CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 7 



described later. The Crustacea also differ from other Arthropoda 

 in having two pairs of antennae. Rudiments of two pairs of antenna 

 have been observed in the embryos of many other arthropods ; but 

 in these cases one or the other of the two pairs of antennae fail 

 to develop. 



The examples of crustaceans named above are the more con- 

 spicuous members of the class; but many other smaller forms abound 

 both in the sea and in fresh water. Some of the more minute fresh- 

 water forms are almost sure to occur in any fresh- water aquarium. 



In Figure 8 are repre- 

 sented three of these 

 greatly enlarged. The 

 minute crustaceans form 

 an important element in 

 the food of fishes. 



Some crustaceans live 

 in damp places on land, 

 and are often found by 

 collectors of insects; 

 those most often ob- 

 served are the sow-bugs 

 (Oniscoida), which fre- 

 quently occur about 



Fig. 9. A sow-bug, Cylis'icus convexus (From water-soaked wood. 

 Richardson after Sars). Figure 9 represents onr 



of these. 



As there are several, most excellent text books devoted to the 

 Crustacea, it is unnecessary to discuss farther this class in this place. 



