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- 



FIG. 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 



