ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 363 



row^ terminating posteriorly with an unpaired plate or meta- 

 stomum. 



As in all true Vertebrates, the eyes are found on the same 

 side as the paired cranial plate, we may conclude that the 

 real break between Arthropods and Vertebrates is made by 

 the transference of the eyes to the neural surface. Since their 



Fig. 14.— Diagram of the circulation (S) in the thorax of an Arachnid, 

 and ( V) in the head of a Vertebrate. 



eyes are situated on the haemal surface, we may conclude that 

 Pterichthys and Bothriolepis, &c., are nearer related to the 

 Arachnids than to the Vertebrates. The position of the eyes 

 undoubtedly depends largely on the position in swimming. In 

 Pterygotus, for example, where locomotion was probably largely 

 effected by swimming on the haemal surface, the eyes have 

 already become lateral — a position very unusual in Arachnids. 

 This change is readily explained, since the original position of 

 the eyes in the embryo of all Arthropods is neural ; moreover, 

 this history of Arthropod eyes shows conclusively that they can 

 assume any position the method of locomotion may demand. 



The above view of the Vertebrate cranium explains why 

 the cephalic bucklers of many primitive fossil fishes are fre- 

 quently divided into distinct dorsal and ventral shields, and 

 also why the abocular or neural surface is so imperfectly 

 preserved. 



