LIST OF ILLUSTRATIONS. 



FIGURE PAGE 



1. Diagram to illustrate possible origin of a sense-organ, c, Cuticle; 



A, cellular or hypodermic layer .. .. .. .. 3 



2. Diagram to illustrate possible origin of a sense-organ, c, Cuticle ; 



A, cellular or hypodermic layer .. .. .. ,. 3 



3. Diagram to illustrate possible origin of a sense-organ, c. Cuticle ; 



h, hypoderm ; n, nerve . . . . . . , . . . 4 



4. Diagram of further stage in the origin of a sense-organ .. 4 



5. Diagram illustrating a second possible origin of a sense-organ 5 



6. Diagram of further stage iu the origin of a sense-organ . . 5 



7. Section through the simple eye of a young Dytiscus larva. A, 



Hypoderm ; ?, lens ; o, optic nerve ; g, p, modified hypodermic 

 cells ; r, retina . . . . , . . . . . . . . . 6 



8. Auditory vesicle of Ontochis .. .. .. .. .. 6 



9. Pacinian corpuscle, a, Neurilemma ; 6, nerve-fibril ; c, capsule ; 



d, peculiar 6bres ; e, central cylinder . . . . . . . . 8 



10. Papilla from the surface of the hand, X 350. a, Cone-like body ; 



6, nerve ; c, end of nerve . . . . . . . . . . 8 



11. Portion of the skin of the back of the hand. The centre figui'e 



represents the arrangement of the hairs ; CJPj the cold-points ; 

 WP, the warmth-points . . . . . . . . . . . 10 



12. Half a cross section through the brain and hinder pair of eyes of 



Nereis cultrifera, 1, Hypoderm ; 2, cuticle ; 3, retina ; 4, 

 outer corneal cells ; 5, inner corneal cells ; 6, brain ; 8, 8a, 

 two places to which the brain sends large nerves, but where 

 the cuticle is unaltered ; g, gelatinous body . . . . . . 12 



13. Part of upper nerve-ring and tactile epithelium of Lizzia. a, 



Tactile epithelium ; g, ganglionic cell ; nr^, upper nerve-ring 12 



14. Diagram of part of the skin of a sea-anemone (Actinia), dz, 



Glandular cell ; nz, nervous cell . . . . . . . . 13 



15. Anterior part of body of Bohemilla comata, lb, Tactile hair; 



