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 Muller.) 
Very little is known as yet concerning the systematic growth of 
the Ostracoda. The genus Cypris, a species of which ( Cypris fusca) 
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 
Nauplius 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 E 7 itromostraca 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 
E E 2 
