Palaeozoic Arachnida of North America. 



17 



dence for the assertion that Arachnida and Diplopoda have only two 

 prostho meres while the Crustacea, insects and Chilopoda have three. 

 It seems to me, on te contrary, that in Arachnida and insects more 

 than three prosthomeres went into the formation of the prae-oral 

 part of the head. The anterior middle eyes of Arachnida correspona 

 both in structure and nerve supply to the simple eyes of insects (ocelli) 

 and did not originally belong to the same segment as the side eyes. 

 In the following tables I give for comparison the segmentation in 

 arthropods according to E. R. Lankester, and of the homologuos 

 organs of the head, as it appears from a comparative study of the 

 development and structure of the brain. I want to state, however, 

 that the second table is only a tentative one and that further research 

 will be necessary before the true segmentation of the head in arthro- 

 pods may be clearly understood. 



TABLE I. 



Segmentation of the head in different classes of arthropods according to 

 E. R. Lankesteri 



TABLE II. 



Homologous structures in the head of some arthropods according to my 



own interpretation 



1 This table was not taken from any of Lankester' s writings, but 

 composed by me on the basis of his ideas. 



Trans. Conn. Acad., Vol. XVIII. 2 June, 1918. 



