CLADOCERA. 



203 



ventral chain of ganglia, the presence of a long chambered dorsal vessel, 

 and perhaps also the lamellate character of the limbs. The fact that 

 the great groups of the Malacostraca through their most primitive 

 member (Nebalia) join on to the Branchiopoda, will still further 

 incline us to see in the latter the now living representative of a series 

 of forms which are least removed from the hypothetical ancestor of 

 the Crustacea (Dohrn, No. 9). On the 

 other hand, Ave must not forget that 

 the existing Branchiopoda (Phyllopoda) 

 in many respects seem undoubtedly to 

 have undergone great secondary modifi- 

 cation. In the first place we must here 

 recall the reduced form of the oral limbs 

 (mandibles and maxillae). With regard 

 to these we shall have to turn to the 

 larvae of the Copepoda and the Mala- 

 costraca, so as rightly to complete the 

 picture of the hypothetical racial form. 

 Nevertheless, in forming a judgment of 

 the phylogenetic relations of the Crus- 

 tacea, we shall often have to return to the 

 Phyllopoda as a very primitive group. 



Fig. 96. — Dorsal aspect of the 

 larva of Limnadia (after Lere- 

 boullet). a", base of the 

 second antenna ; d, intestine ; 

 md, mandible ; ol, upper lip ; 

 s, rudiment of the right shell- 

 valve. 



B. Cladocera. 



Whereas the Branchiopoda pass through 

 a metamorphosis characterised by many 



moults, larval ecdyses are altogether wanting in their relations, the 

 Cladocera, the young animal leaving the egg with a shape resembling 

 that of the adult. The whole modelling of the body is here shifted 

 back to the embryonic stages, among which a distinct Nauplius 

 stage can be recognised ; this is in many cases even marked by a 

 moult — development of the Xauplius cuticle — (Fig. 72, p. 147). 



Special peculiarities of shape, however, are met with in the newly- 

 hatched young of the remarkable genus Leptodora. Whereas the 

 young coming from the summer egg resemble the adult in shape 

 (P. E. Muller), the larva hatched from the winter egg resemble an 

 advanced Metanauplius (Fig. 98), and have therefore another stage 

 to pass through (G. 0. Sars, No. 29). The body in this Meta- 

 nauplius is long, without outwardly recognisable segmentation, and 

 ends posteriorly in the two furcal processes. The first antennae (a') 

 are short and club-shaped, the second (a") long biramose swimming 



