SUB-CLASS: ZOOMASTIGINA 281 



gina. The two merge into one another, and certain forms which are 

 usually placed in the one order might with equal justification be trans- 

 ferred to the other. It would seem better, therefore, to consider most 

 of the flagellates usually included in these two orders as belonging to 

 one order, Protomonadida, and to reserve an order, Polymonadida, for 

 the flagellates belonging to the family Calonymphidse, which includes 

 parasitic forms possessing many nuclei and blepharoplasts from which 

 arise a large number of flagella. 



Hartmann and Chagas (1910a) divide their Protomonadina into two 

 sub-orders — the Monozoa, including forms in which there is only a single 

 nucleus and set of organs; and the Diplozoa, those which have a bilateral 

 symmetry and double set of organs. It seems better, however, as Doflein 

 has done, to separate the Diplozoic forms in another order entirely, for 

 which the name Diplomonadida may be employed. The Zoomastigina 

 can be considered from the point of view of the number of nuclei the 

 adult forms possess, and this is perhaps the best basis for their primary 

 subdivision. The majority of forms possess a single nucleus, and these can 

 be regarded as Monozoic forms; others (Giardia) possess two nuclei, and 

 are therefore Diplozoic; while others again (Calonymphidse Grassi, 1911) 

 have many nuclei, and are therefore Polyzoic (Figs. 291, 301). 



The sub-class Zoomastigina may, therefore, be subdivided as 

 follows: 



A. Monozoic Forms. 



There is a single nucleus and a varying number of flagella and ble- 

 pharoplasts. 



1. Order: PKOTOMONADIDA.— The flagella are few in number (rarely 

 more than six). 



2. Order: HYPEEMASTIGIDA.— The flagella are very numerous. 



3. Order: CYSTOFLAGELLATA Haeckel, 1873.— The body is large and 

 globular, and possesses a peculiar tentacle as well as a single flagellum. 



B. Diplozoic Forms. 



There are two nuclei, while the flagella, blepharoplasts, and other 

 structures are similarly duplicated, giving rise to a bilateral symmetry. 



4. Order: DIPLOMONADIDA.— With the characters of the Diplozoic 

 forms. 



C. Polyzoic Forms. 



There are more than two nuclei and numerous flagella and ble- 

 pharoplasts. 



5. Order: POLYMONADIDA.— With the characters of the Polyzoic 

 forms. 



