26 NERVOUS SYSTEM AND SENSE-ORGANS 
Single epidermal cells or groups of such cells are specialized 
for the reception of stimuli leading to sensations of touch, but 
in such forms as Ccelenterates and Annelids many scattered 
cells of the kind probably minister to other senses besides 
that of touch. And it must be re- 
membered that even the special sense- 
cells of hearing and sight are derived 
from the skin, which is in fact the 
primitive sense-organ. Cells which are 
regarded as tactile, from some of the 
lowest animals, are represented in fig. 
1029. 
The firm external covering with 
which the bodies of Arthropods are 
clothed is naturally a hindrance to the 
Fig. 1030.— Tactile Organs of Insects, é 2 7 a 
greatly enlarged. On the right is a group of reception of stimuli by the underlying 
such srucures, and on the left asingleone, enidermis. The difficulty is got over 
by the existence of little pores in the 
hard investment. Under each pore is an enlarged sense-cell, 
placed at the base of a stiff tactile bristle, with which external 
bodies come into contact (fig. 1030). 
In aquatic Vertebrates the sense-cells of the skin are in direct 
contact with the surrounding medium, although they are not 
infrequently protected by being 
lodged in pits, grooves, or canals 
which open at intervals to the ex- 
terior. But in terrestrial Verte- 
brates there are special end-organs 
of touch which have sunk below the 
epidermis, though they remain suf- 
Fig. 1031.—Organs of Touch - 
A, Small piece of the skin of a Frog, in vertical ficiently near to the surface to be 
section, enlarged; £/., epidermis; 7.C., touch-cor- stimulated when the body comes into 
puscles; .V.F., nerve-fibres. 3B, Touch-corpuscle 
from the bill of a Duck, much enlarged; S.C., sense- contact with surrounding objects. 
cells; V.F,, nerve-fibre; S%., fibrous sheath. 
Such are the groups of touch-cor- 
puscles which are to be found in the skin of the Frog, and 
around the edge of the Duck’s bill (fig. 1031). The latter animal 
feeds upon small worms, &c., which live in the mud that is 
strained through its bill, and such special arrangements are 
clearly necessary to aid in the discrimination between what is 
edible and what is not. Another example is afforded by the 
