THE COPEPODA. 467 



although the number of limbs and body segments is still much less. 

 For only the rudiments of the third and fourth pairs of natatory feet 

 have made their appearance in the form of cushions fringed with 

 setae, and the body consists of an oval cephalo-thorax (head and 

 thorax combined), the second, third, and fourth thoracic segments, 

 and an elongated terminal joint. Many forms of the Crustacea 

 which are parasitic upon other creatures do not attain a further 

 development than this, although they can reproduce their kind, and 

 some do not even reach this stage of, comparatively speaking, 

 "under" development. But all the rest, and most of the parasites, 

 pass through a longer or shorter series of stages of development, in 

 which the limbs acquire a higher stage of division into joints. The 

 posterior pairs of feet are developed, and the last thoracic and 

 the different abdominal segments are successively separated from 

 the common terminal portion. (Claus and Fritz Miiller.) 



Very little is known as yet concerning the systematic growth of 

 the Ostracoda. The genus Cypris, a species of which {Cypris fused) 

 is represented on page 465, is found in fresh water, and nearly all of 

 the members of the genus Cythera frequent the sea. All have a 

 horny cretaceous covering of two valves exactly resembling a mi- 

 nute mussel shell (Baird). Claus has shown that the eggs produce 

 Natiplius forms, which are, however, shell bearing, and that, after 

 many moultings, the adult shape is attained. The eye, the two 

 pairs of limbs, the antennae, and the tail-end of the Cypris fusca, 

 represented in the engraving, belong to a full-grown adult. The 

 general shape of these Entromostraca should be kept in mind, for 

 it will be referred to when considering the metamorphosis of the 

 Cirripedes. 



Many Crustacea have the mouth prolonged in the shape of a 

 sucker, and one order of these {Haustellata) not only has kinds 

 classified under it which are very unusual and extraordinary in 

 shape, but the limbs even are more or less rudimentary. 



The metamorphoses of the sucking and parasitic Crustacea 

 belonging to this order, which frequent salt and fresh water fish 

 of all kinds, are very remarkable, not only because the change 

 of external appearance is great during the successive phases of 

 Nauplius, Zoea, and adult life, but because there is a manifest 

 retrograde transformation ; the structures of the active larva being 



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