NATURAL CLASSIFICATION OF THE TRILOBITES 115 



Comparative Morphology of Crustacea. 



Sub-class I. Trilobita. 



Sub-class II. Entomostraca. 



Sub-class III. Malacostraca. 



1. All marine. 



2. Free. 



.3. Body longitudinally 



tri-regional. 

 4. Larva a protonau- 



plius. 



5. Number of segments 

 variable. 



6. Cranidiinn of five 

 fused segments. 



7. Ocelli rarely present. 



8. Paired compound ses- 

 sile eye.s on cheek pieces 

 usually present. 



9. Thorax distinct ; 

 number of segments 

 variable, all free. 



10. Alidomen distinct ; 

 variable number of 

 fused segments. 



11. All segments of cra- 

 nidiinn, tliorax, and al)- 

 dumen, except the anal 

 segment, carry paired 

 ap])endages. 



12. All appendages bira- 

 mons except anteu- 

 nules. 



13. Appendages typically 

 phyllo])odiform. Exop- 

 odite a swimming leg; 

 endopodite modified 

 into a crawling leg. 



14. All appendages of the 

 head except autennules 

 pediform. 



15. Thoracic appendages 

 ambulatory and swim- 

 ming. 



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 jjresent throughout 

 life. 



Paired compound eyes 

 usually present ; stalked 

 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 

 birainous structure. 



Appendages generally 

 greatly chaiij^ed in most 

 orders ; pbyllojwdiform 

 in young forms and 

 throughout life in Phyl- 

 lopoda. 



Some appendages of the 

 head modified into row- 

 ing organs, man(lil)les, 

 or suckers. 



Thoracic appendages am- 

 bulatory, swimming, 

 and seizing. 



Marine and freshwater. 

 Free and parasitic. 



Various. 



Larva generally a zoea, 

 a nauplius stage being 

 often developed before 

 hatching, except in Eu- 

 phnusia and Peneus. 



Definite number of seg- 

 ments. 



Head of five fused seg- 

 ments to which one or 

 more, or all of the tho- 

 racic segments may 

 unite, forming a more 

 or less complete ceph- 

 alothorax. 



Ocelli absent in adult 

 forms. 



Paired compound eyes 

 usually present; stalked 

 or sessile. 



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 excej)t the 

 last one or two. 



Some appendages have 

 lost birainous structure. 



Appendages typically 

 phyllopodiform, but 

 greatly modified in all 

 but the lowest order 

 {Neballa). 



Some appendages of the 

 head modified into man- 

 d 11 lies, or organs for 

 seizing food. 



Thoracic ajjpendages am- 

 bulatory, swimming, 

 and seizing. 



