STUDIES OF THE OYSTER DRILL 
93 
The preceding data show that the salinity death point of Urosalpinx cinerea is 
influenced to a great extent by the evironmental salinity. Snails collected at the 
Norfolk Point localities having salinities of approximately 15 and 20 parts per mille 
show lethal salinities of approximately 12.5 (Table 2) and 11.7 parts per mille 
(Experiment No. VIII, Table 4). On the other hand snails collected at Beaufort 
with an environmental salinity of approximately that of sea water (over 30 parts per 
mille) for most of the year, show a lethal salinity of 15.6 (Table 3) and 17.6 parts per 
mille (Experiments V, VI, and VII, Table 4). 
In this connection it is interesting to draw attention to one fact which these 
results bring out. The extent of adaptability, or the salinity factor of safety, becomes 
smaller as the animal becomes adjusted to lower salinities. Thus at Hampton 
Roads a drop of from 8 to 9 parts per mille (at the maximum, from 20 to 12) was 
fatal, while at Beaufort a decrease of over 15 parts per mille in the salinity of the 
waters was necessary for death (from 30 to approximately 16). The adaptation to 
lower salinity seems to bring about a reduction of the salinity safety factor. This 
lower factor of safety in an animal which has become adapted to lower salinities 
may become of biological importance in its distribution. For instance, heavy rains 
in the area drained by the Elizabeth, Nansemond, and James Rivers might reduce 
the salinity of the Hampton Roads region sufficiently so that Urosalpinx cinerea 
infesting oyster beds in this locality would be killed in great numbers. 
CREEPING AND MIGRATIONS 
In the majority of gastropods locomotion is dependent on the formation of 
successive pedal waves (Parker, 1911, 1914); and according to Parker (1911, 1914) 
and Olmsted (1917) the pedal wave is an area lifted from the substratum in which 
movement takes place, the remainder of the foot being stationary. Other investi- 
gators have claimed that the wave is an area of convexity (von Uexkull,1909; van 
Rijnberk, 1918-19; ten Cate, 1923). Some few gastropods have also been described 
as showing no pedal waves during locomotion. Parker (1911) and Crozier (1919) 
believe that where no cilia are present this type of locomotion is due to an 
“arhythmic” type of pedal waves; and Dubois and Vies (1907) have shown that 
even though the pedal surface is ciliated, locomotion depends on muscular activity 
alone. More recently Copeland (1919, 1922) has maintained that locomotion due 
to cilia does obtain among gastropods (Alectrion and Polynices), a conclusion that 
does not seem to be altogether proven by his results. The mechanism by which 
the pedal wave is produced has been studied, and several theories have been brought 
forth; but the subject does not properly belong here. Anyone interested in it will 
find a complete bibliography in van Rijnberk’s paper (1918-19). 
In Urosalpinx cinerea no pedal waves can be demonstrated during locomotion. 
It moves by a smooth, gliding motion comparable to that of Nassa obsoleta (Parker, 
1911; Copeland, 1919) and Conus agassizii (Crozier, 1919). Furthermore the pedal 
surface is covered with cilia whose effective stroke is backward, leading one to sup- 
pose that locomotion is due to their activity. 
A study of the pedal surface both moving and at rest gave some interesting 
results. At rest, the animal is attached to the substratum by means of the posterior 
part of the foot. When movement is to take place, the anterior margin of the foot 
is thrust forward and attached. Until this part of the foot is in contact with the 
substratum no translatory movement can occur. Cessation of locomotion occurs 
only after the anterior margin has been lifted from the substratum. Thus there is 
