HIGH SCHOOL ZOOLOGY. 



195 



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well in the chelae and elsewhere. Similar ingi-owtha protect 

 the ventral nervous system. 



8. We distinguish in the nervous system, 

 the brain and ventral nerve-cord, the latter 

 composed of a chain of paired ganglia, con- 

 nected by longitudinal commissures. Of such 

 ganglia the last eleven segments in front of 



J the telson have each one pair, but the ganglia 

 of the five segments in front of these have 

 coalesced into an infracesophageal ganglionic 

 mass. This is luiited to the brain, or supra- 

 oesophageal ganglia, by commissures which 

 lie at either side of the oesophagus. The 

 brain supplies nen^es to the eyes and an- 

 tennae. All of the nerves in the crayfish, as 

 well as other Invertebrates, are of the non- 

 meduUated type. 



9. An examination of the sense-orerans 

 shows that they differ both in position and 

 structure from those of Vertebrates. The 

 eyes are elevated in this order of Crustacea 

 (^Podophtliahnata) on movable stalks, and 

 they are of the compound ty2:»e so character- 

 istic of most Arthropods (Fig. 121). 



II /^ 



The stalk is partly occupied by muscles, but 



oSaldium IrSeS ^"^^'^^ ^^^ ganglionic expansions of the optic nerve, 

 of the Crayfish ; c, cu'ti- from the outermost of which the nerve-fibres pass 

 cular facet formed hy un- ~,,, ,, , , ,., 



derlying hypodermal ott through a basement membrane to end m the 



cells ch";?, pigment cells jnodified epidermal cells which constitute the eyes, 

 surrounding the retino- ^ _ _ •' 



phoral cells and cc, the These cells are disposed in three zones, the outer- 

 crystallinecone; r. retinu- i. c ^ ■ ■> i. ±\. j.- i r j. /• ii 



1* ; rh, rhabdome ; bm niost ot which secrete the cuticular facets of the 



basement membrane ; n, gyg . each facet corresponds to an element (om- 



matidium) of the compound eye, and is formed by 



two cells of the outermost zone ; underneath these are the four retino- 



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