VI COLOURS OF CRUSTACEA AND ECHINODERMA 131 
2. Ophiuroids .—The brittle-stars resemble the 
star-fishes in displaying very considerable brilliancy of 
colour. According to Agassiz’s account the shallow- 
water forms exhibit the greatest variety of colour, 
being blue, green, red or yellow, while deep-sea forms 
are more usually bright orange or red. Agassiz 
(ii. p. 133) makes the interesting observation that 
the colours of the deep-sea forms fade much more 
rapidly in alcohol, than those of the denizens of 
shallow water. This strongly recalls the conditions 
already emphasised for Crustacea, and is probably 
due in the same way either to a combination between 
the pigment and lime, or to the want of penetration 
of the alcohol into lime-containing tissues. The 
brittle-stars not infrequently resemble their surround¬ 
ings in colour ; the common “ Sand-stars ” of our 
own shores are good examples of this, and various 
observers speak of the resemblances in colour between 
the forms living among gorgonians and corals, and 
their organic surroundings. This is, however, in 
curious contrast with a statement made by Kiikenthal, 
to the effect that forms living in the interstices of 
coral-reefs are well protected by their surroundings, 
do not need protective tints, and are therefore “mostly 
of a blackish colour.” 
3. Sea-urchins. — In these we have the same 
difference in permanence between the colours of the 
deep-sea and shallow-water forms as in the brittle- 
stars. The colours seem to be usually reddish, some 
are violet or claret-coloured, others brown or orange. 
A form (Diadema setosa ) which is found on coral- 
reefs has five bright ultramarine blue spots arranged 
round the aboral aperture, while Astropyga freuden- 
