LATERAL AND CENTRAL EYES OF SCORPTO AND LIMULUS. 201 



The Lateral Eyes of the American King Crab, 



LiMULUS POLYPHEMUS, Latr. 



The two lateral eyes of the King Crab exist on either side 

 of the prosoinatic shield, each as a reniform, smoothly polished 

 protuberance. The protuberance is the cuticular lens; it is of 

 great thickness, smooth on its surface, and produced on its 

 inner surface into a number of conical processes, each of which 

 is to be regarded as a secondary or segregate lens. The general 

 features of the structure of this eye have been made known by 

 Grenacher; we have only to add certain details to the de- 

 scription and figures given by him. 



In PI. XI, fig. 18, a diagram is given representing a portion 

 of the polymeniscous lens with its subjacent ommateutn. 



Making use of the terminology which has been explained in 

 the course of this paper, we can briefly describe the structure 

 of the ommateum. Corresponding to each conical facet of the 

 polymeniscous lens is a retinula [Rn). The ommateum is (as 

 observed by Grenacher) essentially monostichous. The reti- 

 nulse correspond in position to the apices of the conical 

 secondary lenses. 



That part of the ommateum which clothes the sides of the 

 lens-cones consists of simple cylindrical cells corresponding to 

 the perineural cells of the lateral eye of Scorpions, and in the 

 valleys between neighbouring lens-cones, the ommateal cells are 

 not to be distinguished from ordinary pigmentiferous hvpo- 

 dermis cells. 



The nerve-end cells, which are combined to form retinulee, 

 are of very large size, as much as ^ J,^th long. Transverse sec- 

 tions of the ommateum show that ten cells are united in each 

 retinula. The difficulties of observation are here even greater 

 than in the eyes of Scorpions (owing to the thickness and 

 density of the cuticular lens, which prevents the preparation of 

 satisfactory sections); but weare inclined to think that ten nerve- 

 end cells, being double the number present in the Scorpion's 

 retinula, is the rule, although Grenacher has figured a retinula 



