1908.1 



NATURAL SCIENCES OF PHILADELPHIA. 



417 



up SO that each piece contained an equal number of snails and placed at 

 once under the conditions of the experiment. As there was less 

 mortality by letting the young snails escape from the case normally, 

 they were not placed under the conditions of the experiment until a day 

 after they escaped from the egg case. 



(7) In those jars in which I have placed water plant I have tried to 

 add pieces of water plant of equal length and foliage. 



(8) Where sediment was needed approximately the same amount 

 was added to each jar. 



Measurements. — To measure growth several methods have been 

 used. It is possible to 

 measure volume, weight, 

 a lineal dimension of some 

 part, or the number of 

 successively arising homo- 

 dynamous structures. In 

 the case of the pond snail 

 the lineal dimension of the 

 length of the shell at once 

 suggests itself. To meas- 

 ure this the following ap- 

 paratus was arranged. A 

 is a compound microscope 

 with about a 7-inch work- 

 ing distance magnifying 

 the object about three 

 times (fig. 1 ) . O is a snail 

 on a thin glass sHde that 

 was placed over a piece of 

 paper ruled in millimeters.. 

 The snail was placed with 

 the aperture flat on the 

 glass and the shell would 

 be projected on the ruled 

 lines. Millimeters were 

 then read off on the 

 paper and tenths estimated. This is sufficiently accurate where the 

 work is purely qualitative. A Vernier caliper could not be used on 

 account of the delicacy of the shell. 



In some cases the weight is given. This is not alwaj^s satisfactory. 

 As growth is a three dimensional phenomenon weight more nearly 



Fig. 1. 



