XATUUAL CLASSIFICATION OF THE TRILOBITES 115 



Comparative Morphology of Cnixfacea. 



Sub-closa I. Trilobita. 



1. All marine. 

 ■2 Free. 



.3. Body longitudinally 



tri-regional. 

 4. Larva a protonau- 



plius. 



5. Number of segments 

 variable. 



6. Cranidium of five 

 fused segments. 



7. Ocelli rarely present. 



8. Paired compound ses- 

 sile eyes on cheek pieces 

 usually present. 



9. Thorax distinct ; 

 number of segments 

 variable, all free. 



10. Abdomen distinct ; 

 variable number of 

 fused .segments. 



11. All segments of cra- 

 nidium, thorax, and ab- 

 domen, exce[it the anal 

 segment, carry paired 

 appendages. 



12. AH appendages bira- 

 mous except anteu- 

 uules. 



13. Appendages typically 

 phyllopodiform. Exop- 

 odite a awimming leg ; 

 endopodite modified 

 into a crawling leg. 



14. All appendages of the 

 head except antennules 

 pediform. 



15. Thoracic appendages 

 ambulatory and swim- 

 ming. 



Sub-claas II. Entomoetraca. 



Marine and freshwater. 

 Free, parasitic, and at- 

 tached. 

 Various. 



Larva almost universally a 

 nauplius. 



Number of segments vari- 

 able. 



Head of five fused seg- 

 ments to which, rarely, 

 a thoracic segment is 

 added. 



Ocelli present throughout 



life. 

 Paired compound eyes 



usually present ; stalker! 



or sessile. Absent in 



adult Cirripedia and 



some Copepoda. 

 Thorax with variable 



number of segments. 



Abdomen with variable 

 number of separate 

 segments. 



Some segments without 

 appendages. 



Some appendages are 

 mollified and have lost 

 biramous structure. 



Appendages generally 

 greatly changed in most 

 orders; phyllopodiform 

 in young forms and 

 throughout life in Phyl- 

 lopoda. 



Some appendages of the 

 head modified into row- 

 ing organs, mandibles, 

 or suckers. 



Thoracic appendages am- 

 bulatory, swimming, 

 and seizing. 



Sub-class III. Malacostraca. 



-.Marine and freshwater. 

 Free and parasitic. 



Various. 



Larva generally a zoea, 

 a nauplius stage being 

 often developed before 

 hatching, except in Fa- 

 phausia and Peneus. 



Definite number of seg- 

 ments. 



Head "f five fused seg- 

 ments to which one or 

 more, or all of the tlio- 

 racic segments may 

 unite, forming a more 

 or less complete ceph- 

 alothorax. 



Ocelli abfsent in adult 

 forms. 



Paired compound eyes 

 usually present; stalked 

 or ses.sile. 



Thorax with eight seg- 

 ments, some of which 

 are generally united 

 with the head. 



Abdomen of seven gen- 

 erally free segments ; 

 eight in Leptostraca. 



All segments usually carry 

 appendages except the 

 last one or two. 



Some appendages have 

 lost biramous structure. 



Appendages typically 

 phyllopodiform, but 

 greatly modified in all 

 but the lowest order 

 (Nebalia). 



Some appendages of the 

 head modified into man- 

 dibles, or organs for 

 seizing food. 



Thoracic appendages am- 

 bulatory, swimming, 

 and seizing. 



