TOUCH 25 
sensations of which we can form little if any idea. And even 
when with reasonable certainty we can correlate sense-organs 
possessed by such animals with some of our own, it by no means 
follows that the vange of a given sensation is the same for one 
of them as for ourselves. As regards hearing, for example, 
there is reason to think that some animals can hear sounds which 
are pitched much higher than any by which we are affected; nor 
is this very surprising when we reflect that the range of hearing 
is not the same in all human beings. Many persons, for example, 
cannot hear the high and piercing sounds made by Bats. These 
remarks are made as a warning against applying the results of 
human physiology to lower animals with too great assurance. 
TOUCH 
Undoubtedly the most primitive of all the senses is that of 
Touch, and we may broadly state that the skin is the Tactile 
Organ, remembering that its outer layer, commonly known as 
the epidermis, is no other than the ectoderm or outer cell-layer 
of the embryo. We must also include here the cellular lining 
of the mouth-cavity and, when such exist, the nasal cavities, 
WGA 
Mis 
oN 
Th Rrain 
Fig. 1029.—Tactile Organs. a, Cells from the ectoderm of a Sea-Anemone (Actizia); 7., a touch-cell, with outer 
end produced into a stiff process; S¢., stinging cell, with sensitive trigger-hair (77.); GZ, glandular cell. 3B, Head of 
a Freshwater Annelid (Bokemilla comata), seen from above, and showing epidermis in optical section, enlarged; 
T., tactile processes of some of the epidermic cells, which are continuous internally with nerve-fibres. 
since these have been developed as in-pushings of the ectoderm. 
The external agents of stimuli which by their action upon the 
skin evoke sensations of touch are of two sorts. There are, in 
the first place, mechanical agents, such as contact or pressure, 
and, in the second place, heat-rays. The sensations which result 
are respectively known as haptzc and thermal. 
