PHYLUM ARTHROPODA 



291 



called the chelicerae. The first three pairs of appendages behind the 

 mouth usually serve together with others as walking legs, but they are 

 sometimes modified as grasping or tactile limbs. While the posterior 

 limbs are usually biramous, the anterior limbs are always uniramous. 

 Most of the living species are carnivores, although king crabs are 

 scavengers and many mites are herbivorous. 



The subphylum Crustacea includes the single class Crustacea, the 

 dominant living aquatic arthropods (Fig- 16.2). The second and third 

 segments each have a pair of antennae. The first pair of postoral ap- 

 pendages are short, stout mandibles, or jaws. The appendages of the 

 fifth and sixth segments are maxillae, modified to aid the jaws by 

 holding and manipulating the food. Many of these appendages are 

 biramous. Crustaceans ha\e invaded a variety of acjuatic habitats; some 

 crawl over the bottom while others swim or drift with the current. Many 

 of the species are extremely abiuulant. Probably more protoplasm is 

 embodied in crustaceans as a whole than in any other class of animals. 



The fourth subphylum, Labiata, includes millipedes, centipedes 







^^■^•^ 



I 

 I 



I 



-Antenna 



Chelicera."^ 



TRIIOBITA 



MACMIQMORPHA 



Anbz.nnz. 



'sm':'mmmkW.-.:'-:'Mi!Ms:m.^'ssssA 



Figure 16.2. Representatives of the four subphyla of the Arthropoda. (After Parker and 



Haswell.) 



