LOWER COASTAL ANIMALS 



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metres, in places where the hard rock does not protrude, 

 one will normally find much oozy or muddy bottom, if 

 not pure clay. Here, however, the fauna changes charac- 

 ter under the influence of the lower temperature and 

 poorer food supplies, for which reason we cannot expect 

 such profilic hauls as over the shelf. Perhaps starfish ex- 

 emplify this most clearly. In shallow northern waters oc- 

 cur the common starfishes (Asterias) and certain multi- 

 armed starfishes (Coscinasterias), both notable for the 

 well-developed sucking discs on their rays. They are able 

 to turn their stomachs inside out thorugh their mouths, 

 and so digest prey which may be too big to swallow. But 

 prey so large as to require such extraordinary methods 

 are mainly found in shallow water. Now starfish are in- 

 capable of detecting their prey at a distance and can catch 

 only what they happen to touch as they roam about. The 

 common starfish does not normally move at more than 

 four or five metres an hour (other species up to nine 

 metres), and so suitable food must not be too widely 

 scattered. 



Unlike the shallow-water species, deep-sea starfishes 

 (Astropecten, etc.) have no sucking discs on the rays, 

 which in their case are pointed. On a clay bed they can 

 move very swiftly, many of them at the rate of more than 

 30 metres an hour, and they thus have a far wider range. 

 They cannot turn out their stomachs, but then the like- 

 lihood of encountering large prey is slight because deep- 

 water animals, on the average, are smaller and more 

 widely scattered. These factors explain — at least partly 

 — why it is the Astropecten group of starfishes which we 

 find among the commonest and most characteristic ani- 

 mals at depths of 200 metres and more, down to the 

 deep sea proper. 



Outstanding among other representatives of the fauna 

 at these depths — over the continental slope — are some 

 remarkable sea-urchins (EchinothuridcB). Ordinary sea- 

 urchins are supported by a strong skeleton, which not 

 only gives them their circular form but also holds up the 

 body wall. The animal is thus roughly the shape of a 

 tomato with the stalk side downwards. But when Echi- 



Silicious sponge (Hyalonema), with 

 stalk covered with a colony of sea- 

 anemones (Polythoa). 



