28 EVOLUTION AND GENETICS 



it exerts a reciprocal influence of great value as an illustration of 

 the results of evolution. 



Classification of Plants. The most common classification of 

 plants is based upon four major groups. The third has been di- 

 vided into three, which are indicated here merely as components 

 of the one division. 



1. Thallophyta. The algae, fungi and hchens. 



2. Bryophyta. Liverworts and mosses. 



3. Pteridophyta. Ferns, horse-tails or scouring rushes, and club 



mosses. 



4. Spermatophyta. The seed-bearing plants. 



Classification of Animals. Some variation occurs in the classi- 

 fication of animals as various writers estimate differently the 

 importance of certain characters. The following outline includes 

 the major subdivisions, or ph3da, which are commonly recog- 

 nized. 



1. Protozoa. Single-celled animals. 



2. Porifera. The sponges. 



3. Coelenterata. The hydroids, jellyfishes, sea anemones and 



corals. 



4. Ctenophora. Comb-jeUies or sea walnuts. 



5. Platyhclminthes. Flatworms: free-living forms and para- 



sitic flukes and tapeworms. 



6. Nemathelminthes. Roundworms, including many parasitic 



forms found in man as well as free-living forms. 



7. Rotatoria. Wheel animalcules. 



8. Bryozoa. The moss animals. 



9. Brachiopoda. Tongue or lamp shells. 



10. Echinodermata. Starfishes, sea urchins, sea cucumbers. 



11. Annelida. Jointed worms: earthworms and leeches. 



12. Arthropoda. Crustacea, including lobsters, etc. Myria- 



poda, spiders, insects, etc. 



13. MoUusca. Snails, mussels, squids, etc. 



14. Chordata. Several obscure worm-like animals, the tunicates 



and salpians, lancelets, round-mouthed eels, fishes, am- 

 phibia, reptiles, birds, and mammals. 

 The tree of life, adapted to modern classification from the 

 original conception of Lamarck, shows graphically the relation- 

 ship of these phyla and some of their more important subdivisions 

 (Fig. 11). 



