o b 



ARTHROrODA 



thickoiiing sharply marked off from the rest of the epithelium. These 

 eyes, hke those of \\^rtebrates. form iiiwrted images. 



Spiders have two kinds of occlU \h'^. 3 7i\ ihc ehief eves and the aeeessory. 

 In the first ^.1) the rhabdomes are next ihe \alreous bod\' and in Iront of the 

 miek'i of the eells. In the other lA'l the reUttions are reversed, the struetiire 

 reeahitiL; the inwrted e\-es of \ertelirates. .\p[iarently the tirsl are for distant 

 \"ision, the others for near objeets. 



The cotiipound e_\'es are mueli larger. They owe their rtame faceted 

 C}'es to tlie fact tliat the cuticle over thetii is di\ided iiUo polygoiial (usually 

 hexagoiral) areas or facets, luich facet corresponds 

 to a stnall cliititious lens (the luimlier of wluch varies, 

 it! dilTeretit species, between a dozen and several 

 thousaiid), and boitnds llie eye externally, whence 

 tJiis layer is called tJie cornea (lig. 37;). The part 

 of the eye beneath tlie cornea consists of radially 

 arranged pvramidal parts or oiiuiiatidia wliich corre- 

 sporid in nunrber arid position to the facets, t.heir 

 broader ends being placed beneath the facets, tiieir 

 narrower internal ends conneclirig wit.h libres of the 

 optic rterve v.duch go to the braiir. Each omma- 

 tidium (fig. 373) has essentially t-he structure of an 

 ocellus: (i) the lens (1) with its epithelium; (2) the 

 virtreous body {kz)\ (0 the retinula {rz). Tlie 

 vitreous bodv is usuallv composed of four cells 

 wliich, in the so-called ciicoiious e\es, surround a 

 tratrspareiit body, the cn'stallitie cone [k"), secreted 

 by these cells. ddie retinular cells are almost 

 always seven in rtumber, each bearing on its iiiirer 

 surface a rhabdome (/•"), the se\-en rhabdomes fre- 

 quently fusing itito a commort mass, hatch omma- 

 tidiutiT is surrounded by a pigment sheath, isolating 

 it optically from its fellows. 



Fig. 37j;. — A sintjlc 

 onunalidium (witti 

 sections) of a com- 

 pound eye. k, crys- 

 tnUine cone: hz, cone 

 cells; /, lens with 

 h\'iioilcrmis; r, rhnh- 

 (loincs; rz, retinular 

 cell. 



There arc really two pigment zones, an anterior which corresponds function- 

 ally to the vertehrale iris, and a posterior eoniparahk' lo tapetnin and chorioid. 

 The iris pi.t^ment lies in speei:il eells :tnd is most alumdant at tlie line between 

 the vitreous body and retiiutla. The posterior pigment is in the relitailar cells. 



From tliis it appears that the compound eye may be regarded as a 

 complex of ocelli. This anatomical cotiception must not, lunvever, ob- 

 scure the physiological. The whole compound eye forms but a single 

 erect picture composed of separate images of small area formed bv the 



