78 NATURAL HISTORY OF OUR SHORES 



more elevated in proportion to its size than spatangus. 

 Its colour is usually greyish brown, sometimes buff. Its 

 spines are not so rigid as those of the other heart urchins, 

 but are almost silky. 



A rarer species, the " Fiddle Urchin " (Brissus lyrifer), is 

 sometimes met with in the littoral, but it belongs to deeper 

 water. 



These heart urchins often betray their presence in places 

 where they were not even suspected, by coming out of their 

 hiding below the surface when the tide begins to return. 



I have often watched a smooth expanse of flat shell 

 gravel at this period, and seen the surface suddenly rise 

 into little mounds like molehills on all sides. Then from 

 each mound a spatangus or an amphidotus would tumble 

 out, often turning upside down in the process. 



Many other burrowing animals take part in this little 

 resurrection on the turn of the tide cockles, mactras, 

 various sand crabs e.g. Thia polita, Portunus marmoreus 

 (which we shall come to presently), and many other forms. 

 The tide turn gives a good, but very brief, collecting time. 



There are one or two other species of heart urchin, 

 but they are too rare to be hoped for by the occasional 

 collector. 



There is one, however, which must not be passed over. 

 This is the little " Pea Urchin " (Echinocyamus pusillus). It 

 is the smallest of the family, a full-grown one only measur- 

 ing about half-an-inch in length. Its colour that is, the 

 colour of its little spines, which look like the pile on furniture 

 velvet are a deep green. It is found in sandy and muddy 

 localities, usually close to the fields of Zostera. It must 

 abound in some parts of the Channel, for on some shell 

 beaches its little test, denuded and bleached to white- 

 ness, is very abundant. 



While the " regular " sea-urchins are predaceous, cutting 

 up and eating any organic substance they light upon, from 



