452 THE AMERICAN NATURALIST [Vol. XLYI 



tions. Moreover, transition forms (in every feature ex- 

 cept the trophi) do occur between them under given 

 though unusual conditions. The phenomena thus fall 

 more naturally under the rubric of dimorphism than 

 under that of mutation; although, as will be apparent 

 later, the phenomena with this rotifer bring the two con- 

 cepts into closer relation than they have perhaps ever 

 been brought before. 



The second general result of work with the living ani- 

 mals was the surprising discovery that the species is not 

 only markedly dimorphic, but trimorphic, possessing a 

 third form — a smaller saccate type which differs from 

 both the humped and the campanulate types quite as 

 much as these differ from each other, and which, curi- 

 ously enough, differs far more, in external appearance 

 at least, from these other forms of its own species, than 

 it does from an allied but distinct species, Asplanchna 

 brightwelli. 



I shall return to this latter point of specific distinctness 

 again. But by way of description I will here state that 

 this saccate type, as I shall call it, although showing a 

 considerable range of fluctuating variability, is a rather 

 primitive but typical Asplanchna. In size it averages 

 much smaller than either of the other types dealt with, 

 reproductive individuals ranging from about 500/* to 

 1,200/* in length. In outward form it resembles closely, 

 now A. brightwelli, now A. priodonta. The corona, when 

 the animal is seen from the end, is always circular, as is 

 typical for the genus, never showing the strong dorso- 

 ventral flattening which is the invariable condition in 

 the humped and campanulate types. Correlated with 

 this circular corona is the general cylindrical shape of the 

 animal. When placed on a slide in a shallow drop of 

 water these saccates characteristically rest on their sides 

 instead of their dorsal or ventral surfaces as do both of 

 the larger types. The trophi are of the same type as are 

 those of the humped Asplanchna, except that they are a 

 little smaller and proportionally a little more robust. 



