498 ARTIIROPODA. 



thorax tlioso used in luconiotioii (})ereioiioda), the al)domen the 

 swimmerets (ple-poda), or it lacks appendages. 



7. By fusion of head and thorax a eephalothorax i^ produced; 

 a postabdomen may be separated from the abdomen. 



8. The eyes are either ocelli or compound eyes. 



0. nerniaphroJitisni is rare; reproduction is by eggs; fre- 

 quently there is parthenogenesis, rarely iiasdogenesis. The eggs 

 usually have a superficial segmentation. 



10. The Arthropoda are divided into Crustacea, Acerata, 

 Malacoj^oda, Insecta, and Diplopoda. 



11. The Crfstacea respire by gills; they usually have two 

 ])airs of antenna, and usually biramous feet; the reproductive ducts 

 open near the middle of the body. 



12. The Crustacea are divided into Trilobitaj, Phyllopoda, 

 Copepoda, Ostracoda, Cirripedia, and Malacostraca. 



13. Phyllopoda, C!opepoda, Ostracoda, and Cirripedia are fre- 

 quently called Entomostraca; they have a shell gland and the 

 nauplius as a larval stage. 



14. The Trilnbike are extinct forms with one pair of antennas, 

 and the body divided by longitudinal grooves into three regions. 



1.5. The Phyllopoda have variable segments and primitive leaf- 

 like feet recalling the parapodia of the annelids. 



10. The Copepoda -Ave ^\\i\\ovii shells and have biramous feet. 



17. The Ostracoda have reduced bodies enclosed in a bivalve 

 shell. 



18. The Cirri'pedia are usually hermaphroditic and are sessile. 

 10. The Malacostraca have 30 (21) segments, of which 7 (8) 



are abdominal; the male sexual openings are on the 13th, the 

 female on the 11th, segment; the excretory organ is the antennal 

 gland; the larva is a zoea, rarely a nauplius, 



20. The Malacostraca are divided into Leptostraca, Thoracos- 

 traca., and Arthrostraca. 



21. The Leptostraca have tAventj'-one somites; they are closely 

 related to the Pliyllojioda. 



22. The Ttioracostraca or Fodopt/tltidmia (Schizopoda, Stoma- 

 topoda, Decapoda) have stalked eyes and some or all of the tho- 

 racic somites fused with the head to a eephalothorax. 



23. The Arttivostraca or Edrioptiltialnn.a have sessile eyes and 

 have seven free thoracic segments. They are divided into Am- 

 phipoda and Tso2)oda. 



24. The Acerata lack antenna^; the body is divided into eeph- 

 alothorax and abdomen; the cejthalothorax bears six pairs of 



