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 Kukenthal, 

 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- 



