ARTHROPODA 
trial in their habits, living on the land, usually in damp places; 
in several respects their structure approaches 
that of the Insects; the mandible has no 
palp, and the exopodites of two or more of the 
abdominal limbs are sometimes provided with 
tubular air passages, which are respiratory ; 
they show, however, no detailed resemblance 
to the tracheae of the higher Arthropods. 
The first antennae are in many species quite 
rudimentary. Many of them, like the SpHAE- 
ROMIDAE, possess the power of rolling themselves 
up in a ball when disturbed. 
Fic. 173.—Oniscus 
asellus, the com- 
mon Wood-louse. 
