IN I. AN I) FISHERIES. 95 



Having attained a lodgment in the sand, all clams observed at 

 once proceed to form a byssus thread, which is attached to sand 

 grains and pebbles. This tends to secure the creature, so that, 

 even if water currents or the action of the waves should dislodge 

 it from its burrow, it would not be carried so far from its original 

 position as would otherwise occur. 



Of their own accord these clams frequently leave the first 

 burrow, wander about, and form another, some individuals repeat- 

 ing the process many times. A time finally conies when they dig 

 into the sand to remain permanently. 



DESCRIPTION OF FIGURES. 



Figure 1. Mya arenar'ui. Ten camera outlines of shells varying 

 in length from A, of a millimeter to 1\ millimeters. They are 

 intended to illustrate the change from a rounded outline in 

 the smaller individuals to the elongated condition of older 

 forms. In this there is at first a more rapid posterior, and 

 subsequently a more rapid anterior, growth of the shell, which 

 causes the relative position of the umbo to shift forward, and 

 then back to a position midway between the two extremities 

 of the shell. 



Figure 2. Mya areiuma. An individual with shell tit of a milli- 

 meter in length. Removed from attachment to seaweed 

 {Enteroriiorplia) and showing the single, liranched byssus 

 thread (b) arising from a byssus gland at the base of the foot 

 (f). The filmy siphons (s) are shown protracted. 



Figure 3. Mya arenaria. Form 2to millimeters long (drawn on 

 smaller scale than Figure 2), removed from burrow in sand, 

 and showing attachment of byssus (b) to numerous sand 

 grains (s. g.). 



