PELECYPODA. 33 



The Mytilidce., or Mussels, are too well known to need descrip- Cases 142- 

 tion. The small foot, which is brown in the common species, is 

 not much used in creeping about, but has -p-^ 27 * 



the power of spinning a byssus or bundle 

 of tough threads, by means of which the 

 animals attach themselves to rocks and one 

 another, forming colonies of vast numbers. 

 Mussels have always been much eaten in 

 this and other maritime countries, and large 

 quantities are brought to the London market 

 from the Dutch coast. Ki times they are 

 unwholesome ; but all the exact causes of 

 this are not known. Mussels seem to be 

 found on every shore, and some of the Trigonia margan'tacea. 



species are very widely distributed — the Case I4i. 



common edible Mussel, 31. edulis, being found on every European 

 coast, on the shores of North and South America, in the Arctic 

 and Antarctic Oceans, and probably on the coasts of Australia. 



One group of Mussels (^Lithodotnus, Case l-li) burrow in rocks and 

 other shells, forming holes just large enough to contain their shells. 

 L. dactijlus is sold as an article of diet on the shores of the 

 Mediterranean. 



The large family of PteriidcB includes the " Wing-shells " Cfi®^ ^*^" 

 (Pteria), the " Pearl " and " Hammer-Oysters " {Margaritifera 

 and Malleus). 



The " Hammer-Oyster " {Malleus) is so called from its rude 

 resemblance to a hammer. The " Pearl-Oysters " {Margaritifera mar- 

 garitifera, Fig. 28) possess rather heavy strong shells, lined with very 

 thick layers of "mother-o'-pearl." Hundreds of tons of these shells 

 are annually collected at the great pearl-fisheries of North and West 

 Australia, and imported into Europe. The pearl-oyster of Ceylon 

 {M. vulgaris. Case 147) is a smaller species, and collected more for the 

 pearls than the shells. The origin and formation of pearls has from 

 ancient times, even until now, been a subject of much discussion, 

 and many theories upon this subject have been propounded. Perhaps 

 that most generally accepted has been the " grain-of-sand " theory. 

 A particle of sand or other foreign substance getting between the 

 animal and its shell, or in the soft parts of the animal itself, was 

 supposed to produce irritation, and thus cause deposits of nacre upon 



* From ' The Cambridge Natural History.' Messrs. Macmillan & Co. 



J) 



147. 



