102 
BULLETIN OP THE BUREAU OF FISHERIES 
days it would resume its egg laying. In many cases the snail would return to its 
“original” spawning grounds. Such a response if the animal’s previous history were 
unknown might give the impression that spawning occurred more than once during 
a single season. 
Oviposition lasts for various lengths of time, depending on whether the animal is 
disturbed. Sudden drops in temperature or lifting from the substratum bring about 
cessation of the spawning reaction, so that it is difficult to determine accurately the 
duration of spawning. During 1927, at Hampton Roads, 6 animals were observed 
in which spawning was accomplished without any apparent interruption. In these 
the average time was approximately 7 days, during which time an average of 28.8 
cases were laid per female. As is shown later these figures agree very closely with 
those obtained from an analysis of the rate of oviposition and the number of egg 
cases laid per female. The rate of oviposition was determined by observing 19 
females that spawned 127 egg cases in 32 days, an average per female per 24 hours of 
3.9 egg cases. This figure is very close to that obtained from the data on the duration 
of spawning in which a female spawned 28.8 egg cases in 7 days or 4.1 egg cases per 
female per 24 hours. 
In order to determine the number of egg cases one female lays in a season, several 
animals were isolated and the number of egg cases laid by each one noted. These 
females were kept isolated throughout the whole summer in order to be sure that 
no further spawning took place. The data obtained from these observations support 
the figures previously given for the duration of oviposition. The average rate of 
oviposition is 3.9 egg cases per day, and the average number of egg cases laid per 
female is 28; thus each female, if undisturbed, would require approximately 7 
days to complete her spawning. 
Examination of 727 capsules collected during the summer of 1927 at Hampton 
Roads gave an average of 8.8 eggs per egg case. The smallest number of eggs per 
capsule was 3; the largest 22. 
In order to determine the incubation period of the drill, freshly laid egg capsules 
were isolated and the time when hatching occurred noted. These experiments 
were necessarily performed in the laboratory. Eleven different batches of egg cases 
were thus isolated during May, June, July, and August. The following table (7) 
summarizes the results. The average incubation period obtained from these figures 
is approximately 40 days. 
Table 7. — Summary of the results on the incubation period of Urosalpinx cinerea obtained in the 
laboratory at Hampton Roads during summer, 1927 
Time egg case was laid 
Incuba- 
tion 
period 
Minimum 
and maxi- 
mum tem- 
peratures 
during 
incubation 
period 1 
Time egg case was laid 
Incuba- 
tion 
period 
Minimum 
and maxi- 
mum tem- 
peratures 
during 
incubation 
period 1 
May 20 
Days 
36 
° C. 
18. 0-26. 0 
June 23 
Days 
37 
° C . 
22. 0- 32. 0 
24. 0- 32. 0 
Do 
38 
18. 0-26. 0 
July 9 
44 
May 23 — - 
38 
18. 0-26. 0 
July 12 
42 
24. 0-32. 0 
June 20 
39 
21. 0-32. 0 
July 16.. 
40 
24.0- 32.0 
20. 0- 30. 0 
June 22 
43 
22. 0-32. 0 
Aug. 11. . 
44 
Do — - 
41 
22. 0-32. 0 
1 See Figure 5 for daily temperatures during these periods, 
