THE EYES OF LIMULUS 



133 



crustaceans. The retinulas in the median eyes of scorpions differ, how- 

 ever, in being much less compact and cylindrical and less highly organized 

 than in the higher types of insects and Crustacea. They consist of sensory 

 cells which terminate at their proximal end in one of the fibres of the optic 

 nerve ; at the outer ends of these cells are elliptical refractile elements, 

 the rhabdomes, and there are also interstitial connective tissue cells 

 containing pigment and an outer layer of epidermal cell-elements next 



vihr. 



nerv.c. 



nerv. 



Fig. 95. — Section through Median Eye of Euscorpius. 



A single lens is present in both the central and lateral eyes of Euscorpius. In the 

 central eye the hypodermal cells are continued across the eye as a vitreous 

 layer which lies between the retinulas and cuticular lens. 



lens : cuticular lens. 

 nerv. c. : terminal nerve cells. 

 nerv.f. : fibres of optic nerve. 

 pigm. : cells containing pigment. 

 rh. : rhabdomes. 

 vitr. : vitreous layer. 

 (After Lankester and Bourne.) 



the basement membrane. The eyes are generally regarded as intermediate 

 in type between the simple, upright eye of the ocellar type, with a single 

 cuticular lens, and the composite and compound faceted types such as 

 are found in certain Crustacea. Lankester considered that the resem- 

 blance of the scorpion's eyes, both lateral and medial, to those of Limulus 

 was important evidence in favour of the latter being more closely related 

 to the arachnids than to the crustaceans. 



The Eyes of Limulus Polyphemus 



Limulus, or the American king crab, is the only known living representa- 

 tive of the extinct palaeozoic forms of the gigantic marine scorpions such as 

 Eurypterus and Pterygotus which inhabited the sea in the Silurian period. 

 The body consists of two parts separated by a movable articulation, 



