CRUSTACEA. Ill 



eye-stalks; 2, the upper antennae (or jointed 

 horns) ; 3. the lower antennas ; 4. the mandibles (or 

 jaws) ; 5, 6. the two pairs of maxillae (or secon- 

 dary jaws) ; 7, 8, 9. the three pairs of foot-jaws, 

 one of which belongs to the head, the other two 

 to the thorax; 10 14. the five pairs of true feet; 

 15 19. the five pairs of false or swimming feet ; 

 20. the swimming plates of the fan-like tail; 21. 

 the minute styles at the very extremity of the body. 

 All these members can be separated in the case of 

 the common Prawn.* 



The body in these animals is encased in a skin 

 which, from the deposition in its substance of salts 

 of lime, acquires a considerable consistence. In 

 some of the lower forms, it may be compared to 

 very thin horn ; but in the larger Brachyura, as in 

 the Eatable Crab, every one knows that it is almost 

 of a stony hardness. Hence the name of CRUS- 

 TACEA. The strength of resistance thus obtained 

 is niost concentrated in the carapace, that great 

 buckler which in the most familiar species covers 

 the thorax, and protects the vital organs. In many 

 forms, however, as all those which constitute the 

 Order Edriophthalma, there is no carapace. 



Theoretically, every segment is formed of two 

 arches or semicircles, each of which consists of a 

 pair of central and a pair of lateral pieces. Of 

 these it is necessary to specify here only the 

 lateral pieces of the upper arch, called epimera; 

 since they are often well-marked, and furnish im- 

 portant distinctive characters, as in the Tribe 

 Gammaracea. 



Almost all the members of the Class, and, of 



* See Bell's " British Crustacea " (Introduction), p. xx. 



