We find the least development of the sense of sight in those animals which are unable to per ceive either images, movements, or direction, but can only dis tinguish light and darkness.
They react mostly to altera tions in the light intensity to which they are accustomed. A very common phenomenon of this kind is the "shadow reflex." Many of the lower animals, particularly those which dwell in shells or shelters of some kind, into which they can withdraw, do so directly a shadow falls on them.
An animal particularly suitable for the demonstration of this reflex is the vineyard (Roman) snail, Helix pomatia, which has a wide distribution in Europe. If we cause a shadow to fall upon its head, it withdraws its horns very quickly, or completely re treats into its shell. Among marine animals may be mentioned the small sessile crustacean, Balanus, which, every time a shadow falls on it, withdraws its feet, which normally move in a constant rhythm. Further, many tubicolous worms react in a similar way, and many bivalves close their shells, if a shadow falls on them.
In short, the phenomenon is unusually widespread. The biologi cal significance of this shadow reflex most certainly is that by this means the animal hides from danger, since the shadow may perhaps indicate the approach of some animal. This is, of course, a very poor expedient, since, in the first place, it fails to give protection from enemies which are in an opposite direction to the sun, and, in the second place, it causes the animal to withdraw unnecessarily thousands of times. Animals which do not possess any well-developed sense organs to give them certain warning of the approach of their enemies, are obliged, however, to help themselves even in this primitive manner.
The animals here mentioned, which respond to the slightest shadow, very frequently do not react in any way if we increase the degree of illumination. Other animals withdraw only in the latter circumstance, and are not disturbed by any shadows. Among these is the sand-dwelling "Ilya arenaria, which is found on the coasts of Europe and America. It lives in the sand at a depth of about 20 cm., and makes a shaft which leads upwards in a
vertical direction, in which lies its tubular siphon, the organ by means of which it takes in food and oxygen. If this fleshy siphon were pushed so far out of the shaft that it stretched out into the water, fishes would immediately come and bite it off. The light reflex serves to prevent this; as soon as the siphon pushes out into the water, it is exposed to stronger light, and the reflex movement of withdrawal promptly takes place.
It is just these simple phenomena which are frequently of in terest to the naturalist, since by their means we may easily understand the laws which govern the perceptual life of animals, and of mankind also. Weber's Law is very well known. This states that a man is first sensitive to a new stimulus when it stands in a particular numerical ratio to that already present.
Thus, if originally light "L" prevails, an animal sensitive to shadow first withdraws when the light is shaded by about L/2o. In this case the value of L is quite immaterial.
The table is compiled from the results of the author's experi ments on Balanus. indicates the original light, L2 the greatest amount of light which yet gives rise to the shadow reflex. is, as can be seen, roughly constant.
universally, was convinced that the lower organisms also possessed a sense of colour similar to his own. Later we went to the opposite extreme. Hess pronounced the definite opinion that all inverte brates and fishes were totally colour-blind, and that only the highest organisms, such as mammals, birds, reptiles and amphib ians, were able to enjoy the colours of nature. This sweeping generalisation was contradicted, and so gave rise to a very active investigation of the whole subject. The result of these labours may be summarised thus :—very many of the lower animals, such as cuttlefishes, insects and the higher Crustacea, do indeed possess a sense of colour. Further, in cases in which it has not yet been proved to exist we hesitate to draw the conclusion that it is absent.