170 ''WE ANIMAL KINGDOM 



not include tlie larger, pigmentless groups such as the choanoflage!- 

 lates and many ol the jxnasitic groups. Zoologists generally claim all of 

 the flagellates, even the groui:)s that are completely autotrophic. In- 

 clusion ot the latter, with the Phytomonadina as an example, is probably 

 not defensible but persists through custom. A good argument for keep- 

 ing all of the flagellates together is that the transition from autotrophic 

 to holozoic nutrition appears to have occurred independently in differ- 

 ent groups. 



Sarcodinids are related to the flagellates through several genera of 

 ameboid organisms that have flagella and through several forms that 

 resemble typical flagellates in open water but which lose their flagella 

 and creep like amebas when they are next to solid surfaces. In fact, the 

 existence of so many intergrades suggests that sarcodinids may have 

 evolved several different times from the flagellates. A further tie relating 

 the groups is found in the gametes of foraminiferans, each of which has 

 two tiny flagella. 



The ciliates are a distinct group and probably arose only once. 

 Cilia are structurally like flagella and are considered to have evolved 

 from them by extensive duplication and diminution. A significant step 

 is the independence of the basal granules from the centriole. The evo- 

 lutionary origin of the macronucleus is unknown. During the conjuga- 

 tion of most ciliates a bit of protoplasm is transferred along with the 

 migrating nuclei. In one species, the heterotrich Cycloposthiiim, each 

 migrating nucleus and its bit of protoplasm separates in the mouth cavity 

 as a distinct gamete with a long tail. The two gametes then move past 

 each other to the opposite side. It has been suggested that this is similar 

 to sperm formation in other organisms, and that it may reflect a flagellate 

 ancestry. Suctorians are easily derived from the ciliates by a modification 

 of the adult stage. 



The sporozoa are probably a composite group. Some species show 

 affinities with the flagellates while others more nearly resemble sar- 

 codinids. Multiple fission may be regarded as an adaptation to para- 

 sitism and may well have developed independently in several groups of 

 flagellates and sarcodinids. 



It is, of course, a challenge to the systematist that the group di- 

 visions are not sharp and clear, either between plant and animal flagel- 

 lates, between flagellates and sarcodinids, or between both of these and 

 the sporozoans. Actually the number of evolutionary changes necessary 

 to develop one group from another is not great, and it is likely that 

 the course of evolution is obscured as much by repetition as by the loss 

 of intermediate forms. 



Questions 



1 . Name the five classes of protozoans and make a sketch of an example from each. 



2. Compare organs and organelles. 



3. What is a basal body? 



4. Compare movement, nutrition and asexual reproduction in Euglena, Paramecium, 

 and the ameba. 



