180 



ONYCHOPHORA. 



occurs early, is introduced by the shifting forward of the first pair 

 of primitive segments to the extreme anterior end of the body, 

 where they become considerably enlarged. A pair of large swellings 

 (cephalic lobes) thus arise at the anterior end ; these soon become 

 marked off from the body by a transverse furrow, and thus constitute 

 the cephalic segment. On the ventral side of these lobes is the oral 

 aperture ; on the dorsal side a pair of prominences appear (Fig. 86 

 A and B) which soon increase in size and become recognisable as the 

 rudiments of the antennae. In P. ccqiensis these are said to appear 

 before the limbs (Sedgwick), but this distinction seems to be of no 

 great significance ; in P. Edwardsii the antennae are said to appear 

 simultaneously with the rudiments of the truncated legs, which they 

 closely resemble. They are, however, distinguished from the latter 

 by their more dorsal and pre-oral position (Figs. 86, 90, and 91). 



In front of the rudiments of the antennae, and lying more 

 medianly, there are, at an earlier stage, two small prominences 

 (Fig. 90, x), which shift later towards the anterior margin of the 

 head (Fig. 94 A and B). These prominences, which were observed 

 by v. Kennel in P. Edwardsii, and the nature of which is as yet 

 unknown, can still be recognised at a later stage than that depicted 



in Fig. 94 B, and disappear 

 ■a from view only when folds 



begin to form in the cephalic 

 integument. We shall refer 

 to them again at the end of 

 this section (p. 187). 



The limbs arise as latero- 

 ventral outgrowths of the 

 segments consecutively from 

 before backward (Figs. 86, 90, 

 and 91). The segmentation 

 of the body is brought about 

 chiefly by the outgrowth later- 

 ally of the primitive segments. 

 The embryo, especially in its 

 lateral parts, thus appears 

 notched (Figs. 86 and 90). 

 The paired nature of the germ-band is still indicated by the presence 

 of a median ventral furrow (Fig. 89). This especially applies to 

 P. Edwardsii, in which also the limbs appear later than in the 

 African and Australian species. This retardation is no doubt due 



K. 



-m. 



— n.. 



_/— . H. 



Fig. 80.— Embryo of P. Edwardsii. A, ventral, 

 and B, lateral aspect (after v. Kennel), a, 

 anus ; W, blastopore ; m, mouth ; n, umbilical 

 cord. 



