That at least some of these races adjust to transplantation to different coast 

 lines of the world is amply demonstrated by the success of Urosalpinx introduced 

 in England and on the west coast of America 



In summary it may be stated that Stauber's hypothesis of the existence of 

 physiological races receives added support from the laboratory experiments of 

 Loosanoff and Davis and from some of the additional information in Table 13, 

 though it is evident that with refinement of experimental technics and more careful 

 observations on the quantity of heat necessary to initiate physiological activities 

 the lower thermal limits at which these physiological activities begin in drills from 

 different regions may be shifted. This should not alter Stauber's evidence for the 

 existence of physiological races. On the other hand, Jenner (pers. com.) points 

 out that the evidence for the occurierce of physiological races as presented by 

 Stauber (1950) is based on the assumption that temperature is the controlling factor; 

 if it should turn out that a factor such as day length is critically important, then 

 Stauber's conclusions would not be supported. 



This portion of this review has indicated a number of suggestions for future 

 research on physiological races of the drill Since in the laboratory, organisms, 

 particularly those from temperate regions, exposed to variable temperatures 

 frequently show accelerated development as compared with those held at a constant 

 temperature of the same mean value, other conditions remaining the same (Allee 

 et ah, 1949), it is important that activities of Urosalpinx under both sets of condi- 

 tions be compared. The difference in the onset of oviposition obtained by Loosanoff 

 and Davis (1950-51) may be explained m part by the constant thermal conditions in 

 the laboratory and the fluctuating thermal conditions in the field. In addition com- 

 parative studies on the rate of oviposition in drills will not be dependable until the 

 ratio of females to males used is accurately determined. This points to the need 

 for a method of reliable sex determination in living drills; or less desirably, as 

 Jenner (pers com.) has suggested, that the sex of the experimental animals be 

 determined by anatomical examination at the conclusion of the experiments . Finally, 

 since oviposition in Uros alpinx may occur only after the summation of a specific 

 quantity of heat energy, it would seem necessary to design experiments with this in 

 mind . On the other hand length of day may be an important factor in influencing 

 the reproductive cycle of the drill, so the study of oviposition under artificially 

 manipulated photoperrods should be undertaken (Jenner, 1954). No investigations of 

 this nature have been reported for the oyster drill. 



The existence of morphological races has also been reported. Walter (1910) 

 in a comprehensive study of the relative morphological variability expressed by 

 U. cinerea living in native habitats and when introduced to new environments, 

 recorded the ratio of the maximum dimension of the shell aperture to the total shell 

 height in a total of 50, 424 drills collected in San Francisco Bay, California; Princes 

 Bay, Staten Island, New York; Norwalk Harbor, Connecticut, Cold Spring Harbor. 



87 



