ARTHROPODA 



299 



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 c[uite 



rudimentary. Many of them, like the Sphae- 



EOMiDAE, possess the power of rolling themselves 



up in a ball when disturbed. 



Fig. 173. — Oniscus 

 ocellus, the com- 

 mon Wood-louse. 



