204 



CRUSTACEA. 



appendages, which however are devoid of the special masticatory 

 hook found in the Branchiopodan Nauplius. The mandibles {md} 

 consist of the basal blade and a long unjointed oar-shaped palp. 

 The oral aperture is overhung by a large upper lip {pi). In the 



thoracic section of the body the 

 {P T -P VI ) are visible in the form 



Fig. 97.— Nauplius stage of Limnetis brachyura (after Grube). a", 

 second antenna ; k, masticatory process of the same ; md, 

 mandible. 



rudiments of six' pairs of limbs 

 of transverse swellings. In the 

 cephalic region,, 

 the Nauplius eye 

 alone is recog- 

 nisable, the com- 

 pound eyes de- 

 veloping at a 

 later stage. Fur- 

 ther metamor- 

 phosis takes place- 

 by the develop- 

 ment of the limbs 

 and of the body- 

 segments, the 

 growth of the 

 dorsal shield, the diminution of the upper lip, the degeneration of 

 the mandibular palp, and the acquisition of the characteristic 

 curvature of the body. The Nauplius eye persists in the generation 

 coming from the winter egg, whereas it is wanting in individuals 

 developing from summer eggs. 



The genus Leptodora, in the metamorphosis of the winter eggs 

 and in the 

 Branchiopoda- 

 like Metanau- 

 plius stage, as 

 well as in many 

 other respects, 

 has thus re- 

 tained primi- 

 tive characters 

 among the 

 Cladocera. 



Fio. 98. — Metanauplius stage of Leptodora hyalina developed from a 

 winter egg (after G. O. Saks), a 1 , first, a", second antenna; md, 

 mandible; pl-pVl, rudiments of the first six pairs of thoracic 

 limbs; ol, upper lip. 



The summer 

 and winter eggs 



differ not only with regard to the development of the embryo, but in other 

 points as well. It has already been indicated (p. 114) that, in Bythotrcphes and 

 Leptodora, the type of cleavage of the summer egg differs from that of the winter 



