236 



HISTOLOGY 



rfc. 



FIG. 208. Part of an ommatidium 



from the crustacean, Palamon parallel bristles Of 

 squilla. rh., rhabdomes; /., lens , _,, , 



or crystalline cone; vis.c., visual rOQS. 1 riCSC rOQS 



cell showing a nucleus near its come OUt at right 

 top. (After SCHNEIDER.) ,, 



angles to the axis 



of the cell, and are therefore at right angles to 

 the light-waves. They form the plates spoken 

 of in the first part of this chapter. 



Over the expanded ends of the rhabdomes 

 lies the end of the lens. This is composed of 

 several parts, four in number in most arthro- 

 pods, and these parts are formed by four thin 

 cells which lie just outside of and around 

 them. These are the lens-cells (see Fig. 209). 

 They rest proximally upon the fetinula cells 

 according to Grenacher; while according to 

 Patten they pass down to the basement mem- 



cell-organ, mentioned above as a rhabdo- 

 mere, as a rhabdome). This is the only 

 part of the whole eye that the light-waves 

 can affect so as to produce a stimulation, 

 and the rhabdomes transmit this stimula- 

 tion to the retinula cells as an impulse, 

 which is carried out to the optic nerve 

 centers through the proximally produced 

 cell bodies of the retinulae, or visual 

 cells, which, 

 therefore, serve as 

 nerve fibers. Fine 

 nerve fibrils have 

 been detected in 

 the rhabdomes of 

 Palcemon, and they 

 pass through into 

 the retinula cell 

 (see Fig. 208). In 

 fact, it can be seen 

 in Periplaneta that 

 the whole rhab- 

 dome is nothing 

 but an edge of the 

 retinula cell bear- 

 ing a row of in- 

 numerable tiny 



FIG. 209. Longitudinal section 

 of a single ommatidium of 

 Periplaneta orientalis. b.m., 

 basement membrane ; cu., cu- 

 ticle divided into corneal 

 areas; rt.c., retinula cells or 

 visual cells; rh., rhabdome or 

 cell-organ of light perception ; 

 /., lens; I.e., lens-cells; c.c., 

 corneal cells; sup.c., support- 

 ing cells (slightly modified 

 hypodermal cells); nv.f., 

 nerve fiber. (After R. HESSE 

 vnZeits.f. Wiss. Zool.) 



