80 



VERTEBRATE ZOOLOGY 



arterial systems of each group, and the appendages may be com- 

 pared. There are, again, the very arthropod-like jaws which 

 Patten has demonstrated for the ostracoderm Bothriolepis, a 

 type which, on the other hand, shows many vertebrate-like 

 characteristics; and the general arrangement of the plates by 



pa.ey. Olfactory 

 Coordpa gy 



^'^"^'- Ley., 

 'Masticatory- ' 



ol.o. 



Fia. 42. — Diagram showing the supposed homologies between: A, insect; 

 B, arachnid (merostome) and C, ostracoderm (Bothriolepis). Pro. C, procephalon 

 or primitive head; Di. C, and Mes. C, dicephalon and mesocephalon, usually 

 spoken of as thorax; Met. C, metacephalon or vagus region; Br. C, Branchioeeph- 

 alon, or respiratory region, d. end, ductus endolymphaticus; grist, o, gustatory 

 organ; I. ey, lateral eye; pa. ey, parietal eye. (After Patten's " Evolution of the 

 Vertebrates and their Kin" [P. Blakiston's Sons & Co.].) 



which the cephalothorax is covered is also very similar in the 

 ostracoderms and in contemporary arachnoids, but unfortu- 

 nately for the argument Bothriolepis is a highly specialized end- 

 form from the Upper Devonian. Nevertheless, while the arach- 

 noid theory has been set forth by Gaskell {The Origin of Verte- 

 brates, 1908) and by Patten {The Evolution of the Vertebrates and 

 their Kin, 1912) the main thesis has received thus far but little 



