29 



PLATE XV. 



J_J O -B S T El R — continued. 



Nervous System — 



Fig. 1. Vektkal Chain of 13 Ganglia united by commisstires — 



Cerebral or supra-oesophageal ganglia anterior to gullet, giving off optic nerves. 

 Circum-oesophageal commissures round gullet. 

 Sub-oesophageal ganglia posterior to gullet. 



5 Eemaining thoracic ganglia, united to one another by double nerve-cords. 



6 Abdominal ganglia, united to one another by single nerve-cords. 



iVofe.— The numbers attaclied to ganglia denote the cephalic, thora'.ie, or abdominal segments to which they belon". 

 Fig. 2. Visceral jS^'erves in Elevation and Plan — 

 Nerves arising from — 



Cerebral ganglia— A'zygos — median, and branching posteriorly to lurm lateral azygos. 

 t Mandibular. 



Commissural G.^jglia J Anterc^lateraL 

 \ Jredio-lateral. 

 (. Postero-lateral. 

 Hepatic formed by the union of postero-lateral and lateral azygos. 

 Sensory Organs — 



Figs. 3, 4. General Structure of Eye — 



Coi-'nea, the transparent cuticle divided into a number of four-sided areas or facets (fig. 4). 

 Eadiating fibres (two are coloured red). 

 Optic nerve expanding into optic ganglion. 

 Flexible stalk. 

 Fig. 5. Minute Structure of one of the elements of the Compound Eye — 



Cornea. 

 r Cone. 



Nervous Elements J ?°''- „ 

 \ Spindle. 



V_ Nerve-fibril continuous with ganglion. 

 I Investing nervous elements and ganglion. 

 Sheath J Outer end continuous with cornea. 



J Inner end continuous with adjacent sheaths. 

 V Oval nuclei and pigment layers. 

 DiAG. 2. Formation of the Vertebrate Eye — 



a. Optic vesicle, a hollow process of the brain, involution of integument meeting it and pushin" 



it in. 

 h. Crystalline lens, a detached portion of thickened integument. 

 c. Eetina, the wall of the doubled-up optic vesicle nearest the lens (red). 



Choroid, the wall nearest the brain (blue). 

 Fiod and cone layer of retina embedded in the choroid by obliteration of the cavity between. 

 Diag. 1. Nervous Eli^ents of Eetina compared with nervous elements of fig. 5— 

 Eod, cone, and outer granular layers = cone, rod, and spindle. 

 Nerve-fibrils continuous with optic ganglion in botk 

 FiG. 6. Base of antennule cnt open to show auditory sac. 

 Fig. 7. Section turottgh Auditoey Sac — 

 Auditory opening. 



Sac, an involution of the integument lined by hairlikc filaments and containing fluid and grains 



of sand. 

 Auditory nerve frnm cerebral ganglia. 

 Fig. S. Hairlike filaments magnified. 



