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never burrows and only when young attaches itself by a byssus. The 

 adult spends most of its time merely lying on the surface of the mud 

 or sand. If displaced, or if the conditions become unfavorable, it 

 can regain its position or move from an unfavorable one by a type 

 of swimming unicjue among mollusks. The valves are clapped together 

 and the expelled water forces the animal off in the opposite direction 

 with a brislmess of motion quite belying the conventional idea of the 

 sluggish clam. As with the fixed forms its mantle is also widely open. 



The great majority of the bivalves burrow, and though there are all 

 degrees of the habit it is possible to divide them into two fairly well 

 marked groups. The first group inhabits bottoms that are shifting 

 and in this do not usually buiTow deeply, trusting to their activity in 

 burrowing to escape from their enemies and to maintain themselves 

 when the bottom is changed by waves or currents. A good example 

 of this is the cockle {Cardiuni). It has a long slender foot with which 

 it burrows actively and thus keeps its feeding position at the surface 

 even on exposed coasts. Few forms found in sand or mud have the 

 mantle widely open. Water can enter or leave the mantle cavity 

 freely only at the posterior end which reaches the surface of the mud. 

 Here the mantle edges are partially united, forming two special 

 openings, one for the inhalent and one for the exhalent current; this 

 is the condition found in Cardiuni. But most burrowing species take 

 a deeper position, and though this bring-s increased protection both from 

 foes and from surf or currents, it removes the clam still farther from 

 the water supply. Such supply is insured by the prolongation of the 

 mantle surrounding the openings just mentioned into two tubes or a 

 double tube, the siphon, through which water may still be obtained 

 even though the clam is some distance from the surface. Of the short- 

 siphoned actively burrowing type the Pismo and razor clams are the 

 best examples. 



Perhaps the larger number of clams live in relatively permanent 

 burrows from which the adult never does or never can move. Such 

 burrows, of course, must be deep enough to furnish protection and must 

 be in a relatively permanent bottom. Some, like the great geoduck 

 and the gaper, are found deep in the soft bottoms of sheltered bays; 

 others, like some of the piddocks and Platyodon, dig into hard clays 

 in which burrows may be maintained in more exposed places. Still 

 others bore into rock so firm that heavy surf does not break down the 

 burrows nor strong currents wash them away, such is the rock borer 

 Paraplwlas. In a related form not considered in the present paper the 

 burrow is made in wood to the great damage of piles and ships. This 

 is the familiar teredo. 



From the foregoing types of habit among the bivalves may be seen 

 how widely varying are the ways in which they have adjusted them- 

 selves to the same problem, that of combining the greatest protection 

 from waves and foes with the amplest supply of necessary food-bearing 

 water. Though the huge geoduck, with its great bulk of heavy siphons 

 buried a yard deep in the sheltered mud and quite incapable of any 

 more activity than pulling in its siphons, is very dift'erent in structure 

 and habits from the actively swimming scallop, both have inherited, 

 from lines of successful ancestors, successful though diverse methods 

 of solving the problems of food and protection. 



