THE FROG 



117 



column composed of vertebrae. Although this is sometimes done, 

 those animals that never have a notochord are almost invariably 

 classed together as the Invertebrata, and hence, for contrast, some 

 authors retain the term Vertebrata for the remaining group, and 

 include in it all the Chordata. In the widest use of the cerm 

 Invertebrata it can, of course, include Protozoa and Ccelenterata, 

 but it is convenient to restrict it to invertebrate ccelomates. These 

 two groups differ greatly in relative value, for the Phylum Chordata 

 forms what may be termed a natural group, since all its members 

 are related to one another by community of descent, and all save 

 certain degenerate forms conform to one fundamental plan. The 

 Invertebrata, on the other hand, are simply all the Ccelomates that 

 are not Chordates. It includes various Phyla, each a unit in itself, 

 but not closely or obviously related to one another, and following 

 different plans of organisation. The relations of the main divisions 

 of the animal kingdom, omitting certain small groups, can be 

 expressed schematically as follows : 



/PROTOZOA. PHYLA. 



Protozoa. Amoeba, Par amoecium, 



Monocystis, Malaria, etc. 



CCELENTERATA. Ccelenterata. Hydra, Obelia, etc. 



ANIMAL / / Platyhelminthes. Tape - worms, 



KINGDOM. 1 Liver-flukes, Flat-worms, etc. 



Nemertea. Ribbon-worms. 

 I Nemathelminthes. Round - 



worms. 



Annulata. Lumbricus, other seg- 

 , INVERTE- I mented worms. 

 BRATA. ( Mollusca. Shell-fish, Octopus, etc. 

 Echinodermata. Star-fish, Sea- 

 urchins, Sea-lilies, Sea- 

 cucumbers, etc. 



Afthropoda. Lobsters. Crabs, 

 Spiders, Centipedes, Insects, 

 etc. 



\CHORDATA Chordata. Amphioxus, Sea- 

 (VERTE- squirts, Scyltium, Rana, Rep- 

 BRATA). tiles, Birds, Lepus, Canis, etc. 



In the following chapters we shall deal with representative 

 forms from some of these Phyla ; their position in the above table 

 is indicated by the names in italics. 



METAZOA. 



CCELO- 



