STUDIES OF THE OYSTER DRILL 
103 
Since the period, of oviposition varies greatly the period of hatching is also varied, 
and one finds in a single group of egg cases embryos in different stages of develop- 
ment. This undoubtedly explains the variations in the incubation period (Table 7), 
since it is impossible to tell the exact age of the embryos within the capsule at the 
time they were isolated. 
Brooks (1879) reported that some of the embryos in the capsules of Urosalpinx 
cinerea broke up, the separate cells swam about, and were drawn within the digestive 
cavities of other embryos. This suggested that some observations should be made 
on the number of larvae hatching from the egg cases. Although the data given below 
are not many, they serve to indicate the magnitude of the percentage of drills hatching. 
Twenty-eight capsules gave rise to 144 larvae, an average of 5.1 larvae per capsule. 
Since the average number of eggs per egg case was determined as 8.8, the average 
percentage of eggs hatching into larvae is about 58 per cent. 
Besides these observations it seemed desirable to study the effect of salinity on 
the spawning of the animal, since in the transplanting of oysters, TJrosalpinx cinerea 
is sometimes subjected to waters of very different salinities. The study was under- 
taken to show: (1) The effect of salinity upon the number of egg cases laid, and (2) 
the effect of salinity on the number of eggs per egg case. Between May 5 and May 26, 
1927, at Hampton Roads 13 crates made of fine mesh wire and each containing 50 
drills and about 1 bushel of oysters were distributed in various places of salinities 
varying from 4 to 20 parts per mille. Two crates were planted at each station; one at 
the surface, the other at the bottom. These were visited at definite intervals, egg 
cases collected, and the condition of spawning noted. The work was onty partially 
successful because by July 1 the crates had all disappeared. The results shown in 
Table 8 are significant in that they show that the oyster drill reproduces wherever it 
survives. Salinity does have some effect on the number of eggs per egg case as is 
evident from the table. A salinity of 17 parts per mille seems the optimum salinity 
for the number of eggs per egg case at Hampton Roads. 
Table 8. — Effect of salinity on the spawning of Urosalpinx cinerea at Hampton Roads, 1927 
The study of the natural history of any animal must be supplemented by laboratory 
experiments, where conditions can be controlled and the behavior of the organism 
more easily analyzed and interpreted. Although it is true that conditons in the 
laboratory never exactly simulate the field or the natural environment, laboratory 
results are important in permitting one to predict what the animal will do when 
subjected to certain isolated stimuli. After all, the behavior in the field is only the 
result of several reactions working simultaneously. In the laboratory these reactions 
are isolated and studied individually. If incbnsistencies are found between the results 
obtained in the laboratory and the responses in the field, one must not condemn lab- 
oratory work as the fault usually lies in insufficient data. In order to arrive at some 
means of controlling the oyster drill it is necessary to know something of its responses 
