200 PROFESSOR E. RAY LANKESTER AND A. G. BOURNE. 



" autochromic," whilst one which is penetrated by pigmenti- 

 ferous connective tissue is " exochromic." 



The central eye of the Scorpion is seen from what has been 

 said above to take a position among Arthropod eyes very dis- 

 tinct from that occupied by the lateral eyes. It is diplostichous, 

 it is definitely retinulate, it is exochromic, and only agrees in 

 fact with the lateral eye in being monomeniscous. As com- 

 pared with all other Arthropod eyes hitherto described, the 

 central eyes of the Scorpion stand alone in being definitely 

 retinulate, whilst retaining the primitive monomeniscous 

 character of lens. 



It is highly noteworthy that the central eyes of Scorpions, 

 which in position and naked-eye appearance agree with the 

 centrally grouped eyes of the Scorpion's congeners, the Spiders, 

 should nevertheless present such marked differences from those 

 eyes as appear from a comparison of the description given in 

 this memoir of the one and by Grenacher of the other. The 

 Spiders have apparently simple axial rhabdomeres in their nerve- 

 end cells in place of laterally developed rhabdomeres, uniting 

 to form rhabdoms, as in Scorpions. Further, it would appear 

 from Grenacher's description that the Spider^s eyes are auto- 

 chromic, whilst the Scorpion's central eyes are exochromic — 

 i.e. the ommateum is penetrated by intrusive connective 

 tissue. 



Having thus noted the peculiarities of the Scorpion's lateral 

 and central eyes, we are in a position to compare them with 

 the lateral and central eyes of Limulus, of which we shall 

 now give a description. It will be seen that though differing 

 in some important details, there are, taking all things into 

 consideration, no Arthropod eyes which so closely agree in 

 their plan of structure with those of the Scorpions as do those 

 of Limulus. Whilst this conclusion might be impugned were 

 we to separate the consideration of the two kinds of eyes, the 

 lateral and the central, it is indisputable if we compare the 

 whole set of optical organs in the one animal Avith the whole 

 set iu the other. 



