ZOOPLANKTON I 



themselves, they are preyed upon by many other creatures from the big 

 toothed whales down. Squid indeed is one of the best foods for animals kept 

 in marine aquaria and is good crab bait. Few indeed are the animals in the 

 sea that eat animal tissue, alive or dead, that do not take it, or even prefer it 

 to their normal food. Squid, and octopus too, are of course a popular food 

 in the Mediterranean countries and boxes of squid are now landed at the 

 British fish markets for overland transport across the continent, thus bringing 

 in a cash return for something that was previously dumped. One of these 

 days the British housewife and her family will realize its excellent taste and its 

 value as food and prices will go up as the demand increases. 



One of the interesting features of these cephalopods, big or small, is their 

 ability to change colour very rapidly indeed. This they do by the expansion 

 and contraction of pigment spots. When fully contracted the spots are minute 

 and far between so that the animal is almost pure white, or the smaller ones 

 very nearly transparent. As the spots expand their colour shows and the 

 animal becomes at first speckled and then completely coloured, usually a 

 brownish-reci. Not all the pigment spots need expand at the same time, and 

 as the expansion need take only seconds, waves of colour come and go in 

 different directions, making it very difficult for a predator to be precisely 

 sure where his prey is. Many are brightly luminescent at night with rows of 

 light-producing photophores arranged in characteristic patterns. Some are 

 illustrated in colour in Plate XV. 



Squid and octopus are particularly fond of crab and other crustacean 

 food, and to aid the capture of these active and well-armoured animals the 

 salivary glands of cephalopods produce a crab-paralysing agent called 

 cephalotoxin. 



63 



