580 A MANUAL OF BACTERIOLOGY 



those which form the outer body wall. The cells of these two 

 layers are not interchangeable, but are fundamentally different 

 in properties and structure " (Kay Lankester). In some in- 

 stances there may be a difficulty in deciding whether an organ- 

 ism is vegetable or animal, and Haeckel proposed to include all 

 indeterminate unicellular organisms in a distinct kingdom, the 

 Protista. 



The cytoplasm of a protozoon is commonly differentiated into 

 an outer, clearer, denser layer, the ectosarc or ectoplasm, and an 

 inner, granular, more fluid portion, the endosarc or endoplasm. 

 The cytoplasm is sometimes naked, or may be covered with a 

 cuticle, usually protein in nature. The cytoplasm contains a 

 well-marked nucleus, sometimes a secondary nucleus and occasion- 

 ally subsidiary chromatin particles or chromidia. A contractile 

 vacuole, which is an excretory organ, is frequently present. 



In most protozoa reproduction takes place by simple division 

 or fission, and by a process of spore-formation ; in others repro- 

 duction is exclusively by spores, which are often formed by a 

 complicated process of development. In many of the protozoa 

 a simple form of sexual reproduction by conjugation occurs. 

 Two dissimilar cells (gametes) are produced, the larger comparable 

 to female cells or ova and termed macrogametes , the smaller 

 comparable to male elements or spermatozoa and termed micro- 

 gametes. The cells from which the gametes are derived are known 

 as gametocytes. The gametes conjugate and form a zygote, 

 which usually divides into a number of spores from which the 

 adult is reproduced. 



In certain cases sexually differentiated individuals reproduce 

 by fission without conjugation ; this phenomenon is termed 

 parthenogenesis . 



Various classifications of the Protozoa have been suggested. 

 Biitschli divides them into four classes : I. The Sarkodina ; II. The 

 Infusoria (p. 588) ; III. The Mastigophora (p. 589) ; and IV. The 

 Sporozoa (p. 632). 



Glass I. Sarkodina 



These are Protozoa in which the cell protoplasm is naked, and 

 locomotion and ingestion of food are performed by means of 

 temporary protoplasmic processes or pseudopodia. 



