STUDIES OF THE OYSTER DRILL 
111 
substratum by means of a solid expanded foot. Each female spawns only once a season 
which at Hampton Roads is from May 15 to September 30. About 28 egg cases are 
laid per female, and each capsule averages 9 eggs. After approximately 6 weeks 
there being no free-living larval stage, the young, about 1 millimeter in length, escape 
from the capsules through a cap at the top of the egg case and begin to feed imme- 
diately. Approximately 60 per cent of the eggs hatch into larvse. Studies on the 
relation between salinity and spawning showed that the a nim al will spawn at any 
salinity at which it will live. 
8. Studies on the tropistic behavior of the snail were limited to a study of its 
geotropism, rheotropism, and phototropism. (a) During the months when the drill 
is active, it shows a very definite and marked negative geotropism which is important 
in its spawning reactions. Because of this, the animal climbs up to the higher levels 
to lay its eggs, and thus they are not covered with mud and suffocated. Experiments 
have shown that this is a true geotropic reaction and not one in response to oxygen 
content or light. (&) If the snail is placed in a current of water it will orient itself 
so that its siphon is pointing upstream. It will then creep in that direction. Studies 
on the relation between current velocity, and the rate of creeping showed that creep- 
ing is independent of current velocity. The rate of turning (orientation) is a 
function of the current velocity. Light does not affect the movements of the drill. 
9. The oyster drill can be greatly reduced in numbers provided the oyster planter 
is willing to make a little effort. Because the snail is practically nonmigratory its 
further distribution can be checked. If an area, at present infested is cleaned of this* 
pest, there is no reason why it should not stay so, provided the planter is careful not 
to reinfest it with oysters from an infested area. The oyster planter must be careful 
of two things: (a) He must not move oysters from an infested area to a noninfested 
area without first attempting to clean the infested oysters. This can be done by any 
of the following methods : The use of forks, screening, and floating. ( b ) He must avoid 
planting uninfested oysters on infested bottoms. The infested bottoms can be cleaned 
by the use of a trap dredge, the use of small concrete pillars, and the use of an oyster 
dredge fitted with a bag of small mesh. If these recommendations are adhered to 
the oyster drill pest will gradually decrease in importance. 
LITERATURE CITED 
Abet, Leslie B., and W. J. Crozieb. 
1919. The sensory responses of Chiton. Journal of Experimental Zoology, vol. 29 No. 2 
October, 1919, pp. 157-260, 8 figs. Philadelphia. 
1921. On the natural history of Onchidium. Journal of Experimental Zoology, vol. 32 
No. 3, April, 1921, pp. 443-502. Philadelphia. 
Baunacke, W. 
1913. Studien zur Frage nach der Statocystenfunktion. Biologisches Centralblatt, Bd. 33, 
No. 7, July 20, 1913, S. 427-452, 11 figs. Leipzig. 
Belding, David L. 
1910. A report upon the scallop fishery of Massachusetts. The Commonwealth of Massa- 
chusetts, 150 pp., 118 figs. Boston. 
Bonnier, Pierbe. 
1896. Sur les sens lateral. Comptes rendus Hebdomadaires des stances et memoires de la 
soci6t6 de biologie, serie 10, Tome 3, pp. 917-919. Paris. 
Brooks, W. K. 
1879. Preliminary observations upon the development of the marine Prosobranchiate Gaster- 
opods. Studies from the Biological Laboratory, Johns Hopkins University, 1877-78 
(1879), 16 pp. Baltimore. 
