28 



BIOLOGY OF THE PROTOZOA 



food conditions, are frequently responsible for changes in size and 

 character of a species, often rendering them difficult to recognize 

 and affording tempting opportunities for swelling the list of syn- 

 onyms by new names for the abnormal forms. Thus Dileptvs 

 gigas when starved has a very different size and character from 

 the normal form (Fig. 6). Again, different normal stages in the life 

 history of a given species are not infrequently mistaken for different 

 species, largely because of difference in size. Thus Uroleptus metritis 

 (see Fig. 1), in its adult vegetative condition, measures about 

 150 (J., but immediately after conjugation not only is it reduced 



Fig. 7. — Uroleptus mobilis Engelm. Old age specimens showing degeneration of 

 macronneleus M and loss of micronuclei. See frontispiece. (After Calkins.) 



by one-third in size, but its internal structure is entirely different 

 from that of the usual form, while during the period of old age it 

 frequently measures less than 75 // (Fig. 7), and has a different 

 appearance from the more youthful stages. 



Even more striking examples of normal dimorphism are shown 

 by the rhizopod Dimastig amoeba and by the ciliate Glaucoma (Dalla- 

 sia) frontata. Species of the former usually appear as small earth- 

 dwelling ameboid rhizopods, but with the addition of water they 

 develop flagella and become actively moving ellipsoidal flagellates. 

 Glaucoma frontata in its usual vegetative state is a more or less 

 quiescent tailed form (Fig. 8), but under certain environmental 



