XI 



PHYLUM ARTHROPODA 



545 



en,. 4- ' 



considered as endopodite and exopodite. But in all the other limbs, 

 as we have seen, the exopodite springs from the second segment 

 of the axis, and the probabilities are 

 that there is no exact correspondence 

 between the parts of the antennule and 

 those of the remaining appendages. 



The eye-stalks, already noticed, 

 arise just above the antennules and 

 are formed each of a small proximal 

 and a large distal segment. They are 

 sometimes counted as appendages 

 serially homologous with the an- 

 tennae, legs, &c. But, as we have seen 

 in the case of Apus, the appendages 

 of Crustacea are always formed in 

 regular order from before backwards ; 

 the eye-stalks, on the other hand, 

 always appear later, both in individual 

 development and in the Crustacean 

 series, than the normal anterior ap- 

 pendages. They are therefore more 

 properly to be looked upon as arti- 

 culated processes of the prostomium, 

 developed in connection with the need 

 for an increased range of vision. As- 

 suming this to be the case, it will be 

 seen that the body of the Crayfish 

 consists of a prostomium, nineteen 

 metameres, and a telson. The pro- 

 stomium bears eye-stalks: the first 

 five metameres are fused with the 

 prostomium to form the head, and 

 bear the antennules, antenna?, man- 

 dibles, first maxillae, and second 

 maxilla? : the next eight metameres 

 (5th-12th) constitute the thorax, and 

 bear the three pairs of maxillipeds and 

 the five pairs of legs : the remaining 

 six metameres (13th-18th), together 

 with the telson, constitute the ab- 

 domen, and bears five pairs of pleo- 

 pods and one of uropods. 



The articulation of the various 

 podomeres of the appendages is on 



the same plan as that of the abdominal segments (p. 541). The 

 podomeres are, it must be remembered, rigid tubes : they are 

 connected with one another by flexible articular membranes 



VOL. I N N 



Fir.. 432. Portion of a legof Astacus, 

 with the exoskeleton partly re- 

 moved, showing articulations and 

 muscles, art. m. articular mem- 

 brane ; en. 2 5, podomeres of endo- 

 podite ; I'.i-t. extensor muscles ; I. 

 flexors ; h. hinge. 



