4 : 4 
bulletin of the bureau of FISHERIES. 
where it approaches the edge of the mantle. The cilia within this groove beat toward 
the posterior to produce currents that continually remove foreign bodies from the 
mantle chamber. Any large particle of dirt or sand which enters the mantle chamber 
soon finds its way into this ciliated passage and is carried forthwith to the exterior. 
BURROWING. 
In order that Atrina may maintain its position in the mud throughout life, it must 
burrow more or less. The bottom about it is sure to be shifted considerably by the tides, 
thus tending to uproot or cover up fixed objects on its surface. This shifting of the 
bottom was observed last year on the very beds where Atrina was found most abundant. 
During the summer of 1908 these beds were covered by eel grass, while a year later this 
grass had entirely disappeared and the character and depth of the bottom had changed 
to a noticeable extent. Atrina while undisturbed in its natural surroundings was never 
seen to burrow. But the method of burrowing was frequently observed when the 
animal was removed and again partially buried with the anterior end downward. The 
shell valves were opened wide by the relaxation of the adductor muscle, and the edges 
of the posterior part of the mantle lobes were brought together firmly to prevent the 
escape of water in this direction. Then followed the contraction of the adductor, 
forcing water from the mantle chamber at the anterior end. The force of the expelled 
current makes the water fairly boil, washing up quantities of sand and mud from 
beneath. This process was usually repeated several times at intervals of four or five 
seconds and then there followed a period of rest during which the sand and mud which 
had entered the mantle chamber during the burrowing movements was removed through 
the ciliated groove. This heavy material was expelled over the posterior edge of the 
shell in surprisingly large quantities in a short time. This accomplished, the burrowing 
movements were resumed. The settling of the specimen was very gradual, but in the 
course of an hour one could see that it had sunk 3 or 4 inches. Although the ciliated 
groove is of service in removing solid particles which enter the mantle chamber with 
the respiratory current, I think it is an especial adaptation for removing the heavier 
bodies which enter the mantle chamber during burrowing movements. 
A number of Atrina individuals were laid upon their sides to see if they would 
bury themselves. The results were practically negative, for although they were left for 
weeks in this position not one made any attempt to bury itself. They seemed to thrive 
as well in this position as in any other, and none were seen to make movements which 
could be construed as an attempt to assume the normal position. Mr. Charles Hatsel, 
official collector at Beaufort, in whose charge certain experiments were left during the 
winter, reported that one specimen buried itself as far as the box in which it was kept 
would permit. 
REGENERATION AND GROWTH OF SHELL. 
Atrina is a particularly good subject for experiments upon the growth and regenera- 
tion of the shell because of the great rapidity with which this is produced. When one 
breaks a piece from the posterior or ventral edge of the shell, the mantle in this region 
becomes particularly active in mending the breach, a strip one-tenth of an inch in width 
