COMPO UND EYES OF AR THRO PODS. 307 



siirrouiulinj^ it a circular row of ((iIuiiiiku- pii^mented epithelium. 

 The loiis is secreti'd by the surrouiuliu;:; roluninar cells. Around 

 the whole omniatidiuiii tiiere exists a rather thick, well defined 

 basement membrane {B. M) which separates the ommatidium 

 from the mesodormic tissues {Mti). 



In {/}) Fig. 6a, a section at about the level of the letters AV, in 

 Fig. 6, passing through the sensory portion is shown. The main 

 contents of the ommatidial capstile {B. M) are the retinula cells 

 {lit). The axial moiety of each retinula cell is free from the 

 pigment granules, and a tiiiii cuticular covering is secreted 

 around this portion of the cill. This cuticular covering consti- 

 tutes the rhabdomere (Bh). Sections of a numlier of pigmented 

 cells are seen lying between the rctinulae and the basement 

 membrane; these are sections of the perineural epithelial cells. 



In (f). Fig. 6a, a section passing through the body of the 

 ganglion cell {G) is shown ; the ganglion cell is surrounded by 

 a circle of retinula cells, which are now gradually tapering, each 

 into a slender nerve process. The ganglion cell takes a more 

 eccentric position in {d) than in (c). The retinula cells {Jit) in 

 (c) and {(1) are represented by the circle of nerve fibres sur- 

 rounding the ganglion cell {G). 



In {e), Fig. Qa, the section passes through the bottom of the 

 ommatidium ; all of the sensory cells are reduced into nerve 

 fibres which proceed one from the bottom of each cell. The 

 basement membrane (/>. J/) is still distinctly seen, forming the 

 sheath of the nerve bundle. The interspaces of the adjacent 

 ommatidia are occupied by the mesodermic connective tissue. 



If we plot out the arrangement of the component elements in 

 the ommatidium of Limulus, the appearance shown in Fig. 7 

 will be presented. The epithelial cells {Eji) forming the walls 

 of the pit are drawn with their chitin-secreting surfaces turned 

 towards the axis of the ommatidium, which is morphologically 

 the exterior surface of the body. 



c. The Development of the Compound Eye. 



The first rudiments of the compound eye appear extremely 



early in the eml)ryonic stage as a pair of ectodermal thickenings 



above the level of the " dorsal organ." In Fig. 39, PI. 



XXXII, * shows the place where the compound eye makes its 



