ON THE OEIGIN OF VERTEBEATES FEOM ARACHNIDS. 337 



lioD; strictly speaking, does not belong to the lateral eyes, 

 for they develop on the third thoracic segment, but to a 

 small sense-organ lying on the outer edge of the invagination 

 (Fig. 7, A, e^.) ; consequently this sense-organ and not the 

 lateral eye, is homologous with a lateral eye of Scorpio. In 

 the larvae it is connected by a nerve with the ganglion of the 

 lateral eyes (Fig. 10, e^.); it is there deeply pigmented, and 

 counected with a branching plexus of pigment-cells. In this 

 stage it has been seen and described as a mere pigment-spot 

 (Brooks). In the adult this simple eye stands close to its 

 fellow in the median line, in front of the mouth ; the overlying 

 cuticula is there clear and transparent, forming two rudi- 

 mentary lenses. These facts establish beyond doubt the 

 visual character of the organ and its serial homology with the 

 other eyes. 



Passing backward into the thorax, we find that each line of 

 the ganglionic pits just described is continued into a seg- 

 mentally deepened furrow extending the whole length of the 

 thorax (Fig. 9, e, /. /., and Fig. 16). On the lateral side of 

 this furrow there is a thickened band of ectoderm, which in 

 each thoracic segment contains a broad shallow depression, 

 undoubtedly of a sensory nature. The sense-organ of the 

 second or third thoracic segment (I could not determine with 

 certainty which) gives rise to the lateral eye, /. e. ; that in the 

 fourth is very large, and has erroneously been supposed to give 

 rise to the lateral eyes (Kingsley), or to represent a dorsal 

 organ (Watase). It is a true sense-organ, and is connected by 

 a nerve-bundle with the cord of ganglion-cells arising from 

 the lateral furrow ; it persists until the end of the first larval 

 moult, and in this period is covered by a great disc-like thick- 

 ening of the cuticula, which, judging from its shape and 

 transparency, undoubtedly represents a rudimentary lens. 



The nerve to the lateral eyes arises, like the lateral cord of 

 ganglion-cells, from or near the lateral furrow, and may be 

 regarded as a specialisation of that part of the cord extending 

 from the third segment of the brain to the second or third 

 segment of the thorax. It is accordingly unlike any other 



