NUCLEOPLASMIC RELATIONS IN ARCELLA 61 



b. Three small families (ap. 38, ap. 39, and ap. 69) were reared 

 which exhibited to a marked degree this difference in diameter 

 and nuclear number between families. Family ap. 38, whose 

 members possessed 8 nuclei, had a mean diameter of 27.26 units; 

 family ap. 39, whose members possessed 7 nuclei, had a mean 

 diameter of 25.91 units, and family ap. 69, whose members pos- 

 sessed 6 nuclei, had a mean diameter of 34.22 units. These 

 data should be compared with those obtained from family ap. 5, 

 in which the specimens that possessed 7 nuclei had a mean diam- 

 eter of 30.22 units. 



c. Extended observations were made on family ap. 34. Here, 

 as in families ap. 38 and ap. 39, the diameter associated with a 

 particular number of nuclei was less than that in family ap. 5, 

 and the number of nuclei ranged from 5 to 10, whereas in family 

 ap. 5 the range (except when offspring were obtained from pieces) 

 was from 3 to 7, Two branches of famly 34 whose members 

 possessed 7 nuclei differed in mean diameter by 3.28 units; this 

 indicates that lines permanently diverse in diameter but with 

 the same number of nuclei might easily be isolated within this 

 family. 



7. A comparison of the relations between the diameter of the 

 shell and the diameter of the mouth opening within different 

 families and in wild populations indicates that groups of multi- 

 nucleate specimens exist in nature that differ in the percentage 

 of the diameter of the shell occupied by the mouth. 



4. OBSERVATIONS AND EXPERIMENTS ON ARCELLA DISCOIDES 



a. Pure lines 



Many specimens of Arcellas that corresponded to the descrip- 

 tions given by Leidy and others of A. discoides (fig. 40) were 

 found in the same habitats where the multinucleate Arcellas 

 were obtained. These were of many different sizes, but all re- 

 sembled one another in possessing only 2 nuclei. Fifteen wild 

 specimens of various sizes were chosen from a large number col- 

 lected on duckweed and 7 families were reared from them. The 

 distribution of the diameters of these families is given in table 



