ABYSMAL FAUNA. 243 
reigns. One result of this may be the interesting fact, that in certain 
sessile animals, whose “strength is to sit still,” such as the Monax- 
onid Sponges, a perfectly definite symmetry of form is developed, 
aud they differ in this from their shallow-water allies, which are 
exposed to varying currents and conditions, and which are of any 
shape provided only it be asymmetrical. Curiously enough, the 
absence of movement seems to produce the contrary effect in certain 
radial but non-sessile animals. The Holothurians of the shallow seas, 
rolled about in the varying tides, turn now one radius now another, 
to the substratum. But the Holothurians of the deep, the Zzerorpz, 
unmoved by ebb or flow, presenting one face perpetually to the 
bottom, have developed a secondary bilateral symmetry, and pro- 
trude their tube-feet from one surface alone. 
No light from the sun penetrates the deep sea. There is no day 
and night. In connection with this absence of light from without 
certain animals, notably the Fishes, Crustacea, some Echinoderms, and 
Worms, have developed phosphorescent organs, but the part. they play 
in illuminating the depths can hardly be greater than that of the 
policeman’s bull’s-eye in lighting up London during a November fog. 
Corresponding with this darkness, lit up by an occasional phosphores- 
cent flash, the animals of the depths have either lost, or are losing, 
their visual organs, or have developed enormous eyes. With the 
absence of light may again be associated the uniformity of colouring 
of the denizens of the depths. Many of them are brightly coloured ; 
reds abound in the Crustacea, purple and green in the Holothurians, 
yellow and browns in the Crinoids, violet and orange in the deep-sea 
Meduse; a large number are uniformly white or whitish-yellow, or 
grey, and sometimes black; blue alone is rare. It has been suggested 
by more than one observer that the colours of some animals, e.g. the 
Crustacea, change as they are dragged up from the bottom to the 
surface and that some of the bright red forms as we see them are in 
the abysses of the ocean of a blue colour. But, whatever be the colour, 
it is, as a rule, uniform, and there is a marked absence of those bands, 
stripes and spots, which play so large a part in the life of dwellers 
on the land or in the shallow waters. The large size attained by 
certain groups, such as the Isopods, seems to be more nearly asso- 
ciated with a polar distribution than with the great depths. 
In the abysses of the ocean there is no sound. The organs which 
in Invertebrata are usually associated with the perception of sound, 
as a rule only reach a biyh degree of development in those forms 
which move actively about, and it is most probable that they act as 
balancing organs, not as hearing organs. The necessity for organs 
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