LATERAL AND CENTRAL EYES OF SCORPIO AND LIMULUS. 205 



important to notice that the number varies even in individuals, 

 and that supernumerary eyelets of small size are distinguished 

 as of irregular occurrence by the side of the larger eyelets, thus 

 seeming to indicate that we have in this region of the Scor- 

 pion's prosomatic shield an *' ocular area," which by reversion 

 may occaionally reproduce the more numerous and closely set 

 eyelets into which the single ancestral eye of the common 

 parent of Scorpio and Limulus was divided in the Scorpionid 

 line of descent.^ 



The Central Eyes of the American King Crab, 

 Limulus polyphemus, Latr. 



The central eyes of Limulus have not hitherto been ex- 

 amined. Grenacher appears not to have had an opportunity 

 of studying them, although he has figured sections of the 

 lateral eyes. Packard has published some figures relating to 

 them, which are valueless, because he did not attempt to re- 

 move the pigment which necessarily obscures their structure. 



Hence we have here a perfectly novel subject to deal with. 



These eyes are very difficult to study on account of the great 

 strength and thickness of the lenses and adjacent cuticle. The 

 ommateum must necessarily be drawn away from the lens in 

 the fresh state, and then hardened and cut. Our conclusions 

 are founded on the examination of the central eyes of four fresh 

 specimens of Limulus. 



The anticipation which immediately forces itself on the 

 mind is, that the central eye of Limulus will prove — if the 

 assimilation of Limulus and Scorpio be justified — to be, like 

 that of Scorpio, diplostichous, monomeniscous, and retinulate, 

 with a more or less abundant intrusive connective tissue in the 



' It is important to note the following difference between the lateral eyelet 

 of a Scorpion and a single element of the King Crab's lateral eye — in the 

 former the ommateum contains more than one retinula, it is retinulate, in 

 the latter it contains but one group of nerve-end cells, truly a retinula when 

 the whole eye-group is considered but in itself uon-retinulate. Thus the 

 eyelet of the Scorpion is morphologically more (a larger segment of the 

 original ocular area) than the lens-cone element or eyelet of Limulus. 



