. Principles of modern biology. Biology. The Animal Kingdom - 625 THE PROTOZOA1 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
. Principles of modern biology. Biology. The Animal Kingdom - 625 THE PROTOZOA1 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 beau
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