CHAPTER XIII 

 THE INVERTEBRATES 



The invertebrate or backboneless animals include all of 

 the great branches, or phyla, into which the animal king- 

 dom is divided, except one, the branch Chordata. Ac- 

 cording to our table of animal classification (see pp. ii6, 117) 

 there are twelve of these invertebrate branches, one of 

 which, the Protozoa, or one-celled animals, we have already 

 briefly discussed. Included in the other eleven branches 

 are all the sponges, sea-anemones, corals and jellyfishes, 

 all the animals we commonly know as worms and a host of 

 less familiar worm-like others, all the starfishes, sea-urchins 

 and sea-cucumbers, the crabs, the centipedes, the insects 

 and spiders and all the shell-fish and other creatures grouped 

 together as molluscs. The backboneless animals out- 

 number by far in species and in individuals the backboned 

 animals. The insects alone, which compose but a single 

 class, the Insecta, of the great branch Arthropoda, include 

 a greater number of kinds than all the other animal classes 

 and branches together. But just the same the interest of 

 most of us is held more by the backboned animals, the fishes, 

 batrachians, reptiles, birds and animals; those animals 

 with which our own bodies may be most readily compared 

 and among whom we find our most valuable and entertaining 

 and friendly companions and aids in life. 



However, the five hundred thousand known species, 

 more or less, of invertebrate animals now living include a 

 host of kinds whose lives are of great interest and of great 

 immediate importance to us. Some of them help build 



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