328 THE PINEAL ORGAN 



fishes and the main or parent stem from which certain extinct and living 

 arthropods have descended. The resemblances are found chiefly in the 

 head region and are most evident in the Eurypterida and Trilobites among 

 the extinct classes, and in Limulus, Apus, and Lepidurus among living 

 species. But the fundamental differences in the relative positions of the 

 thoracic and abdominal viscera between vertebrates and invertebrates 

 (Fig. 68, Chap, n, p. 106) precludes the assumption of there being any 



Fig. 228. — Restoration of Cephalic Shield of Ki^eraspis auchenaspidoides. 

 Dorsal View. (After Stensio.) 



c. : cornu. na 1 : opening of hypophyseal sac. 



dsf. : dorsal electric field. na 2 : nasal opening. 



d.sp. : dorsal spine. orb. : orbital opening. 



/. en. : fossa circumnasalis. ps. : pectoral sinus. 



Isf. : lateral electrical field. pin. : pineal foramen. 



close relationship between these divergent types ; and they indicate 

 that the period during which the divergence of the vertebrate from the 

 invertebrate stock and the primary changes in the evolution of their 

 respective types of lateral eyes took place must have been infinitely 

 remote and probably occurred in a very simple form of animal showing 

 bilateral symmetry and paired ocelli of the simple upright type. Thus it 



