THE EYES OF SPIDERS 123 



types of eye there has been a change not only in the position and direction 

 of the retinal cells with respect to the incidence of light, but in the verte- 

 brate or inverted types of eye, owing to the evolution of a single biconvex 

 lens in a position which will cause an inverted image to be thrown upon the 

 retina, as indicated in Fig. 83, the rays coming from the upper part of 

 the object fall upon the lower part of the retina, whereas in the upright, 

 aggregate type of eye, of insects and Crustacea, the retinal image is not 

 inverted (Fig. 84), since each ommatidium can only transmit those rays 

 of light which fall upon the surface of the eye in the direction of its longi- 

 tudinal axis, and the retinal image is not inverted. A decussation of the 

 fibres of the optic nerve, however, takes place in the chiasma externa in the 

 higher arthropods (Fig. 85), and in the Cephalopods (Fig. 122, Chap. 12, 

 p. 166), whereby the cortical impressions received in the optic lobe form, 

 it may be presumed, a mental picture which is in the same position as the 

 object viewed — this picture, it is believed, being similar as regards orienta- 

 tion to that formed in the visual cortex of the human brain. 



In the higher Mollusca the lateral eyes resemble the vertebrate type 

 with regard to the position of the lens and iris relative to the retina, but 

 differ in having an upright retina and in other respects which will be 

 mentioned in Chapter 12. 



The Eyes of Spiders — Arachnida 



The Class Arachnida includes, in addition to the various forms of 

 spider, the scorpions, mites and ticks, king crabs and the extinct Eury- 

 pterida. The Class is much less homogeneous than the Insecta, and there 

 is considerable variety in the number of eyes present in each animal 

 and in the type of eye. As in insects, many of the spiders have both 

 median and lateral eyes. The lateral eyes of the adult animal are, how- 

 ever, not compound, as in many of the insects, but are of the ocellar 

 type, having a single, non-faceted cuticular or corneal lens. As a rule 

 the upright eye is simple in type with respect to the retina, but the central 

 eyes of the scorpion differ in having the sensory cells grouped together in 

 a form resembling somewhat the retinulae of the compound eyes of insects 

 and crustaceans . Compound eyes are also present in Limulus, in which the 

 retinulae of the lateral pair of eyes (Fig. 86), are barrel-shaped and lie beneath 

 papilliform downgrowths of the cutis, each of which is believed to serve as a 

 separate lens for the retinula which lies beneath it. The central eyes of 

 Limulus (Fig. 87) also show a grouping of the sensory cells into units 

 which have been described as resembling retinula;, and have a marked 

 general resemblance to those found in the central eyes of Euscorpius. 

 These tulip-like buds (Figs. 86 and 87), however, differ markedly from the 

 compact cylindrical retinula; of the eyes of insects and Crustacea, and 



