THE SENSE-ORGANS 467 



a support and protection for the essential elements of the sense- 

 organ, which may thus attain a high degree of sensitiveness. 



Regarding the differentiation of the sensory cells themselves, 

 they may present at their free end a ciliated or simple sur- 

 face, or may bear one or more flagella. In the most special- 

 ized types the flagella themselves may be modified for the better 

 reception of certain definite forms of impression, as in the 

 case of the rods and cones of the retina, or the acoustic hairs 

 of the inner ear ; these types, however, owing to the extreme 

 delicacy of the projecting parts, can exist only upon an external 

 surface bathed by water, as in many aquatic invertebrates, or 

 upon a surface that faces some internal cavity furnished with 

 an artificial fluid or semi-fluid. 



Accessory organs for the reception and intensification of the 

 external stimuli or for the protection and care of the essential 

 parts are the rule in the case of the more specialized and com- 

 plex organs, but are not employed to assist in the reception of 

 general tactile impressions save in certain invertebrates with 

 a thick and hard exo-skeleton which would naturally prevent 

 such impressions from reaching the interior. In this latter 

 case the sensations are transmitted by sensory hairs which are 

 protruded through pores in the external armor, and communi- 

 cate with underlying sense-organs. In those vertebrates in 

 which the integument is covered by non-sensitive parts, such 

 as scales, feathers, or hairs, often necessary to protect the ani- 

 mal from serious injury, the sensory organs are developed 

 between them, or may be situated about their bases, when they 

 are stimulated indirectly through the movements of the insen- 

 sitive outer parts, much as in the previous case. 



Concerning the actual sensations produced by the different 

 kinds of sense-organs found among vertebrates we know very 

 little, and inferences must be made with extreme caution. Al- 

 though something can be deduced from the mechanical struc- 

 ture of a terminal organ, especially from that of its accessory 

 parts, and although from- the physiological side something can 

 be learned from the behavior and responses of an animal under 

 observation, the only certainty concerns our own sense-organs 



