Classification 45 



level, while the separation of these two from the grasshopper occurs at 

 the phylum. The relationships of these animals are clearly shown by 

 this classification. 



Some Phyla of the Animal Kingdom. — The many different forms 

 of animal life have been grouped into a relatively small number of phyla. 

 These phyla are comparatively clear-cut groups which are easily recog- 

 nized. In the latter portion of this book, these different phyla shall be 

 studied in greater detail, but at present there shall be only a brief survey. 



Protozoa. The protozoans. Single-celled animals which are included in the 

 subkingdom of the same name. Chapter 20. 



Porifera. The sponges. Attached animals which consist of loosely organized 

 cells. They live chiefly in the ocean and have but few fresh-water representatives. 

 Chapter 21. 



Coelentcrata. Hydra, jellyfish, corals, and sea anemones. These also live 

 chiefly in the ocean, but have a few fresh-water representatives. Chapter 21. 



Ctenophora. The comb jellies. These are entirely marine animals. Chap- 

 ter 21. 



Platyhelminthes. The flukes, tapeworms, and planaria. These are the flat- 

 worms, many of which are parasitic. Chapter 22. 



Nemathelminthes. The roundworms. Many of this group are parasitic, but 

 the vast majority are free living. They dwell in all possible habitats. • Chapter 23. 



Annelida. Earthworms, leeches, and sea worms. These worms have seg- 

 mented bodies and a body cavity lined with tissue. Chapter 24. 



Arthropoda. Insects, spiders, crayfish, etc. This phylum contains the great- 

 est number of individual species. Chapters 26 and 27. 



Mollusca. Snails, clams, squids, octopuses. Usually these animals have shells. 

 Chapter 25. 



Echinodermata. Starfishes sand dollars, sea urchins, sea cucumbers, crinoids. 

 All marine forms. Chapter 28. 



Chordata. All higher animals that have a strengthening skeletal structure 

 in the back. This phylum includes man, his domestic animals, fish, birds, etc. 

 They are found in nearly all possible habitats. Chapters 29, 30, and 31. 



The Classes of the Phylum Chordata.— Since the phylum 

 Chordata is to be considered in greater detail than the others, it is well 

 to consider its subdivisions. There are two principal subgroups : the 

 protochordates, small marine forms that are chiefly of interest because 

 they show possible vertebrate ancestry ; and the vertebrates. The mem- 

 bers of the vertebrates are all included in a single subphylum, the Verte- 

 brata. This contains all the animals with true backbones. 



The classes of the vertebrates are as follows : 



