94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Surface views at Stage 3 (Figure 3) are given in Figures 9, 10, and 

 11, Ibr. 



A sagittal section at this stage shows (Plate 3, Figure 13) an elon- 

 gation of the labral fundament, and demonstrates its origin from the 

 germ band by simple evagiuation. The posterior surface of the labrum 

 is now the anterior wall of a distinct invagination (or.), the fundament 

 of the storaodseum. 



At Stage 4 (Figure 4) the labrum is longer (Plate 3, Figure 19, Z6r.) 

 and its long axis has swung backward, probably on account of the ex- 

 cessive elongation of the anterior labral surface. A ventral aspect of 

 the germ band (Figure 12) shows the labrum to be approximately oval 

 in cross-section, but with a more rounded anterior surface. 



At Stage 5 (Figure 5) the labrum (Plate 3, Figure 20, Ibr.) is de- 

 cidedly longer. The basal part of the labral fundament represents the 

 clypeus, with which the lateral folds, or mouth-folds (Figure 2\,pli.or.) 

 are now confluent ; overhung by the end of the labrum is the distinct 

 stomodaeum. 



At Stage 7 (Figure 7) a distinct depression (Plate 5, Figure 31, dep.) 

 separates the clypeus from the procephalic lobes ; the depression, in 

 fact, maybe seen as early as Stage 1, for it simply forms the angle be- 

 tween the labral fundament and the procephalic lobes. Although the 

 clypeus merges insensibly into the cheeks, the labrum is a free trapezoi- 

 dal plate, as in the adult (Plate 6, Figure 40, Ihr.). The antennae are 

 now inserted (Plate 4, Figure 24, Plate 5, Figure 30, at.) almost ex- 

 actly opposite the base of the labrum. At this stage the clypeo-labral 

 suture is not distinctly indicated (Figure 24), but in Stage 8 an invagi- 

 nation occurs to form the labral hinge of the adult (Plate 6, Figure 40, 

 ate). 



In Stage 8 the only other important changes in the labrum are the 

 evagination of single hypodermis cells to form the external setae, and 

 the formation of trivial cuticular folds which represent the rudiments 

 of the epipharynx. In Anurida, as in Orchesella, the epipharynx is 

 purely a cuticular structure and unconnected with the central nervous 

 system. 



In the adult Anurida a shallow clypeo-frontal groove is distinguish- 

 able (Plate 6, Figure 40, suL), but does not amount to a suture, and 

 the clypeus is not laterally demarcated from the groove. In Orchesella 

 and Tomocerus, however, the clypeus is a distinct sclerite. In none of 

 the Collembola that I have studied is tliere any distinction between 

 clypeus and labrum on the roof of the pharynx. 



