422 



GENERAL ZOOLOGY 



marine; Cyclops viridis, a fresh-water copepod; Eucypris 

 virens, a fresh-water ostracod; Balanus balanoides, the rock 

 barnacle. 

 SUBCLASS 2. MALACOSTRACA. Usually with 20 body 

 segments: head 5, thorax 8, and abdomen 7; and 19 pairs of 

 appendages: head 5, thorax 8, and abdomen 6. Head and 

 thorax generally combined in a cephalothorax. 



Examples: Porcellio scaber, a sow bug, terrestrial; 

 Palaemonetes vulgaris, a common shrimp, marine; Homarus 

 americanus, American lobster; Cambarus bartoni, American 

 crayfish; Cancer irroratus, a common New England crab; 



Libinia emarginata, a spider crab 

 (Fig. 245). 

 CLASS II. ARACHNOIDEA. 

 No antennae are present. There 

 are two body divisions, the 

 cephalothorax, with six paired 

 appendages, and the abdomen, 

 without appendages, except in the 

 horseshoe crab. 

 SUBCLASS 1. XIPHOSURA. A 

 horseshoe-shaped cephalothorax, 

 six platelike paired appendages 

 on the abdomen, and a spikelike 

 tail. 



Example : Limulus polyphemus, 

 the American horseshoe crab. 

 SUBCLASS 2. ARACHNIDA. The cephalothorax bears six 

 pairs of appendages : the mandibles or chelicerae, the pedipalps, 

 and four pairs of walking legs. The last three pairs of abdom- 

 inal appendages of spiders are modified into spinnerets. 

 Mostly terrestrial and breathe by modified gills known as 

 lungs. Some have a tracheal system. 



Examples: Diplocentus whitei, a scorpion of the South- 

 western United States; Argiope aurantia, an orb-weaving spider 

 (all spiders belong to the Arachnida); Phalangium opilio 

 (daddy-long legs). Has a tracheal system with a single pair of 

 spiracles. Does not spin a web. Sarcoptes scabiei, one of the 

 itch mites, causing itch in man and mange in pigs. Margaropus 

 annulatus, the Texas cattle tick. The bite of the tick inoculates 

 cattle with Babesia bigemina, a sporozoan which causes Texas 



Fig. 245. — Libinia, a spider crab, 

 sketched devouring a sea urchin. 



