344 



PALAEOSTKACA. 



mouth changes its position, shifting further back, so that it comes 

 to lie behind the chelicerae, between the second pair of limbs 

 (Fig. 156). Between the oral and anal apertures, which, according 

 to Kingsley (No. 14) are peculiarly elongate, runs the neural groove 

 (Fig. 155, ?i), which occupies the place of the primitive groove, that 

 has now disappeared. The whole of the long germ-disc is bordered 

 laterally by a thickened wall (Fig. 156, r), in which we recognise 

 the first rudiment of the cephalo-thoracic shield. The first rudi- 

 ments of the most anterior pair of limbs of the abdomen (a,, 

 operculum) very soon appear. Even at this stage, on either side 

 and beyond the limits of the germ-disc, on the level of the fourth 

 thoracic segment, two rounded thickenings (Fig. 155 B, do), can 

 be recognised; these probably correspond to the so-called dorsal 

 organ (Watase). 



The following stage (Fig. 156) shows the thoracic limbs more strongly 

 developed and each bent on itself inwards and downwards. There 

 are now the rudiments of two pairs of leaf-like abdominal limbs 



(a v a 2 ) which are distin- 

 guished from the thoracic 

 limbs by their form, by 

 their position near the 

 median line, and by the 

 manner of their appearance. 

 They become separated 

 from the germ-disc by an 

 infolding of the body-sur- 

 face taking place below and 

 behind them (Kingsley). 

 At this stage, Packard 

 thought that he could 

 recognise an indication of 

 the boundaries of the seg- 

 ments on the lateral parts 

 of the yolk near the 

 germ-disc ; this, however, 

 Kingsley was unable to 

 confirm. 

 The embryonic rudiment lies, up to this time, as a gradually 

 extending flat disc on the sphere of yolk. The latter now begins 

 to change into the dorsal half of the embryo by the absorption of 

 the nutritive materia] (Fig. 157). The abdominal region (ab) is 



PlO. r,<;.— Embryo of Limulus (after Dohrn). 1-6, 

 the six pairs of thoracic limbs ; a,, fust abdominal 

 limb (operculum); a.,, second abdominal limb ; 06, 

 Abdomen flexed rentrally; brj > ventral chain of 

 ganglia ; c, blastodermic cuticle ; d, chilaria ; w, 

 mouth ; r, margin of the future cepbalo-thoracic 

 shi.ld ; x, exopodite of the sixth thoracic limb. 



