24 William Patten. 



E. The Median Eye. The three anterior median openings are compared with the tri- 

 ocular median eye of Limulus, Apiis, Trilobites, Merostomata and other Arthropods, but 

 not with the three frontal ocelli of adult insects. In Limulus, Patten '89, and '93, where the 

 structure and development of this organ is best known, the retinas of the median ocelli 

 arise from two pairs of segmental sense organs, that during the closing in of the brain 

 migrate from the margins of the cephalic lobes to the roof of the fore-brain vesicle. Here 

 the ocelli come to lie at the blind end of a long tube-like outgrowth of the brain roof. The 

 distal end of the tube divides into two vesicles, lying in the median line, one in front of 

 the other. The retinas of one pair of ocelli unite to form one of the vesicles, and, in a dege- 

 nerate condition, lie deeply buried beneath a median tubercle on the dorsal surface of the 

 head. The other two retinas lie close together in the second vesicle beneath two median 

 lenses. These two terminal vesicles are found in a more or less modified form in many 

 Crustacea, and without doubt in the Trilobites and Merostomata also, since the arrange- 

 ment of their surface lenses is precisely the same as in Limulus. The Median Eye of Tre- 

 mataspis, like that of Limulus, probably consists of a complex group of three ocelli derived 

 from the incomplete fusion of two pairs. They were true cerebral eyes lying at the end of 

 a tubular outgrowth of the brain. The distal end of this tube was probably bifurcate, as in 

 Limulus, the anterior vesicle containing one pair of ocelli lying beneath, or in, the median 

 pit, and the posterior vesicle lying in the paired median orbits. The anterior and posterior 

 vesicles of Limulus and Tremataspis are represented in true Vertebrates by either the 

 vesicular ends of two separate outgrowths from the brain roof, one behind the other, or by 

 two terminal vesicles, one in front of the other, arising fiom a common tubular out- 

 growth. 



According to this view, the visual organs of Vertebrates are derived from three pairs 

 of segmental sense organs, originally situated near the margins of the cephalic lobes. The 

 median eyes, which were originally the most anterior in position, were the first to be con- 

 verted into cerebral eyes of the Vertebrate type. This change took place in the Arthropods, 

 and the various steps in the process are clearly seen in Insects, Crustacea, and Arachnids. 

 The transfer of the lateral eyes to the cerebral vesicles and their consequent inversion took 

 place much later, probably in the intermediate type of animals to which the Tremataspidae 

 belong. It was during this period that the median eyes reached their highest development 

 and the lateral eyes degenerated, or in some cases disappeared completely from the surface. 

 We have no means of knowing whether their temporary decline in functional importance 

 was the cause, or the result, of their transformation into eyes of the cerebral type. 



F. The Post-orbital Opening contained the forerunner of the Vertebrate olfactory organ, I 

 have identified it with the frontal organ of Limulus, Branchipus, Apus and other Arthropods. 

 This organ in the Arthropods presents extraordinary variation in its position, but it can 

 always be identified by the peculiar histological structure of the terminal organ and 



