The Animal Kingdom - 625 



THE PROTOZOA 1 

 ANIMALS) 



(LITERALLY, FIRST 



This fairly large (about 20,000 species) 

 phylum includes all one-celled animal or- 

 ganisms. Most of the species are of solitary 

 habit; but there are a number of colonial 

 forms. Typically the nutrition is holozoic; 

 but one group, the Sporozoa, is entirely para- 

 sitic; and there are some saprophytic species. 

 Four main subgroups (here designated as 

 classes) have been evolved: 



Class 1. The Sarcodina (Fig. 32-1). Pro- 

 tozoa with pseudopodia. 

 Class 2. The Mastigophora (Fig. 32-2). 

 Protozoa with flagella (some- 

 times called the Flagellata). 

 Class 3. The Ciliophora (Fig. 32-4). Pro- 

 tozoa with cilia (at least during 

 some part of the life cycle). 

 Class 4. The Sporozoa (Fig. 30-7). Para- 

 sitic Protozoa, without distinc- 

 tive locomotor organs. 



1 Many zoologists believe that the status of the Pro- 

 tozoa should be raised to that of a Subkingdom of 

 the Animalia. In this case, the four classes (see below) 

 of the Protozoa must be designated as phyla. In any 

 event, the paleontological evidence indicates that the 

 four groups of Protozoa initiated separate lines of 

 evolution at a very early date (Fig. 29-11). 



ACTINOSPHAERIUM 



The Sarcodina. All these one-celled animals 

 possess pseudopodia. By definition, a pseudo- 

 podium is an elongate retractable protrusion 

 of the cytoplasm, which, in most cases, serves 

 for locomotion. However, pseudopodia dis- 

 play a wide variety of forms. They may be 

 blunt and fingerlike (as in Amoeba and Dif- 

 flugia, Figure 32-1); slender and needlelike 

 (as in Actinosphaerium and Heliospaera, 

 Fig. 32-1); stiff or flexible; branched or un- 

 branched; clear or granular. But always a 

 pseudopodium is a temporary protrusion, 

 capable of retracting and reforming from 

 time to time. 



The Sarcodina are quite numerous and 

 varied. Some, like the Foraminifera, inhabit 

 perforated calcareous shells; others, like the 

 Radiolaria (for example, Heliosphaera, Fig. 

 32-1), build beautiful intracytoplasmic skele- 

 tons of siliceous material; and others, like 

 the Amoeba, are unprotected except by a 

 very delicate pellicle. 



During past ages, Radiolaria and Forami- 

 nifera populated the sea in great abundance. 

 In fact, the skeletal accumulations of these 

 organisms, collecting on the floor of the 

 ocean, played a dominant part in the deposi- 

 tion of certain layers of sedimentary rock. 

 Accumulations of Foraminifera form a gray- 



PSEUDOPODS 



!7milm^ :y 



,?Joww,'ft 



AMOEBA 



Fig. 32-1. Protozoa with pseudopodia: Class Sarcodina. 



