THE ANIMAL KINGDOM 3 



is populifolia, because it has leaves that twinkle in the wind like those of a 

 poplar tree. 



The common cat is Felis domestica; the lion, Felis leo; the tiger, 

 Felis tigris; and there are, in all, some forty species more in the genus 

 Felis. Linne called us Homo sapiens. We belong to the family Homi- 

 nidae (of which we are the only living species) to the order Primates, the 

 class MammaUa, the phylum Chordata, and the animal kingdom. In a 

 general way, for the larger and more famiUar animals and plants, the 

 vernacular name, such as pine or elephant, refers to the genus. 



Phylum I. Protozoa 



The protozoa are the simplest of animals, their bodies usually con- 

 sisting of only a single cell. Since they reproduce by simple fission of the 



^^If-^^ ^ - ^ ' ^ ~^FOOD VACUOLE 



PSEUDOPODIUM- 



AMOEBA. 



Fig. 2. — Amoeba proteus. Amoeba illustrates the characteristic features of the 



Protozoa. 



parent body there is no natural death in the group and no protozoon has 

 ever lost an ancestor. 



There are, at least, ten thousand species. Most protozoa are micro- 

 scopic; some are just visible to the unaided eye; a few colonies are milli- 

 meters in diameter; the slime molds, which are giant reticulated amoebas 

 and neither quite individuals nor quite colonies, are thin sheets some 

 centimeters in extent. 



Among the simplest are the amoebas, which to the superficial observer 

 look like blobs of jelly with no particular shape. They are without 

 skeletons. But some protozoa, such as the not uncommon fresh water 

 diffiugia, though otherwise hke amoebas, have single-chambered shells of 

 chitin or sihca. The foraminifera have more elaborate shells, calcareous 

 in most forms, which have accumulated through geologic ages into beds 

 of present-day limestone. The White Chalk Cliffs of Dover, which are 

 composed mostly of protozoan skeletons, are four or five hundred feet 

 thick, and extend nearly to Paris. 



More familiar are the forms with cilia or flagella — the sUpper animal- 

 cule, Paramecium, the trumpet animalcule, Stentor, and the host of 



