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 (dermatoptic 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 IOSI.-A, Euglena viridit, enlarged; fi., ^ t proved with SOttlC 



nagellum; m., mouth; N., nucleus; p.v., pulsating 



vacuole. B, Eye-spot, greatly enlarged, showing meanS of detecting the proximity 



six rounded lenses resting on a mass of pigment. . * 



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 (Euglena 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 



