598 



ZOOLOGY 



SECT. 



which afterwards spreads over the whole yolk. But in most 

 cases the egg is centrolecithal and segmentation superficial, as 

 in Astacus. 



Development is always accompanied by more or less metamor- 

 phosis. In most Branchiopoda the young is hatched in the form of 

 a nauplius (Fig. 429, A), and further changes are of the same char- 

 acter as in Apus. In Cladocera development is direct, the nauplius- 

 stage being passed through in the egg, and the young hatched in 

 a form closely resembling the adult. In one of the Cladocera, 

 however, Lcplodora (Fig. 449, 3), while development of the summer 

 eggs is indirect, the winter eggs give rise to free nauplii. In the 

 Ostracoda the nauplius is peculiar in having a bivalved shell and 

 all three pairs of appendages uniramous. In all the Copepoda 

 there is a free nauplius, which, in the parasitic forms, leads a 

 free existence for a time, and then attaches itself to its particular 

 host and undergoes retrograde metamorphosis. 



In the Cirripedia, also, there is a free nauplius, the body of which 

 is often produced into long spines. After several moults, the 



FIG. 474. Cypris-stage of Lepas fascicularis. a'j. abdomen ; pa. paired eye ; rj, thoracic 

 feet ; ua, unpaired eye ; 1, antenmile. (From Lang's Comparative Anatomy, after Claus.) 



nauplius passes into a form called the Cypris-stage (Fig. 474), 

 characterised by the presence of a bivalved shell, like that of an 

 Ostracod: the antennules (1) also have become modified into organs 

 of adhesion by the development of the penultimate segment into a 

 disc, the antennae have disappeared, and six pairs of swimming-feet 

 like those of a Copepod have made their appearance : there are 

 paired compound eyes, and the shell is closed by an adductor 

 muscle. After leading a free existence for a time, the Cypris- 

 larva attaches itself by its antennules, aided by the secretion 

 of cement-glands, and becomes a pupa : the carina, terga, and 

 scuta appear beneath the shell, and within the skin of the mouth- 

 parts and legs of the pupa appear the corresponding appendages 



