3S2 LOWER FORMS OF MARINE LIFE 



all round Australia; and is of special interest, not only for the antiquity of its 

 genus, but from being one of the few genera in which the beaks of the shell point 

 backwards. Another family comprises the mussels and allied species (Mytilidce), 

 distinguished by their equivalved but inequilateral shells in which the beaks are 

 incurved, and the horny ligament connecting the two valves occupies a groove and 

 is of unusual thinness. Mussels are inhabitants of all seas, from Iceland to New 

 Zealand, and from Alaska to Cape Horn, as might, indeed, have been expected 

 from their anchoring themselves, by means of the adhesive tuft termed the byssus, 

 to the bottom of ships and floating timber (as well, of course, as to rocks and piles), 

 and being thereby carried from one ocean to another. In certain localities the 

 ordinary Mytilus eclulis is cultivated for food in what are termed mussel-farms, 

 while it is also extensively reared for bait. 



In an entirely distinct section of bivalves an important position is occupied by 

 the typical pearl-oyster {Meleagrina margaritifera) and its relatives, which belong 

 to the family Aviculiclce, typified by the so-called wing-shells (a name also applied 

 to the members of the genus Strombus). Pearl-oysters, some of which attain 

 large dimensions, measuring as much as ten inches in the longer diameter, are 

 essentially tropical bivalves, and have a wide distribution equatorially. Pearl- 

 fisheries exist in many parts of the tropics, one of the oldest and most famous 

 being the one in the Gulf of Manar, where the pearl-banks cover an area of more 

 than 700 square miles. 



The Ceylon pearl-oyster {Meleagrina fucata), in which the sexes are separate, 

 thrives best in the open sea at a depth of about six fathoms in place of in intertidal 

 waters. At the present time the Ceylon pearl-banks are practically unproductive ; 

 and it seems that there are periodical spells of barrenness, when not a single adult 

 oyster is to be found over the whole area. During such a period the banks may, 

 however, become suddenly replenished and covered in countless numbers with the 

 ' spat ' or spawn over several square miles, and the problem awaiting solution is 

 the origin of this presumably foreign spat. Another problem is connected with 

 the disappearance of the oysters, both old and young. Although it has been proved 

 that predaceous fish and boring molluscs have a share in this destruction, there 

 remains a considerable percentage of oysters which die for some unknown reason, 

 although epidemic disease may be the cause. The Ceylon banks are dredged solely 

 for the pearls contained in the oysters, the shells of which are too small to be 

 of much, if any, commercial value. On the other hand, the pearl-oysters of the 

 Japanese and Philippine seas are of large size, and are dredged mainly for the 

 sake of the shells, which form the chief source of commercial mother-of-pearl. In 

 the Philippines there are two kinds of these large pearl-oysters, the golden-lipped 

 M. maxima and the black-lipped M. margaritifera, of which the former is by 

 far the more valuable ; and it is probable that the same two species also occur in " 

 Japanese waters. The Philippine oysters are much less prolific in their yield of 

 pearls than the small Ceylonese species ; nevertheless Philippine pearls are some of 

 the finest and most beautiful known, so that there could not be a better locality 

 for an attempt to produce pearls by semi-artificial means. For hundreds, if not 

 thousands, of years the Chinese have been in the habit of introducing small rough 

 images of Buddha into the shells of a river-mussel (Dipsas plicata), which are then 



