CHAPTER XVII 



THE ECHINODERMS 



THE Echinoderms, or prickly-skinned animals, are exclusively 

 marine, so that they have no representatives in the freshwater 

 aquarium. At the present day five kinds occur in our seas. Of 

 these, the sea-urchins, the starfishes, the brittle-stars and the 

 sea-cucumbers, are all represented in the shore pools, but the 

 fifth kind, the beautiful sea-lilies, are only to be found in deep 

 water. On some parts of our coast, however, fossil sea-lilies are 

 to be found abundantly in the shore rocks, even in such rocks 

 as shale, which is merely the hardened mud of an old sea beach. 

 This shows that in the geological past the sea-lilies must have 

 lived in shallow water, and the fact that at the present time the 

 vast majority live in the great ocean depths, suggests that they 

 have been thrust out of the shallow water by the competition of 

 modern, highly specialised forms like the brittle-stars. Some 

 interesting facts are therefore suggested by putting into the 

 aquarium a piece of shale with fossil sea-lilies (Crinoids) in com- 

 pany with living starfish and brittle-stars. 



If we begin with the sea-urchins we find that the common 

 urchin (Echinus esculentus) is nowhere frequent in the shore pools, 

 though it may be occasionally taken there ; its habitat is normally 

 just below low- tide mark. In any case it is too large to be 

 readily kept alive in confinement. On the other hand, its ally 

 the purple-tipped urchin (E. miliaris), which is very much smaller, 

 is really common in the pools, and may be kept without difficulty 

 in the aquarium. It should be looked for in the Laminarian 

 zone, where so many interesting animals are to be found, and 

 therefore a low spring tide should be utilised for the expedition 

 in search of it. The diameter of the shell, or test, is often about 

 that of a penny piece, but specimens of a considerably larger size 

 may also be found. The test is depressed and is clothed with 



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