LIFE AT THE SURFACE OF THE SEA, 193 
The Pteropod ooze, which is found in tropical and sub- 
tropical regions at depths of less than 1500 fathoms, is 
composed of the shells of Pteropods, Heteropods, and 
Foraminifera, and of the siliceous skeletons of Radiolaria. 
We see, then, that the surface of the sea, not only near the 
coasts, but over the vast expanses of the oceans, swarms with 
life. We have seen that the fauna of the surface is a highly 
representative one, including examples of some of the highest 
and most specialised species, as well as of the lowest and 
most primitive. We have seen also that many species 
and some classes of animals, which in adult life never appear 
a surface, may yet pass through one or more free- 
swinming larval stages. Nearly all pelagic animals tend to 
occur in enormous swarms, even microscopic species being so 
abundant as to discolour the water for miles. Then, again, 
another very characteristic feature amongst the smaller 
animals is their transparency. Nerves, muscles, skin may 
all be quite transparent, the eyes alone in some instances 
being pigmented. The larger animals, instead of being 
transparent, are often of a blue colour on the upper surface, 
and white or pale underneath. We have also noted that 
the majority of the animals of the Plankton are phospho- 
rescent. Even amongst the fishes we sometimes find 
photogenic organs. Most of the Planktonic forms, again, are 
nocturnal in their habits; or perhaps it would be more 
accurate to say that they seem to love the darkness rather 
than the light, for during the day they sink into deeper 
water, to appear again at the surface after dark. It is an 
interesting and important fact that sunlight penetrates 
only for a short distance into the water, so that, at the 
comparatively slight depth of 50 fathoms, it is practically 
quite dark even during the brightest sunshine. Recent 
observations with very sensitive plates, however, show that 
some rays of light do penetrate to considerably greater 
depths. 
The very wide distribution of many of the pelagic species 
has been referred to, but perhaps it should be specifically 
stated that certain groups are characteristically abundant in 
certain areas. In the Arctic regions, for example, one finds 
especially vast numbers of free-swimming Crustacea, and of 
