90 



NA TURA L HIS TOR Y 



rarely an inch a day ; but time does not trouble the mussel. As he goes 

 higher;, the old byssal threads become useless ; but the mussel is able to cast 

 them off, for otherwise they would soon become an absolute bar against 

 further upward progress. 



The common sea-mussel (Mytilus edulis) is extensively eaten in Europe, 

 but here is almost wholly neglected. On the shores of France there are 

 immense 'parks' devoted to its culture. Stakes are driven into the mud, 

 and between them the space is filled by wicker-work, which serves as a 

 support for the molluscs. Here, nourished by the multitude of micro- 

 scopic forms in the water, they rapidly grow, hanging down in large 

 clusters. At intervals, when the tide is out, they are collected for the 



Fig. 83. — Date-shells (Lithodomus lithophagus) in their rocky burrows. 



market, the fisherman pushing his flat-bottomed boat over the slippery 

 surface of the mud, between the rows of stakes, and picking the larger 

 mussels from the basket-work, much as one would gather grapes from a 

 Still, mussels are not very palatable; indeed, they have been 

 known to cause serious sickness. 



^ The form illustrated in the cut above is a near relative of the species 

 just described. It has received the common name, date-shell, but by natur- 

 alists it is known as Lithodomus (stone-house). This name is very appro- 

 priate ; for while the young Lithodomus anchors itself by a bvssus, like 

 the common mussel, the adult bores a hole for a home in the "soft rocks 

 which are found in the seas where it occurs. How it or the other boring 

 molluscs excavate their burrows is a problem which naturalists have not 



