148 



CRUSTACEA. 



B. Arthrostraca and Cumacea. 



In the Arthrostraca, that region of the egg where later the 

 embryonic rudiment develops is often indicated, even during cleav- 

 age, by the small size of the blastomeres, or by the earlier formation 

 of the blastoderm. A germ-disc forms which is at first round, but 

 soon spreads out into a germ-band covering the whole ventral surface 

 of the egg, and occasionally also extending dorsally over the anterior 



and posterior ex- 

 ZJ Tf ^-«— -^ tremities (Fig. 



74 A) The an- 

 terior end of the 

 germ-band is indi- 

 cated by paired 

 enlargements, the 

 cephalic lobes (Fig. 

 69, o, p. 138), 

 from which the 

 rudiments of the 

 eyes and of the 

 brain arise, while 

 the germ -band 

 itself soon appears 

 divided up by 

 furrows into the 

 separate body-seg- 

 ments (Fig. 74 B). 

 This segmentation 

 and the first ap- 

 pearance of the 

 pairs of limbs 

 takes place from 

 before backwards; 

 the segmentation, 

 however, fre- 

 quently occurs almost simultaneously in all regions of the body. 

 As the lengthening germ-band has no room for free development, 

 a characteristic curvature soon comes about. In the Isopoda, the 

 curvature, which is originally dorsal (Fig. 74 B), only gives place 

 to the reverse curvature at the end of embryonic life, whereas, in 

 the Amphipoda, such a ventral curvature appears at an earlier 



Fio. 73.— Three stages in the development of parasitic Copepoda 

 (after Van Beneden). A, Nauplius stage of Brachiella Thynni. 

 B, older embryo of Anchorella, with the limbs of the first 

 Cyclops stage. C, Cyclops stage of Hcssia colorata. a>, first 

 antenna ; «", second antenna ; d, food-yolk ; en, entoderm (wall 

 of the mid-gut) ; k, germ-band ; m, mandible ; mx, first maxilla ; 

 mfl, first maxillipede ; mf", second maxillipede ; oe, oesophagus ; 

 ol, upper lip ; p', pU, first and second pairs of thoracic (rowing) 

 limbs. 



