THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 183 



eyes (au), as slight ectodermal depressions, are already present as 

 rudiments (P. capensis), situated somewhat ventrally to the antennae. 

 In P. Edivardsii they arise immediately behind the bases of the 

 antennae. 



The further development of the mouth and of the two pairs of 

 limbs connected with it is of special importance. The anterior 

 aperture remaining after the partial closing of the blastopore does 

 not directly give rise to the mouth, but becomes carried inwards by 

 an invagination of the ectoderm, the stomodaeum, and thus forms 

 the aperture between the latter and the enteron.* Neither does this 

 second aperture represent the mouth of the adult, for it becomes 

 covered by various outgrowths of the ectoderm, which form above it 

 a secondary buccal cavity. This process 

 commences by the appearance of a fold 

 on the outer side of the limb next in 

 order to the antenna (Figs. 91 and 92, lc); 

 this fold is closely applied to the limb, 

 and is continued posteriorly along the 

 ventral surface of the embryo (Figs. 91 

 and 93, /, and Fig. 92, p). It appears 

 notched, and, in P. Edivardsii, is repre- 

 sented by a series of papillae lying one close 

 to the other (Figs. 92 and 94). In later 

 stages these two folds shift closer towards 

 the oral aperture, and thus press the 

 limb-rudiments that lie on the inner side 

 of them towards the mouth. As the 

 folds grow still higher, these limbs, to- 

 gether with the stomodaeal aperture, come 



to lie in a cavity, the adult buccal cavity (Fig. 94), the limbs 

 themselves becoming the jaws of the adult. The distal part of each 

 of the limbs, at the time when the formation of the buccal cavity 

 just described begins, appears deeply notched, and the two strong 

 chitinous teeth arise at this part (Fig. 94 A and B). These 

 terminal teeth, which are to be compared with the double claws 

 on each of the legs, prove, even in the adult, that these jaws are 

 true limbs. 



Several other structures contribute to the complete development 

 of the buccal cavity, between the cephalic lobes, and ventrally 

 to them, a somewhat long prominence arises (Fig. 94, ol), which 



* Cf. below, p. 196. 



Fig. 92.— Anterior portion of an 

 embryo of P. Edivardsii, seen 

 from the ventral side (after 

 v. Kennel, from Lang's Text- 

 book of Comp. Anat.). k, jaw ; 

 no, aperture of the nephridium 

 belonging to the segment of 

 the oral papillae (op) ; p, pa 

 pillae of the folds which sur- 

 round the jaws laterally. 



