150 THE LIFE OF CRUSTACEA 



and other pelagic fishes — that prey upon plankton 

 Crustacea appear to swallow them in bulk, with- 

 out much selection ; and the Greenland Whale, as 

 it swims open-mouthed through the sea, is not likely 

 to be guided by the greater or less visibility of the 

 Copepods that it sifts out on its baleen plates. 

 Further, this glass-like transparency is by no means 

 universal, for many plankton Copepoda are brightly 

 coloured. In some, as in the beautiful blue Anomalo- 

 cera, common in British waters, the colour is due to 

 pigment in the fluids and tissues of the body ; in 

 others the feathery hairs on the body and limbs 

 show brilliant metallic colours, produced, like the 

 colours of a peacock's feather, not by pigments, 

 but by the diffraction of light in the texture of 

 the organ. The most beautiful of all Copepoda is 

 Sapphirina, in which the surface of the body 

 absolutely sparkles with iridescent colours. 



The striking phenomenon known as the " phos- 

 phorescence of the sea " is familiar to every ocean 

 voyager, and is seen from time to time on our own 

 coast. On a dark night the crest of every wave 

 often seems to break in a pale glow, the wake of the 

 vessel is a trail of light, and an oar dipped in the 

 water seems on fire. This luminosity is due to 

 the animals of the plankton, largely to the lowly 

 Protozoa and the jellyfishes, but in part also to 

 certain Crustacea. A number of pelagic Copepoda 

 have been shown by Giesbrecht to secrete, from 



