1908. 



NATURAL SCIENCES OF PHILADELPHIA. 



435 



ficial effect on the growing snail. This has been explained in the case 

 of effect of numbers in various ways, but most reasonably by lack of 

 aeration and of composition of the water. It is probable that those 

 two factors work together. 



4. Temperature. — Semper (79) reported that snails chilled were 

 retarded in growth and that growth ceased at 13° C. Walter ('06) 

 found that they became more active in warm water than in cold water. 

 The experiments of Walter ('06) were repeated in the following way: 

 Four large snails 9 mm. and four small snails 1.5 mm. were placed in a 

 glass dish with about 20 cc. of water. Under the dish a piece of cross- 

 section paper was laid, and on another piece of cross-section paper the 

 position of each snail was plotted every five minutes for a period of 

 three hours. 



For the first hour the dish was in a cold room and the temperature 

 fell from 12^° to 6^° C. The dish was then packed around with ice 

 for half an hour until the temperature fell from 6^° to 3^° C. The 

 dish was then placed in a warm room for one hour, the temperature 

 rising from 3^° to 17° C. For the next twenty minutes it was placed 

 near a radiator, and the last ten minutes the dish was placed above the 

 radiator, the temperature rising from 17° to 26° C. Fig. 5 shows how 

 the snails were affected, tt represents the temperature curve; the 

 heavy black line the distance in millimeters that the small snails 

 travelled in periods of five minutes; the dotted line indicates the 

 same thing for the large snails. 



Table XIII. 



The average speed for the small snails closely follows that|for the 

 large ones. The table shows, several things: (1) that cold apphed 



