8 



OUTLINE OF THE ARACHNID THEORY. 



we may study this process of cephalization in detail. At one end of the body we 

 may observe the birth of new, independent metameres, and at the other the gradual 



Pr. C I) i.C. 



Ms. C. 



Br. C 



st.co. 



A 



Pt.C DiC. Ms C. Mt.c. 



E r. C. 



st.co. 



st g: 



Pr.C. DiC. MS.C Mt.C. 



B 



Br.C. 



D 



FIG. ,5. Diagrams showing the five characteristic body regions of arthropods, and their progressive concen- 

 tration to form the head of a vertebrate. The principal points illustrated are: a, The early location of the prin- 

 cipal functions; b, the concentration of the cardiomeres in the branchial region; c, the enlargement and concentra- 

 tion of the anterior cephalic neuromeres; d, the change in position of the optic ganglia and oral arches; e, the 

 closure of the old mouth and the formation of the new one;/, the transfer of locomotor organs from the nieso- 

 cephalon to the postbranchial metameres. /I and B, Insect; C, arachnid; D, vertebrate. 



decline of metamerism, and the incorporation of the old metameres, as specialized 

 subordinate parts, into a new and more highly organized unit. 



B. Origin of the Linear Arrangement of Unlike Cephalic Functions. It is 

 frequently assumed that the primitive vertebrate head consisted of a considerable 



