COLOUR AND PHOSPËORESCENCÈ
183
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for the colour.
These are of several varieties, some being
black and others orange or yellow, while there are also
bundles of crystalline needles which act as reectors; The.
pigment cells can be increased or decreased, contractig
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to tiny points or sending out ramifying branches, and on
the relative degree to which the different kinds are expandédor
cohtracted depends the colour pattern of their
It is perfectly easy to experiment with at—sh on different
‘
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coloured backgrounds, -and the accurate way with which
the sh tone with them is shown in Plate 69…
In this case
light appears to be the deciding inuence in colour change,
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though it never acts directly on' the pigment cells.
A '
blind sh is not able to change colour.
The lack of
_
pigment from the under side of the body is apparently
due to the absence of illumination on this side under normal
‘
conditions, for if the sh are placed in glass bottomed tanks
illuminated €from beneath, they develop colour on the
under, as well as the upper, surface;
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.
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Many marine animals are coloured in such a way as” to
blend with their surroundings, but do not possess the power
of actually changing colour. 'ThlS is especially true Qfmany
of the sea slugs, such as the common sea lemon, Doris,
which lives on rocks and is yellow with mottlings of red
and- other colours, closely
the rock with its
encrusting sponges‘, sea -squirts; and red coralline seaweed ;
an allied sea slug, the red Dori5àmmea, always lives on
red sponges ; there isa small green sea slug named Elysia
which always lives on green weed, and this list could be
extended almost indenitèly.
Many, crustaceans are also
difcult“
detect in their natural surroundings and in
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this group, it may be remembered, there are many instances
of “ masking,” theanimals draping themselves in seaWeed,
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sponge, o;hydroids the better to di‘sguise themselves.
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Inother Cases the often vivid colouring of animals like.
.