excretory system, reproductive system, ova, and egg capsule helps to explain the 

 success and high degree of adaptability of Urosalpinx to a wide range of ecological 

 conditions . 



4 . Noticeable variation in the onset of spawning of the drill may reflect 

 annual differences in spring water temperatures, physiological races, and/cr in- 

 complete information. The average number of egg cases oviposited per season varies 

 from a few to 96 per drill, the number being larger in older drills . The average 

 number of eggs per case varies from 8 to 12. Actual numbers range from to 29, 

 older drills ovipositing more than young sexually mature drills. 



5. Urosalpinx lacks a free swimming larval stage . Development occurs in 

 the egg capsule . A prehatching mortality of 14 to 50% is reported. Duration of 

 incubation is markedly influenced by temperature and may vary from 18 to 56 days. 

 Newly hatched drills are fully shelled and capable of drilling small prey. 



6 . Growth rate data are incomplete . In America it is assumed drills reach 

 a height of 8 to 19 mm. during the first growing season. In England drills are 

 thought to attain a height of 10 to 20 mm . the first year, and to live to a maximum 

 age of 13 to 14 years during which males may reach a height of 39 mm. and females 

 43 mm. "Maximum heights of drills in America (longevity unknown) vary from 27 

 to 40 mm., and a giant subspecies reaches heights of 61 mm. Sexual maturity is 

 said to be attained at ages varying from one to three years and at heights- of 13 to 

 24 mm. 



7 . Urosalpinx displays some discrimination in its choice of food, but feeds 

 upon a wide variety of animal prey, particularly young oysters, edible mussels, 

 and barnacles . Anomia are only infrequently attacked The drilling site is not 

 limited to any specific region on the prey, nor necessarily to portions which are 

 easier to penetrate . Although young oysters are attacked most commonly, those 

 over 8 cm. in length are drilled by large Ur osalpinx . It appears that the thickness 

 of the shell is more important than the length in decreasing the rate of predation. 

 The observation that the oyster drill secretes a toxic substance while drilling which 

 kills its prey has not been confirmed. The number of prey destroyed by drills per 

 given time increases as the size of the prey decreases, and larger drills destroy 

 more prey than do smaller drills. In a temperature range of at least 13 to 24° C 

 the rate of destruction increases with temperature; excessive exposure to an and 



to low salinities curtails drilling; drilling rate mcreases during the breeding season. 

 The maximum average number of small oysters destroyed by small diills is recorded 

 as 34 per week. On the average adult drills destroy oysters 4 to 6 cm. long at the 

 rate of about . 14 to .35 per week . 



130 



