace, and 

 a posterior, the abdomen, which is divided into distinct seg- 

 ments, movable upon one another in a vertical plane. The 

 cephalothorax is again divided into two regions, an anterior, 

 the head (cth\ and a posterior, the thorax (kd), by a trans- 



