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ANIMALS CLASSIFIED 



mentioned above are either very poorly developed or entirely 

 lacking. But in the so-called " higher " animals each of the 

 above-named functions is assigned to a certain organ or group of 

 organs. The work is done better and more quickly than in the 

 " lower " animals. Division of labor is thus a guide in helping 

 us to determine the place of animals in the groups that exist on the 

 earth. 



The Animal Series. — We have found that a one-celled animal 

 can perform certain functions in a rather crude manner. Man 



can perform these same functions 

 in an extremely efficient manner. 

 Division of labor is well worked 

 out, extreme complexity of struc- 

 ture is seen. Between these two 

 extremes are a great many groups 

 of animals which can be arranged 

 more or less as a series, showing 

 the gradual evolution or develop- 

 ment of life on the earth. It 

 will be the purpose of the follow- 

 ing pages to show the chief char- 

 acteristics of the great groups of 

 the animal kingdom. 

 I. Protozoa. — Animals composed of a single cell, reproducing 

 by cell division. 



The following are the principal classes of Protozoa, examples of which we may 



have seen or read about : — 



Class I. Rhizopoda (Greek for root-footed). Having no fixed form, with pseudo- 

 podia. Either naked as Amoeba or building limy (Foraminifera) or glasslike 

 skeletons (Radiolaria) . 



Class II. Infusoria {in infusions). Usually active ciliated Protozoa. Examples, 

 Paramcecium, Vorticella. 



Class III. Sporozoa (spore animals). Parasitic and usually nonactive. Exam- 

 ple, Plasmodium malarioe. 



The giasslike skeleton of h radiolarian, 

 a protozoan. (From model at Ameri- 

 can Museum of Natural History.) 



II. Sponges. — Because the body contains many pores through 

 which water bearing food particles enters, these animals are called 

 Porifera. They are classed according to the skeleton they possess 

 into limy, glasslike, and horny fiber sponges. The latter are 



