xi PHYLUM ARTHROPODA 583 



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, Leptodora (Fig. 456, 3), while development of the summer 

 eggs is direct, 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 

 nauplius passes into a form called the Cypris-stage (Fig. 481), 

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

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



oc 





FIG. 481. Cypris-stage of Lepas fascicularis. add. adductor muscle of carapace ; aU, 

 antennules ; caud.f. caudal styles ; ex. excretory organ (shell-gland) ; fix. disc for fixation ; 

 fix. gl. fixing gland ; gn. jaws ; int. intestine ; lab. labrum ; m. mouth ; oc. 1 , simple eye ; 

 oc. 2 , compound eye ; th. thoracic legs. (From MacBride, after Willemoes-Suhm.) 



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 

 of the adult. In Lepas the anterior region of the head grows out 

 into a peduncle. The pupal integument is then thrown off, the 

 paired eyes disappear, and the adult form is assumed. 



In Sacculina a still more extraordinary metamorphosis takes 

 place. The young is hatched as a nauplius, but without alimentary 

 canal, and passes into a Cypris-stage. In this condition, after a 



p p 2 



