ON THE ORIGIN OP VERTEBRATES FROM ARACHNIDS. 339 



organs or eyes and their ganglionic invagination ; and the 

 lateral cord of ganglion cells may be regarded as a cord of 

 segmental ganglia serially homologous with the optic ganglia. 



Each segmental sense-organ of Limulus is represented in 

 Scorpio by a pair of sense-organs at the base of each thoracic 

 appendage (Pis. XXIII and XXIV). 



If now we compare Acilius, Scorpio, Limulus, and Verte- 



/(X'Xt". 



Fig. 8. — H. Semi-diagrammatic view of median eye of young Limulus 

 larva, seen from the side (compare Woodcut 10). A-F. Section of the 

 eye and its stalk at points A-F. G. Surface view of median eyes. ant. 

 Anterior side of eye. e^. Eye of first brain-segment, e^. Eye of second 

 brain-segment, ec. Ectoderm. n^. Nerve to eyes of first segment. 

 «^ Nerve to eyes of second segment, np. Neuropore. o. ff^-^. Optic 

 ganglia. 



brates, we shall find they present four distinct stages of one 

 process of enclosing the fore-brain — that is, four stages in the 

 extension of the ganglionic invaginations (see Fig. 9). 



In Acilius we have three distinct pairs of invaginations on 

 the outer edge of the cephalic lobes. In Scorpio and 

 Limulus the invaginations of the first two segments unite, 

 forming a continuous, amnion-like fold, the free edge of which 

 grows medianly and backward, infolding not only the optic 

 ganglion, but the eyes and the anterior part of the brain. 



