MUSSELS 439 



of them breathe by gills or branchiae. (Contrast with pulmonate gastro- 

 pods.) Between the leaves of these gills, also, the larvce, after their 

 escape from the eggs, pass through the earlier stages of their develop- 

 ment. They next attach themselves to fish and tadpoles, which they do 

 not leave until some months later, when they have developed into the 

 perfect bivalve. 



Other Lamellibranch Species. 



In ponds we frequently meet with a species of bivalve, the so-called 

 Pond Mussel (Anodonta cygnea), which reaches a length of 7J inches. 

 Inhabiting as it does calm water, its shell is usually thin, and the "hinge" 

 toothless. 



On the other hand, in the Fresh- Water Pearl Mussel (Unis 

 margaritifer), which inhabits rapid mountain streams, we meet with a 

 strong hinge provided with large teeth. This mussel is found in Northern 

 Europe and America, and, like other fresh-water mussels, furnishes pearls 

 (hence name). These owe their origin to the introduction of grains of 

 sand or similar bodies between the mantle and the shell, which act as 

 an irritant on the pallial membrane, and hence become surrounded by 

 nacreous layers. (Compare with the encystation of the Trichina.) 



The pearls of fresh-water mussels are not to be compared either in 

 size, brilliancy or colour with those furnished by the true marine Pearl 

 Oyster (Meleagrina margaritifera) . This animal lives in colonies in 

 different parts of the Indian Ocean on banks at a depth of not more than 

 from 27 to 54 feet from the surface. The pearl fishery is carried on by 

 native divers. These, in order to get rapidly to the bottom, let them- 

 selves down with a stone, gather as many oysters as they can in the 

 short space of a minute for they are unable to stay longer than this 

 below the water and are then drawn up by a rope. The oysters are 

 thrown in heaps and allowed to die, and it is only after they have com- 

 pletely decayed that they are examined for pearls. A closely-allied 

 species on the coasts of the West Indian Islands also furnishes valuable 

 pearls. The shells of many of the pearl-producing bivalves, including 

 those of the fresh-water pearl mussel, are used for the production of 

 many kinds of ornamental articles, buttons, etc. 



Many lamellibranchs, again, are important as food. This especially 

 applies in Northern and Western Europe to the Common Oyster 

 (Ostrea edulis). The larva of this species for a time swims about freely 

 in the sea (dissemination of the species), but finally settles per- 

 manently on sand, stones, or the shells of other molluscs, one of its 

 valves (the left) becoming, in a very short time, firmly cemented to the 

 supporting surface. In this valve, which constantly increases in thick- 



