216 University of California Publications in Zoology [Vol. 20 



The spiral type is represented by Stephanonympha and probably 

 by Calonympha, multicellular parasites of the termites, in which the 

 nueleus-neuromotor units are spirally grouped in the anterior end of 

 the asymmetrical body of the organism. The direction of the spiral 

 is unfortunately not clearly established in Janicki's (1911) figures. 

 It is obvious that this type of organismal symmetry is dominated by 

 the spiral organization typical of the body of the unicellular flagellates. 

 There is, in so far as is known, no fibrillar connection between the 

 cellular units of these somatellas beyond that achieved by the proximity 

 of axostyles. 



The bilateral type found in the Hexamitidae is, on the other hand, 

 a very great morphological departure from the fundamental primi- 

 tive, spiral, organismal organization so widely expressed among the 

 Mastigophora. It involves two fundamental departures from the 

 organization of these spiral somatellas. The first is the existence of 

 the transverse commissure and its derivative, the anterior node, which 

 unite the cells of the right and left sides of the body. The second is 

 the reversal of symmetry of the nucleus and neuromotor system of 

 the one side so that it constitutes a mirror image of these organs of 

 the other side. Given the spirally asymmetrical, uninucleate flagellate, 

 representing the right half of Oiardia, this morphological reversal 

 affords the only method by which a binucleate, bilateral organism 

 (Giardia) might have been evolved from the uninucleate spiral one 

 {Chilomastix) . We do not mean to postulate that Giardia itself was 

 evolved from Cliilomastir, since these two genera are highly differ- 

 entiated representations of their respective groups, but assume that 

 the divergence of the binucleate group occurred in some more primitive 

 condition of ancestral representatives. 



The morphological contrasts involved in the evolution of sinistral 

 and dextral, spiral uninucleate flagellates, as illustrated in certain 

 parasitic flagellates of the termites, such as Dinenympha gracilis 

 Leidy and Personympha flagellata Grassi, are comparable in type with 

 those reversals in stereometric relations which exist between organic 

 compounds such as laevulose and dextrose and between sinistral and 

 dextral albuminoids. Should organ-forming substances be in the 

 control of symmetry in flagellates, their potency in establishing the 

 sinistral and dextral types of organisms is to be sought in their own 

 sinistral and dextral molecular structure. 



The morphological modifications of symmetry involved in the evolu- 

 tion of the Hexamitidae from the Triehomonadidae, or of Giardia from 



