346 WILLIAM PATTEN. 



retina like that in many Vertebrates. Again^ the lateral eye of 

 Limulus arises from the third or fourth thoracic segment, 

 although its nerve and optic ganglia are united with the 

 third brain-segment; if such an eye were involved by ganglionic 

 invagination, it would lie at the end of a long backwardly 

 directed tube, like that of the lateral eye of Vertebrates. 



In other words, we can explain the most remarkable and 

 characteristic features of the lateral eyes of Vertebrates, 

 such as (1) the shape of the retina, (2) its histological 

 structure, (3) the formation of the choroid fissure, (4) the 

 backwardly directed eye-stalk (in marked contrast with the 

 forwardly directed pineal eye), by supposing them to be derived 

 from kidney-shaped thoracic eyes like those in Limulus, 

 Trilobites, and Merostomata. 



The proximal ends, at least, of the optic nerves of Scorpio 

 and Limulus, are probably serially homologous with the pedal or 

 neural nerves of the thorax ; this is shown by their histological 

 structure and by their position. For example, in Limulus 

 both the second and third segments of the fore-brain (and 

 possibly the third in Scorpio) are provided with segmentally 

 arranged motor nerves exactly like the motor nerves of the 

 post-oral segment ; the optic nerves taking the place of, and 

 resembling in structure, the pedal or neural ones of the 

 thorax (Fig. 10). 



In Limulus, one of the motor-like nerves of the fore-brain 

 is connected with the skin about the upper lip; this fact, 

 together with its position, suggests that it may represent an 

 incipient olfactory nerve (Fig. 10, hir.). 



VI. The Cranial Flexure of Vertebrates has been ex- 

 plained by supposing that the fore-brain represents an inverte- 

 brate supra-oesophageal ganglion. But, judging from Kleinen- 

 berg's view and Beard^s speculations, this theory must be 

 abandoned, since the Annelid brain arises independently of the 

 remaining central nervous system, while the Vertebrate fore- 

 brain does not. It is therefore tacitly admitted that the 

 Annelid theory cannot explain the cranial flexure, that 



