536 Phylum Mollusca 



not feed but remains attached to the substratum and, unless disturbed, 

 continues until spawning is completed. Observations in the laboratory 

 show that each snail spawns only once during the summer. 



Although oviposition lasts for various lengths of time, depending on 

 whether the animal is disturbed, the average time is approximately 

 seven days. The rate of oviposition averages 3.9 egg cases per female 

 per 24 hours. According to observations made at Chesapeake Bay, the 

 average number of egg cases per female is 28; Nelson ( 193 1) reports for 

 Delaware Bay an average of 50 egg cases per female. 



Examination of 727 capsules collected during the summer of 1927 at 

 Hampton Roads, Virginia, 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 of hatching noted. The average 

 incubation period obtained is approximately 40 days. 



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 development. This fact undoubtedly explains the 

 variations in the incubation period since it is impossible to tell the exact 

 age of the embryos within the capsule at the time they are isolated. 



Studies on the effect of salinity on the spawning of this species 

 showed (Federighi, 1931a) that reproduction takes place wherever 

 the adult lives. 



At the time of hatching the larvae are approximately 1 mm. in length 

 and are immediately able to feed themselves. These young drills will 

 attack the young oyster spat or the thin shelled Crepidula. Before the 

 next winter they are approximately 1 cm. long. 



In the colder climates (north of and including Chesapeake Bay) 

 where the water reaches a temperature of less than 7°-io° C. the drills 

 become inactive and cease feeding at these temperatures. In the spring 

 the animals again begin feeding and reach adult size the following 



summer. 



Bibliography 

 Brooks, W. K. 1879. Preliminary observations upon the development of the 



marine prosobranchiate gastropods. Stud. Biol. Lab., Johns Hopkins University, 



1877-78 (1879), 16 pp. Baltimore. 

 Colton, Harold Sellers. 1908. How Fulgur and Sycotypus eat oysters, mussels, 



and clams. Proc. Acad. Nat. Sci. Phila. 60:3. 

 Federighi, H. 1930. Salinity and size of Urosalpinx cinerea Say. Amer. Nat. 



64:183. 

 1931a. Studies on the oyster drill (Urosalpinx cinerea, Say). Bull. U. S. 



Bur. Fish. 57: 85. 

 1931b. Further observations 



Conchol. 19:171. 

 Nelson, J. R. 1931. Trapping the oyster drill. 



on the size of Urosalpinx cinerea Say. J. 

 N. J. Agric. Exper. Sta. Bull. 523. 



