OSTRACODA. 



205 



-egg. Sars (No. 30) has pointed out in this connection that the embryos that 

 develop (in ephippia) in the winter eggs remain during the whole duration of 

 development enveloped in the egg-integument ("chorion"), while the summer 

 eggs throw off the very thin vitelline membrane before reaching the last stage of 

 development. 



The number of pairs of thoracic limbs which we found as rudiments in the 

 Metanauplius was six, and this must be considered as the primitive number in 

 the Cladocera. This number is also retained in the Sididae. In the embryo of 

 Lynceus six pairs appear as rudiments, but the last of these degenerates later 

 (Claus, No. 8). 



4. Ostracoda. 



The Ostracoda, which are provided with a bivalve shell, often 

 impregnated with calcareous salts, undergo more or less marked meta- 

 morphosis. These animals were probably derived from Phyllopodan 

 ancestors by a process of degeneration tending in a definite direction, 

 the number of segments being reduced, and the powers of locomotion 



#* SM * 



Fig. 99. — Two larval stages of Cypris (after Claus, from Balfour's Text-book). A, Nauplius 

 stage. B, second larval stage. A ', first, A", second antenna ; /", first pair of feet; Md, 

 mandible ; OL, upper lip ; Mx', first maxilla ; SM, adductor muscle. 



diminished. In Cypridina, the Cytheridae, and the Halocypridae, 

 whose development is as yet unknown to us, the metamorphosis 

 appears to be to some extent abbreviated, since the larva when 

 hatched has almost the form of the adult; the Cypridae, on the 

 contrary, undergo a metamorphosis, passing through many moults 

 from the Nauplius stage to the adult form. This metamorphosis has 

 been observed by Claus (Nos. 32 and 34) in the cases of Cypris 

 fasciata, C. ovum, and G. vidua. Reckoning by the number of 

 consecutive moults, there are here nine different stages, some of 

 which, however, differ only slightly. 



The Nauplius (Fig. 99 A) resembles the Phyllopodan larva at the 

 same stage in that the three pairs of limbs are used as locomotory 

 organs, and in the possession of a large helmet-shaped upper lip {OL), 

 but in having a well-developed bivalve shell provided with an 



