156 PHYLUM: PROTOZOA 



PHYLUM: PROTOZOA GOLDFUSS, 1817. 



The phylum Protozoa^ as defined above, is the subdivision of the animal 

 kingdom in which all unicellular animals are grouped. It may be divided 

 into two sub-phyla, as suggested by Doflein (1901). The first of these 

 is the PLASMODROMA, which includes the forms which have 

 pseudopodia or flagella, and in which syngamy, where it is known to 

 occur, consists in the complete fusion of two gametes. The second sub- 

 phylum is the GILIOPHORA, which comprises those Protozoa 

 which have numerous cilia as motile organs, a special type of binuclearity 

 (macronucleus and micronucleus), and a process of syngamy in which two 

 individuals temporarily associate, undergo exchange of nuclei, and then 

 separate. The class Opalinata, in which syngamy is of the type seen 

 amongst the Plasmodroma while the binuclearity characteristic of the 

 other classes of the Ciliophora is wanting, forms a connecting link between 

 the two sub-phyla. 



A. SUB-PHYLUM: PLASMODROMA DOFLEIN, 1901. 



This, the first of the sub-phyla into which Doflein divides the Protozoa, 

 includes forms which have either pseudopodia or flagella as organs of 

 locomotion, and the parasitic Sporozoa which, owing to their mode of life, 

 have been modified in various ways. There is either a single vesicular 

 nucleus or more than one are present. Syngamy takes place by the 

 complete fusion of gametes, which may be alike (isogamy) or different 

 (anisogamy). In many forms, after a period of asexual reproduction, 

 syngamy, followed by a different method of reproduction, occurs (alterna- 

 tion of generations). 



The sub-phylum contains four classes of Protozoa, two of which include 

 mainly free-living forms, while two contain forms which are exclusively 

 parasitic. One class is characterized by the amoeboid form of the body 

 which produces pseudopodia as organs of locomotion, while in another, 

 though the body may be amoeboid, it possesses one or more flagella. The 

 Protozoa of the first type belong to the class RHIZOPODA, and those 

 of the second to the class M ASTIGOPHORA. The separation of these 

 two classes is rendered difficult by the fact that certain organisms which 

 are amoeboid and devoid of flagella for the greater part of their existence 

 may at certain stages develop flagella, while, conversely, forms which 

 usually possess flagella may have a purely amoeboid phase. 



As regards the parasitic types, many observers have grouped them 

 together in the one class Sporozoa, which was divided by Schaudinn 



