100 ANIMAL FORMS 
some of the head segments give rise to a great fold of the 
body-wall, the carapace, which extends backward and covers 
all or a part of the thorax, with which it may firmly unite, 
as in the crayfish. The appendages are usually highly spe- 
cialized, and are made to perform a variety of functions. 
101. The shrimps.—Among the simplest of these are the 
opossum-shrimps (Fig. 57) and their relatives, small trans- 
Fic. 57.—The opossum-shrimp (Mysis americana). 
parent creatures often seen swimming in great numbers at 
the surface of the sea or hiding among the seaweeds along 
the shore. In general appearance they resemble crayfishes 
or prawns, but are readily distinguished by the two-branched 
thoracic feet. This “split-foot” character also occurs 
among many of the preceding Crustacea, and is generally 
a badge of low organization, tending to disappear in the 
more highly organized forms. In this and other respects 
the shrimps are especially interesting in their relation to 
the preceding Crustacea, and in the fact that they may 
closely resemble the ancestors of the modern prawns (Fig. 
58), lobsters, crayfishes, and crabs. 
102. Crayfishes and lobsters——The last-mentioned spe- 
cies and their allies, usually large and familiar forms, con- 
stitute a group known as the decapods (meaning ten feet), 
referring to the number of thoracic feet. Among the mem- 
bers of this division probably none are more familiar than 
the crayfishes, which occur in most of the larger rivers and 
their tributaries throughout the United States and Europe. 
It is their habit to remain concealed in crevices of rocks 
