322 



ZOOLOGY. 



distinct ; the heart also appears, being a pale streak along 

 the middle of the back extending from the front edge of 

 the head to the base of the abdomen. 



Just before hatching the head-region spreads out, the ab- 

 domen being a little more than half as wide as the cephalo- 

 thorax. The two compound eyes and the pair of ocelli on 

 the front edge of the head are quite distinct ; the append- 

 ages to the gills appear on the two anterior pairs, and the 

 legs are longer. 



The resemblance to a Trilobite is most remarkable, as 

 seen in Fig. 273. It now also closely resembles the fossil 

 king-crabs of the Carboniferous formation (Fig. 274, Prest- 

 wicliia rotundatus, Fig. 275, Euproups Dance). 



Fig. 271. Prestwichia, natural size. 

 After Worthen. 



Fig. 275. Euproops, natural size. 

 After Worthen. 



In about six weeks from the time the eggs are laid the 

 embryo hatches. It now differs chiefly from the previous 

 stage in the abdomen being much larger, scarcely less in 

 size than the cephalo-thorax ; in the obliteration of the seg- 

 ments, except where they are faintly indicated on the car- 

 diac region of the abdomen, while the gills are much larger 

 than before. The abdominal spine is very rudimentary ; it 

 forms the ninth abdominal segment. 



The reader may now compare with our figures of the re- 



