29 



PLATE XV. 



J_lO-DQ J. -ER continued. 



NERVOUS SYSTEM- 

 FIG. 1. VENTRAL CHAIN of 13 GANGLIA united by commissures 



Cerebral or supra-oesopliageal ganglia anterior to gullet, giving olf optic nerves. 

 CircunwEsophageal commissures round gullet. 

 Sub-O3sopliageal ganglia posterior to gullet. 



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



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



Note. The numbers attached to ganglia denote the cephalic, thoracic, or abdominal segments to which they belong. 

 FIG. 2. VISCERAL NERVES in Elevation and Plan 

 Nerves arising from 



Cerebral ganglia A zygos median, and branching posteriorly to form lateral azygos. 

 ( MandiLmlar. 



COMMISSURAI GANGLY J Anteio-latemL 



) Medio-lateral. 

 V. Postero-lateral. 



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

 SENSORY ORGANS 



FIGS. 3, 4. GENERAL STRUCTURE OF EYE 



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

 Radiating 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. 

 / Cone. 



NERVOUS ELEMENTS 



Spindle. 



Nerve-fibril continuous with ganglion. 

 i Investing nervous elements and ganglion. 

 SHEYTII J Outer end continuous with cornea. 



J Inner end continuous with adjacent sheaths. 

 V. Oval nuclei and pigment layers. 

 I)IAG. 2. FORMATION OF THE VERTEBRATE EYE 



a. Optic vesicle, a hollow process of the- brain, involution of integument meeting it and pushing 



it in. 



I). Crystalline lens, a detached portion of thickened integument. 

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



Choroid, the wall nearest the brain (blue). 



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

 DIAG. 1. NERVOUS ELEMENTS OF RETINA compared with nervous elements of fig. 5 

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

 Nerve-fibrils continuous with optic ganglion in both, 

 FIG. 6. Base of antennule cut open to show auditory sac. 

 FIG. 7. SECTION THROUGH AUDITORY SAC 

 Auditory opening. 

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



of sand. 



Auditory nerve from cerebral ganglia. 

 FIG. 8. Hairlike filaments magnified. 



