STRUCTURE AND CLASSIFICATION OP THE ARTHROPODA. 555 



ultimate joint of the ramus, so that the last joint works on it 

 — as^ for instance, in the lobster's claw. Such chelate rami 

 or limb-branches are independently developed in Crustacea 

 and in Arachnida, and are carried by somites of the body 



Fi&. 11. — Diagram to show tlie derivation of tiie unit or "oni- 

 matidium " of the compound eye of Crustacea and Hexapoda, C, 

 from a simple monomeniscous nionosticlious eye resembling the 

 lateral eye of a scorpion, A, or the unit of the compound lateral eye 

 of Limulus (see article Akachnida, Figs. 22 and 23). B repre- 

 sents au intermediate hypothetical form in wiiich the cells beneath 

 the lens are beginning to be superimposed as corneagen, vitrella, 

 and retinula, instead of standing side by side in horizontal series. 

 The black represents the cuticular product of tiie epidermal cells of 

 the ocular area, taking the form either of lens, cl, of crystalline 

 body, cry, or of rhabdom, rhab; hy, hypodermis or epidermal cells; 

 corn, laterally placed cells iu the simpler stage A, which like the 

 nerve-end cells, vit^ and rel^, are corneagens or lens-producing ; 

 cor;;, specialised corneagen or lens-producing cells; vif^, potential 

 vitrella cells with cry, potential crystalline body now indistinguish- 

 able from retinula cells and rhabdomeres ; vit, vitrella cell with cry, 

 its contained cuticular product, the crystalline cone or body ; vit^, 

 rhab^, retinula cells and rhabdom of scorpion undifferentiated from 

 adjacent cells, vii^ ; ref^, retinula cell ; rhab, rhabdom ; «/, optic 

 nerve-fibres. (Modified from Watasse.) 



wliich do not correspond in position in the two groups. The 

 range of modification of which the rami or limb-branches of 

 the limbs of Arthropoda are capable is ver}^ large, and in 

 allied orders, or even families or genera, we often find what 

 is certainly the palp of the same appendage (as determined 



