6 INTRODUCTION 



These four expressions, cell, tissue, organ, and organ sys- 

 tem, describe the materials of plants and animals which 

 carry on the eight life processes referred to above. We 

 shall read more and more about them as our study of 

 biology progresses. 



Classification of Living Things. — Our study of biology 

 cannot progress far before we see the need of classifying 

 animals and plants. Animals are generally grouped in 

 two divisions : invertebrates (animals without backbone) 

 and vertebrates (animals with backbone). Plants are also 

 divided into two groups : cryptogams (flowerless and seed- 

 less plants) and phanerogams (flowering or seed-bearing 

 plants). Below is given a detailed reference table of 

 these classifications. 



I. Invertebrates. Animals without a backbone. 



1. Protozoa. 8000 different kinds. 



a. Rhizopoda. Example, the amoeba. 



b. Ciliata. Example, the paramoecium. 



2. Porifera. Sponges, 2500 different kinds. Example, the bath 



sponge and grantia. 



3. Ccelenterata. Hydra, corals, and jellyfish. 4500 different kinds. 



a. Hydrozoa. Example, the hydra, obelia, pennaria. 



b. Scyphozoa. The large jellyfishes. 



c. Actinozoa. The corals. 



4. Echinoderms. Starfishes and sea urchins. 4000 different kinds. 



5. Worms and wormlike animals. Examples, flat worms, tape 



worms, earthworms. 11,000 different kinds. 



6. Mollusca. The clams and snails. 61,000 different kinds. 



a. Pelecypoda. Example, clams. 



b. Gastropoda. Example, snails. 



c. Cephalopoda. Example, squids, devilfish. 



7. Arthropoda. Crabs and insects. 400,000 different kinds. 



a. Crustacea. Example, crayfish and crabs. 10,000 different 



kinds. 



b. Insecta. Example, grasshopper, flies, butterflies, bees. 



390,000 different kinds. 

 II. Vertebrates. Animals with a backbone. 



1. Fishes. Examples, trout, perch, bass, cod. 13,000 different kinds. 



