ARTHROPODS. CLASS CRUSTACEA 99 
form of minute larve, provided with three pairs of append- 
ages, a median eye (Fig. 56), and a firm external skeleton 
or cuticle. This latter prevents the continuous growth of 
the larve or nauplius, and every few days it is thrown off, 
and while the new one is forming the body enlarges. Dur- 
ing this time new appendages are developed, so that after 
each moult the young crusta- 
cean emerges less like its 
former self and more and more 
like its parents. In the bar- 
nacles, after several moults 
have taken place, the young 
become permanently attached 
by means of their first anten- 
nex, their thoracic feet change 
into feathery appendages, and 
several other changes occur. 
In some of the parasitic bar- 
nacles (Sacculina) the larva 
attaches itself to a crab, throws 
off its various appendages, and, a: 
after other great degenerative Fie. 56.—Development of a barnacle 
5 (Lepas). a, larva; 6, adult. 
changes, enters its host. For 
a time, therefore, their development is toward greater com- 
plexity, but the later stages constitute a retrograde meta- 
morphosis. 
100. More complex types.—The larger, more useful, and 
usually more familiar Crustacea belong to the second divi- 
sion (subclass Malacostraca). It comprises such animals as 
the shrimps, crayfish, lobsters, crabs, and a number of other 
forms which are at once distinguished from the preceding 
by the constant number of segments composing the body. 
Of these, five constitute the head, eight the thorax, and 
seven the abdomen. The head segments are always fused 
together, and with them one or more thoracic segments 
unite to form a more or less complete cephalothorax. Also, 
