The Arthropoda 



323 



— <r. £ 



,-.' 



n. cA 



n.t. — 



Fig. 208. Ommatidium and Central Nervous 

 System. 

 A. An ommatidium or eye-element from 

 the eye t»f the Lobster, c, cornea (cuticle) ; 

 eh., corneal hypodermis, which secretes the 

 cuticle; co., cone cells; cr., crystalline cone; n, 

 nuclei; n.f., nerve fibres; r.d., distal or outer 

 retinula cells; r.p., proximal or inner retinula 

 cells; rh„ rhabdome, (After G. H. Parker.) 



B. A semi-diagrammatic view of central 

 nervous system of a crayfish, ab.l, ab.6, The 

 first and sixth abdominal ganglia; cer., cere- 

 bral ganglion; c.oes., circumcesophageal com- 

 missure; I.e., longitudinal commissures of 

 ventral cord; n.ab.L, nerves to abdominal 

 limbs; n.at.l, nerve to antennule; n.at.2, 

 nerve to antenna; n.ch., nerve to cheliped; 

 n.m., nerves to limbs adjoining the mouth; 

 o.n., optic nerve; s.oes., subcesophageal 

 ganglion; st.a., sternal artery; th.l, th.6, first 

 and sixth thoracic ganglia; v.n., nerve to 

 proventriculus ; v.n'., nerve to hind-gut. 

 (After Borradaile.) 



Thus, three adjoining facets might view the word "Biology" in this way: 



Bio olo ogy. 

 That is, facet one, would see the first three letters, facet two the 

 middle three, and facet three the last three. But since the range of each 

 facet overlaps that of the adjoining, the image formed is actually this : 



Bio ogy 

 olo 

 In other words, instead of an apposition image or mosaic, a super- 

 position image or continuous picture is formed. 1 



1 Microphotographic studies have definitely demonstrated that the account here given is the 

 correct one. 



