DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 345 



still very short, its posterior end being flexed downwards (Dohrx, 

 cf. Fig. 156, ab). The embryo now becomes covered with a delicate 

 cuticular integument, which is cast off by ecdysis in the next stage 

 and then comes to lie within the space which is enclosed by the 

 blastodermic cuticle. 



The next stage (Fig. 158) is characterised by the transverse 

 splitting of the chorion, the two halves of which remain for a long 

 time attached to the egg as a hemispherical shell. The embryo 

 moves within the space enclosed by the blastodermic cuticle, which 

 is now increased by the sea-water taken into it. The cuticle is thus 

 stretched, and the cell-like mosaic on its surface disappears. The 

 limbs now gradually develop more in the direction of their final 

 C 



Fig. 157. — Embryo of Liviulus (after 

 Watase). 1-6, the six thoracic limbs ; 

 o„ first abdominal limb (operculum) ; 

 a z , second abdominal limb; ab, abdo- 

 men ; do, dorsal organ ; an, position of 

 the rudiment of the lateral eye 

 (Watase) ; c, blastodermic cuticle ; x, 

 exopodite of the sixth thoracic limb. 



Fio. 158.— Limulus embryo, lateral view (after 

 Kingsley). 1-6, the six thoracic limbs; «,, 

 first abdominal limb (opt- rculum) ; a 2 , second 

 abdominal limb ; ab, abdomen ; th, chilaria ; 

 do, dorsal organ ; x, exopodite of the sixth 

 thoracic limb. 



shape, their segmentation and the rudiments of the pincers becoming 

 apparent. The outer appendage of the coxa (x) of the sixth pair of 

 limbs, usually regarded as the exopodite, appears. Behind these 

 limbs the rudiments of the paired lower lip, known as the chilaria 

 (ch, metastoma), become apparent close to the middle line of the 

 body ; since this appendage has no ventral ganglion of its own, and 

 does not correspond to any mesoderm-segment (Kingsley, No. 14), 

 it should not be regarded as a limb.* An indication of segmentation 

 now appears in the abdominal region (Figs. 157, 158, ab). 



* [According to Kishinouye, this pair of appendages has a ventral gangHmi ,,| 

 its own, and is therefore to be considered as a pair of limbs. Kingsley (No. III.) 

 believes that Kishinouye mistook the operculum for the metastoma. — Ed.] 



