1058 OYSTERS, AND ALL ABOUT THEM. 



abandoned snail-shells begin to move about, caused by the 

 hermit-crabs (Pagurus bernhardus], which have taken up 

 their residence in them, trying to creep out of the heap 

 with their dwellings Here and there a ring- 

 worm (Nereis pelagica], of a changeable bluish colour, slips 

 out of the mass of partially dead, partially living, animals. 

 . Even the shells of the living oysters are inha- 

 bited. Barnacles (Balanus crena'tus], with tent-shaped, 

 calcareous shells, and tendril-shaped feet, often cover the 

 entire surface of one of the valves. . . . Upon many 

 beds the oysters are covered with thick clumps of sand, 

 which are composed of the tubes of small worms (Sabel- 

 laria anglica]. These tubes, called " sand-rolls," resemble 

 organ-pipes, and are formed from grains of sand cemented 

 into shape by means of slime from the skin of the worm. 

 The shell forms a firm support upon which the worms can 

 thus live close together in a social .community. Upon 

 certain beds near the south point of the island of Sylt, 

 where the finest flavoured oysters of our sea-flats are to be 

 found, there lives upon the oyster shells a species of tube- 

 worm (Promatoceros triqueter}, whose white, calcareous, 

 three-sided tube, is very often twisted about like a great 

 italic S. The shells of many oysters upon these beds also 

 carry what are called " sea-hands " (Alcyonium digitatuni}, 

 which are white or yellow communities of polyps, of the 

 size and shape of a clumsy glove. . . . Within the 

 substance of the shell itself animals are also found (such as 

 the Clione celata], and in the spaces between the layers of 

 the shell in old oysters is found a greenish-brown worm 

 (Dodecaceraea concharuni}, armed with bristles, and bearing 

 twelve large tentacles upon its neck. I once took off and 

 counted, one by one, all the animals living upon two 

 oysters. Upon one I found 104 and upon the other 221 



