44 



NERVOUS SYSTEM AND SENSE-ORGANS 



great differences in detail, and much has yet to be learnt re- 

 garding the exact structure and use of the numerous parts which 

 are present. The most plausible explanation which has yet been 

 given of the mode of action of this sort of eye is that of " mosaic 

 vision ". According to this a visual pyramid is only stimulated by 

 light-rays which exactly correspond in direction with its long axis, 

 and numerous pyramids co-operate so as to enable the shape and 

 colour of surrounding objects to be perceived (fig. 1057). 



Fig. 1058. Sections through the Compound Eye of an Earwig (Forficula, A), and the Camera Eyes of a Spider 

 (Epeira diadema, B), and a Marine Annelid (Alciope, c), enlarged 



In A numerous radiating visual pyramids are seen, ending externally in the facets of the thickened cuticle, and con- 

 nected internally with nerve-branches; one of the pigmented zones is indicated. In B the cuticle is thickened into a 

 rounded lens, and behind this is a transparent layer, upon which abut the retinal cells, continuous with nerve-fibres; 

 each retinal cell contains a refracting rodlet. c is a vesicle, of which the external part is thickened into a spheroidal 

 lens, while the rest constitutes a retina, consisting of an internal refracting layer, separated by pigment from the 

 external sensitive part, into which nerve-fibres are seen running. 



Camera Eyes are found in Annelids, Arthropods, Molluscs, 

 and Vertebrates. Just as in a photographer's camera a picture of 

 external objects is imaged on a sensitive plate by means of a lens, 

 so also in a camera eye do we find refracting structures which 

 focus light-rays on a retina, or layer of sensitive visual cells. 

 Scattering of light is prevented in the former case by a blackened 

 lining, in the latter by a layer of pigment. 



One of the two exceptionally large eyes present on the head 

 of a marine Bristle- Worm (Alciope] is represented in fig. 1058. 

 A Sea-Centipede (Nereis] possesses four smaller and less complex 

 eyes of similar kind on the upper side of its head-lobe, and in 

 some of the tube-inhabiting Bristle-Worms (e.g. Branchiomma and 



