folsom: mouth-parts of axurida mapjtima. 93 



post-retinennes, la lame ganglionnaire, le chiasma externe, la masse 

 medullaire externe, le chiasma interne et la masse medullaire interne. 



" Toutes ces parties, si nettement caracterisees, se retronvent sans 

 modification chez I'lnsecte ; il n'est done pas donteux qu'il existe au 

 moins pour cette premiere region du ganglion optique similitude com- 

 plete entre les deux types que nous cherchons a comparer. . . . Cette 

 similitude a ete reconnue par tous ceux qui se sont occnpes de ce sujet 

 (Berger, Bellonci, Carriere et moi). . . . En somme, au point de vue 

 des parties dont nous venons de parler, il n'existe que des differences 

 bien pen iraportantes entre I'lnsecte et le Crustace : chez le premier, les 

 deux lobes cerebraux sont tres rapproches et se soudent sur la ligne 

 mediane ; chez le second, ces memes parties (appelees balles superieures) 

 sont ecartees, chacune d'elles etant logee daus le pedoncule oculifere 

 correspondant." 



Packard '98, p. 51) says, "Hence the ocular segment, i. e., that 

 bearing the compound and simple eyes, is supposed to represent the first 

 segment of the head. This, however, does not involve the conclusion 

 that the eyes are the homologues of the limbs, however it may be in 

 the Crustacea." As Viallanes has proved the equivalence of protocere- 

 brum and optic nerves in insects with those of Crustacea, and others 

 have shown that the compound eyes of both groups are constructed 

 alike, even to the number of retinal elements, it is proper to infer that 

 the compound eyes of the two groups are homologous. 



The protocerebrum of Collembola and Thysauura agrees in develop- 

 ment and structure with that of other insects and also with decapod 

 Crustacea ; the facetted eyes of Hexapoda and Crustacea are likewise 

 homologous. 



Labrum and Clypeus. 



The labrum is chiefly interesting because it has frequently been held 

 to represent a pair of primary appendages. 



At Stage 1 (Plate 1, Figure 1 ; Plate 2, Figures 8, 8', Ibr.) the 

 labrum (really clypeo-labrum) is a median hemispherical papilla anterior 

 to and distant from the bases of the antennae ; at no period does it give 

 evidence of a paired origin. 



At Stage 2 (Figure 2), while the distances between the labrum and 

 mandibles is precisely the same as in the preceding stages, the antennse 

 are inserted beside the oral region of the upper lip ; the latter is globular 

 and flattened against the egg shell. 



