CEREBRAL EYES OF VERTEBRATES. 127 



therefore, essentially like that of the parietal eye in Limulus and the scorpion. 

 This fact, and many others to be brought out later, demonstrates that the parietal 

 eye of the Crustacea and arachnids is a true cerebral eye in the vertebrate sense, 

 and is identical with the parietal eye of vertebrates. 



The lateral eyes of vertebrates represent the compound or convex eyes of 

 arthropods that have been transferred to the interior of the cerebral vesicle. In 

 the arthropods the lateral eyes lie near the margin of the cephalic lobes, on the 

 outer edge of a deep ganglionic infolding. In vertebrates, they are first seen in a 

 very similar position on the lateral margin of the open medullary plate. Later 

 they are swept into the infolding brain, turning the retinas inside out. They then 

 grow out laterally on the end of membranous tubes, in much the same manner as 

 the median eyes. In arthropods, the lateral eyes usually have a crescentic, or 

 kidney-shaped outline; in vertebrates, this shape is retained, giving the retinas 

 their characteristic crescentic outline during the early stages. When the two 

 limbs of the crescent unite, a circular retina is produced, giving rise to the choroid 

 fissure and the centrally located optic nerve that, together with the inverted 

 rods and cones, have long been such inexplicable features of the lateral eyes in 

 vertebrates. 



The olfactory organ in vertebrates arises from three placodes situated on the 

 anterior margin of the cephalic lobes. They are not drawn into the brain cham- 

 ber, but remain permanently in the surface ectoderm. They move forward 

 along the median line followed by two pairs of olfactory nerves, that in the lower 

 vertebrates may remain separate up to the adult stages. Its structure, develop- 

 ment, and innervation is therefore similar to that of the frontal organs of the 

 Crustacea, and the olfactory organ of Limulus. 



II. THE EYES AS SEGMENTAL SENSE ORGANS. 



The larval ocelli, lateral eyes, auditory organs, stemmata and olfactory or- 

 gans appear to be local modifications of a series of primitive sense organs belong- 

 ing to the procephalic and first six thoracic metameres. 



In insects and arachnids, the larval ocelli of the procephalic lobes present a 

 clearly defined segmental arrangement. (Fig. 14.) In scorpion and Limulus, 

 in addition to these ocelli, there is a transient series of segmental sense organs in 

 the thorax, which appears to be a continuation of that in the forebrain. (Figs. 15, 

 16, 140-142.) 



In Limulus, the first pair of the thoracic series are the lateral eye placodes, I.e. 

 The fourth pair, s.o 4 , are large, circular placodes, distinctly sensory in character, 

 that are retained through the first larval or trilobite stage, after which they dis- 

 appear. This organ is probably the forerunner of the auditory organ of verte- 

 brates for it has the same shape and general appearance as the auditory placode 

 in vertebrate embryos, and as nearly as may be determined, lies on the same 

 segment. The other placodes are less distinct and are visible for a very short 

 period only. 



In scorpions, on the outer margins of each coxal joint (Figs. 15-16), there 



