THE INVERTEBRATES 



To classify all the animal world into Vertebrates and Inverte- 

 brates, that is, into two groups which should be distinguished by 

 either the presence or absence of a spinal column, would seem to 

 us to be a very simple matter. 



But the more the animal world is studied the more difficult 

 classification becomes. With every further step in our acquain- 

 tance with the structure and nature of animal forms the less easy 

 is it to say where the Vertebrates should end and where the 

 Invertebrates should begin; for with some animals it is difficult 

 to say whether the spinal column should be considered to be 

 absent or present. 



The following table of Invertebrates gives us the most modern 

 view of correct classification. This should help us to understand 

 the true positions of the various greater groups of this vast king- 

 dom of animal forms. 



The kingdom of the Invertebrates is divided into the following 

 sub-kingdoms : — 



I. The Arthropoda — Insects, Spiders, Crustaceans. 

 II. The EcHiNODERMATA — Star-fish, Sea-urchins, &c. 



III. The MOLLUSCA — Cuttle-fish, Whelks, Oysters, Mussels, &c. 



IV. MoLLUSCOiDA — Lamp-shells, Corallines. 

 V. Vermes — Worms and their relatives. 



VI. Coelenterata — Jelly-fish, Sea-anemones, Corals. 

 VII. Porifera — Sponges. 

 VIII. Protozoa — The microscopic Foraminifera (single-celled animals). 



Most of us are probably acquainted with certain representatives 

 of all these sub-kingdoms except the last, and that fact alone 



