SURVEY OF THE ARACHNIDAX EYES. 



69 



In tracing back the Arachnid eyes to compound eyes, in order to 

 understand their structure, Ave must first explain the origin of the 

 latter. It facilitates matters if we refer to the probably analogous 

 conditions in the Insecta. The simplest eyes (ocelli) of the latter are 

 pit-like depressions of the hypodermis, over each of which lies a single 

 lens. Corresponding simple eyes are found in the Annelida; these 

 are mere pits in the body-integument, richly pigmented and with a 

 lens lying in each (Diopatra and Onuphis, v. Kennel, Xo. 60). The 

 ocelli of the Insecta are on a somewhat higher level, the hypodermal 

 layer bulging inwards from the sides over the retina, and thus 

 forming the so-called vitreous body beneath the lens (Fig. 36, gh). 

 In the Myriopoda, the Insect larvae, and the Thysanura, a number of 

 such ocelli are found grouped together. If we imagine their number 

 still further increased, and a closer connection formed between them, 

 we have the beginning of a compound facet-eye. In the Myriopoda, 

 especially in Scutigera, such a condition seems actually attained. 

 The eye which has thus arisen has at first no kind of uniformity. 

 Its constituent parts seem still to be independent structures of too 

 complicated a nature to permit of co-operation. A gradual reduction 

 of the elements occurs in the single eyes, and, as it progresses, leads 

 to the result known in the compound eye, viz., to the retention of 

 only a few of the elements of each primarily single eye. The 

 single eye has in this way become an ommatidium. We thus 

 imagine the ommatidia as 



arising out of ocelli. The . , 



rationale of this process, 

 which led simultaneously 

 to the reduction of the 

 elements in the ocellus, 

 and to an increase in 

 number of ocelli, must be 

 sought in the function of 

 the compound eye, which 

 requires the utmost attain- 

 able diminution of the 

 visual surface in the single 

 eye. 



Such very simple eyes as the ocelli of the Insecta are not found in 

 the Arachnida, but we must presuppose the presence of similar 

 structures in their ancestors. The most simple Arachnidan eye is 

 the lateral eye of Scorpio (Fig. 11 B, p. 17), a unilaminar eye, in 



Fig. 36. — Section through the ocellus of a Dytiscus 

 larva (after Grexacher, from Hatschek's Text-book 

 of Zoology). 6, basal membrane ; c, chitinous 

 cuticle ; gk, vitreous body ; h, hypodermis ; /, lens ; 

 2)2, pigment-cells ; /-, retina. 



