SARKODINA 481 



are not interchangeable, but are fundamentally different in proper- 

 ties and structure " (Ray Lankester). It is true that in some 

 instances there may be a difficulty in deciding whether an organism 

 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 or ectosarc, and an inner, granular, 

 more fluid portion, the endosarc. 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 occasionally 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 reproduction 

 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 microgametes. 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 parthe- 

 nogenesis. 



Various classifications of the Protozoa have been suggested. 

 Biitschli divides them into four classes : I. The Sarkodina (p. 481) ; 

 II. The Mastigophora (p. 487) ; III. The Infusoria (p. 507) ; and 

 IV. The Sporozoa (p. 508). 



Class I. Sarkodina 



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

 locomotion and ingestion of food are performed by means of tem- 

 porary protoplasmic processes or pseudopodia. 



The Sarkodina includes a number of forms of very varied mor- 

 phology and habits, such as the Amoebae, Heliozoa, Radiolaria, and 

 Foraminifera, the three latter groups being characterised by the 

 presence of a siliceous or calcareous skeleton or shell. 



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