CRUSTACEA. 



225 



animals, the Trilobites (fig. 53), which recent investiga- 

 tions have shown to be 

 crustaceans, but which can- 

 not be more definitely 

 placed within that group. 

 They agree with neither 

 Entomostraca nor Malacos- 

 traca in their structure. 

 They have a flattened body, 

 in which head, thorax, and 

 abdomen are readily dis- 

 tinguished, and in which 

 both thorax and abdomen 

 consist of an axial portion, 

 and two lateral regions or 

 lobes, whence the name of 

 the group. The head bears 

 a pair of compound eyes, 

 a single pair of antenna?, 

 and four pairs of append- 

 ages, which served at once for walking and for taking 

 food. Each segment of thorax and abdomen supports a 

 pair of two-branched appendages. Trilobites appear in 

 the earliest fossil-bearing rocks, and the group died out 

 soon after the period of coal-formation (in the Permian). 



.! ?. 



FIG. 53. Restoration of the under 

 surface of a Trilobite, showing the 

 appendages. After Beecher. 



SUBCLASS II. MALACOSTRACA. 



This group contains the larger and higher Crustacea, in 

 which the body consists of twenty somites,* all of which 

 except the last (telsori) may bear appendages. Com- 

 pound eyes are usually present; and the nauplius stage 



* Twenty-one in Nebalia. 



