VI 



ARAGHNOIDEA—THE EYES 



521 



III. The Eyes. 



Most Avachnoidea possess eyes. These are unicorneal and are, 

 except in the middle eye of the Scorpions, constructed on the same 

 general plan as the ocelli of the Jntennata. The hypodermis is nearly 

 always continued under the cuticular lens to form the so-called vitreous 

 body. 



Number and Position of the Eyes. — The eyes of the Arachnoidea 

 are sessile and from 2 to 1 2 in number ; they lie symmetrically arranged 

 on the upper side of the cephalo-thorax. 



Solpucfidai : 2 large ocelli on one common prominence. Scorpionidm : 2-6 

 pairs of eyes, one pair of which, the gi-eat middle eyes, are placed close to the median 

 line, and the rest at the anterior edge of the cephalo-thorax. Cheriietidcc : 0, 1 or 2 

 pairs of eyes. Pcdipalpi : i pairs of eyes, the largest lying in the middle, the other 3 



Fig. 307. — A section through a middle eye of Euscorpius italicus (after Carri6re). c, Ohitinous 

 carapace ; I, chitinous lens ; liy, hypodermis, continued as so-called vitreous body under the 

 ohitinous lens ; p, pigment cells ; r, retinulse ; rfc, the proximal nucleated portions of the retinula; ; 

 no, optic nerves. B, A single retinular cell (r), with the rhabdomere (r/i), and the nucleus (fc). 

 C, A retinula with the pigment cells Pi, Pzf V:i, Pt- (-B and C, after Ray Lankester.) 



on the anterior edge of the cephalo-thorax. PJialangidce : usually 1 pair of eyes in 

 the middle of the cephalo-thorax on a prominence. In Cyphophthalmus there is an 

 eye on each side on a prominence ; in G-iibocellum 2 eyes on each side at the edge of the 

 cephalo-thorax, each on a prominence. Araneidce : generally 8, less often 6 or fewer 

 eyes, symmetrically arranged, generally in 2 transverse rows on the cephalo-thorax. 

 The special arrangement is of value for classification. Acarina : eyes are wanting, 

 or present in 1 or 2 pairs. Lingriatulidce : eyes wanting. 



The Structure of the Middle Eye of the Seorpionidse (Fig. 367). 

 — The middle eye of the Scorpion takes, according to its structure, an 

 intermediate place between a simple eye (ocellus) and a compound or 

 facet eye. It agrees with the ocellus in possessing one single cuticular 

 corneal lens, and with the facet eye in its retinal cells (understanding 

 these cells in Grenacher's sense) which form groups, the so-called 

 retinulse. 



