550 



THE AMERICAN NATURALIST [ Vol. XLVI 



body very broadly saccate to broadly campanulate in form, with very 



ing the length of the animal; anterior end of animal, within ^corona, 

 concave instead of convex; flame cells approximately SO ^ to 115 /*; 

 contractile vesicle small. Animal resting when water is withdrawn on 

 dorsal or ventral surface ; tropin very large, typically from 300 p to 340 ft 

 in length, enclosing a narrowly oval area : inner teeth relatively less 



and more firmly fiised with it than in the preceding types; lamellate 

 teeth near tips of rami much developed and meeting, with cutting edges, 

 in middle line; tips of rami not interlocking but shearing past each other 

 when closed. Length of entire animal approximately 1^800 p to 2,500 p. 



In conclusion, it may be pointed out that the type of 

 variation shown by the rotifer here discussed seems 

 somewhat peculiar, in that it lies seemingly on the line 

 between germinal variation and variation which is com- 

 monly supposed to be somatic. To use recent phraseol- 

 ogy, it is difficult to say whether the types which this spe- 

 cies of Asplanchna produces should be called genotypes 

 or phenotypes. 10 They are like genotypes in that when 

 once produced they manifest a marked tendency toward 

 stability, each type reproducing itself through a number 

 or even a multitude of generations after the special con- 

 ditions which favored their origin have ceased to be 

 present. They are to some extent like phenotypes in that 

 this stability is less than that of true species, yielding, 

 though rarely, to degenerating or other modifying con- 

 ditions. 



