376 WILLIAM PATTEN. 



Vertebrata 

 Urocliorda Cephalocliorda 



SiPHONEURA 



Central nervous system tubular. 

 Eyes on neural surface. Retinas to both median and lateral eyes, inverted. 



utu -J (■ Cephalaspis, Pterichthys, 

 chthyoida . [pte^raspis' Botliriolepis, &c. 

 Limulus 

 Eurypterus 



A 1 -J TVT 4. i. C Limulus 



Aracnnida Merostomataj 



Trilobites 



Peltacephala. 

 Head shield-shaped. Eyes on haemal surface. 



SYNCEPHALA. 

 Divided into Head, Body, and Tail. 

 Head-segments 9 — 13; in four groups; first three head-segments 

 devoid of appendages. Terminal segments more or less fused to form 

 aurostyle. Eyes median and lateral. Median eyes 2 — 4, in a common 

 sac or tube ; retinas inverted. 



Insecta Crustacea 



Myriapoda 



PSEUDOCEPHALA. 



Brain segmented ; originally post-oral. 



Annelida 



ENCEPHALA. 



Brain unsegmented ; pre-oral. 



SEGMENTATA 



/ \ 



CCELENTERATA ECHINODERMATA ? 



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PROTOZOA 



