40 NERVOUS SYSTEM AND SENSE-ORGANS 
sudden variations in the amount of illumination. But even this 
limited sort of vision may be of the greatest importance to its 
possessor, since it often gives valuable information about the 
surroundings. In such skin-seeing (dermatoptzc vision) it is usual 
to find colouring-matter or pigment in or below the epidermis, 
which localizes the action of light-rays upon sensitive cells in this 
layer. This is the case, for instance, in Earth-Worms, the safety 
of which must often depend upon their avoidance of light. A 
further and more interesting illustration is afforded by many of the 
bivalve Molluscs which live in sand or mud, and which feed and 
breathe by means of two tubes, the siphons, which project from 
the hinder end of the body (vol. ii, p. 249). Such animals are 
often found hidden in their bur- 
rows with only the extreme tips of 
the siphons projecting. But even 
though thus concealed they would 
more frequently fall victims than 
they do to octopi and fishes, or, in 
the case of those which live between 
tide-marks, to strand-haunting birds, 
Fig. 105t.—A, Euglena viridis, enlarged: # were they not provided with some 
flagellum; 7., mouth; V., nucleus; £.v., pulsating 
Suto ee OF Seen We provi? 
of such enemies. Warning is often 
given by the siphons themselves, which are commonly pigmented 
and sensitive to changes in light-intensity. And experiments 
on specimens kept in aquaria have shown that the fully-extended 
siphons are rapidly drawn in if a shadow is suddenly cast upon 
them, an event that would happen under natural conditions on 
the approach of a voracious fish or too inquisitive bird. 
Eyes.—Localization and improvement of the powers of sight 
have led to the evolution of definite visual organs or eyes, though 
many of the lower Invertebrates have more or less retained the 
old faculty of diffuse skin-seeing. The simplest organs of the 
kind are known as eye-spots, and their presence is marked by 
dense pigment. These are possessed even by some Animalcules, 
e.g. by a little green creature (Zuglena viridis) which often swarms 
in stagnant water (fig. 1051). The eye-spot in this case is marked 
by the presence of a tiny patch of red colouring-matter on which 
rest several little lenses that serve to concentrate the light. 
In some of the Jelly-Fish the margin of the umbrella bears 
