493 ARTHROPODA. 



thorax those used in locomotion (pereiopoda), the abdomen the 

 swimmerets (pleepoda), or it lacks appendages. 



7. By fusion of head and thorax a cephalothorax is produced; 

 a postabdomen may be separated from the abdomen. 



8. The eyes are either ocelli or compound eyes. 



9. Hermaphroditism is rare; reproduction is by eggs; fre- 

 quently there is parthenogenesis, rarely paedogenesis. The eggs 

 usually have a superficial segmentation. 



10. The Arthropoda are divided into Crustacea, Acerata, 

 Malacopoda, Insecta, and Diplopoda. 



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

 pairs of antennae, and usually biramous feet; the reproductive ducts 

 open near the middle of the body. 



12. The Crustacea are divided into Trilobitae, Phyllopoda, 

 Copepoda, Ostracoda, Cirripedia, and Malacostraca. 



13. Phyllopoda, Copepoda, Ostracoda, and Cirripedia are fre- 

 quently called Entomostraca; they have a shell gland and the 

 nauplius as a larval stage. 



14. The Trilobitm are extinct forms with one pair of antennae, 

 and the body divided by longitudinal grooves into three regions. 



15. The Phyllopoda have variable segments and primitive leaf- 

 like feet recalling the parapodia of the annelids. 



16. The Copepoda are without shells and have biramous feet. 



17. The Ostracoda have reduced bodies enclosed in a bivalve 

 shell. 



18. The Cirripedia are usually hermaphroditic and are sessile. 



19. The Malacostraca have 20 (21) segments, of which 7 (8) 

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

 female on the llth, 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 twenty-one somites; they are closely 

 related to the Phyllopoda. 



22. The Thoracostraca or Podophthalmia (Schizopoda, Stoma- 

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

 racic somites fused with the head to a cephalothorax. 



23. The Arthrostraca or Edriophthalmia have sessile eyes and 

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

 phipoda and Isopoda. 



24. The ACERATA lack antennae; the body is divided into ceph 

 alothorax and abdomen; the cephalothorax bears six pairs oi 



