STRUCTURE AND CLASSIFICATION OF THE ARTIIROPODA. 553 



parapodia of Cli;i3fcopoda with cirilii and braiicliial lobe 

 (fig. 7). It is by the specialisation of two ''endites^^ that 

 the endopodite and exopodite of higher Crustacea are formed, 

 whilst a flabelliform exite is the homogen or genetic equiva- 

 lent of the epipodite (see Lankester, '^ Observations and 

 Reflections on Apus cancriformis/' ' Q. J. Micr. Sci.'). 

 The reduction of the outgrowth-bearing " corm ^' of the 

 parapodium of either a Chastopod or an Arthropod to a simple 

 cylindrical stump, devoid of outgrowths, is brought about 



Etv^ 



Ene 



Fig. 9.— The second thoracic (fiftJi post-oral) appendage of the 

 left side of Apus cancriformis, placed with its ventral orneural 

 surface uppermost to compare witli Figs. 7 and 8. 1, 2, the two 

 segments of the axis ; en^ , the gnathobase ; en- to en^\ the live 

 followiug "endites;"/, the flahellum or anterior exite; br, the 

 bract or posterior exite. (After Lankester, ' Q. J. Micr. Sci.,' vol. 

 xxi, 1881.) 



when mechanical conditions favour such a shape. We see it 

 in certain Chastopods (e. g. Hesione) and in the Arthropod 

 Peripatus (fig. 8). The conversion of the Arthropod's limb 

 into a jaw, as a rule, is effected by the development of an 

 endite near its base into a hard, chitiuised, and often toothed, 

 gnathobase (see figs. 9 and 10, en^). It is not true that all 

 the biting processes of the Arthropod limb are thus produced, 

 — for instance, the jaws of Peripatus are formed by the axis 

 or corm itself ; whilst the poison-jaws of Chilopods, as also 

 their maxilla), appear to be formed rather by the apex or 

 terminal region of the ramus of the limb; but the opposino- 

 jaws (= hemignaths) of Crustacea, Arachnida, and Hexapoda 

 are gnathobases, and not the axis or corm. The endopodite 

 (corresponding to the fifth endite of the limb of Apus, see 

 fig. 9) becomes in Crustacea the " walking leg " of the mid- 



VOT-. 47, PART 4. — NKW SERIES. N N 



