THE LOWER FORMS OF LIFE. 19 



PART II. 



THE PKOTOZOON. 



CHAPTEE I. 

 THE PROTOZOA : THE GREGARINA AND INFUSORIA. 



As the simple vegetable cell, the Protophyton, is the type upon which 

 all plants are constructed, so does the Protozoon represent the pri- 

 mitive simple cell, by the aggregation of which all animal structures, 

 whether flesh, bone, skin, hair, nail, feather, or scales, are formed. 

 The animal kingdom is divided into five great sub-kingdoms, viz. : 



1. The Protozoa, comprising the lowest form of animal life of 

 the Protozoon, in fact, in its simple unicellular character. 



2. The Radiata, so called because they have a radiate form, like 

 the sea-anemone and the beautiful zoophytes formerly thought to be 

 flowers. The name, however, is faulty, inasmuch as animals like the 

 star-fish and echinoderm, which have a much higher organisation 

 and belong to a higher sub-kingdom, have marked radiate forms. 

 The difficulty has been fully recognised by naturalists. The term 

 Coelenterata, for reasons we shall see by and by, has on anatomical 

 grounds been applied to the group. 



3. The Mollusca, so called because their bodies are soft, like 

 the oyster. This group includes the great mass of animals that are 

 protected by shells, or tunics of softer material. A great number, 

 however, like the garden slug, are naked. 



4. The Annulosa, so called because the bodies are formed upon the 

 plan of segments, or annular divisions. It includes the echinoderms, 

 worms, crabs, lobsters, spiders, myriapods, and insects. 



5. The Vertebrata, so called because each animal has a vertebral 

 column, formed of a series of bones termed vertebrae. It includes 

 fishes, reptiles, birds, and mammals. 



With this necessary preface, we will now deal with the first and 

 lowest sub-kingdom. 



THE PROTOZOA. 



This group is represented by five classes of animals, all consisting 

 of simple cells, and therefore ranking in organisation nearly parallel 

 to each other. Naturalists, however, have placed them in the 



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