ANIMALS CLASSIFIED 



185 



A common snail, a 

 mollusk. (From a 

 photograph by 

 Davison.) 



than the worms and may be known by the spines in their skin. 

 They show radial symmetry. Starfish or sea urchins are examples. 



VI. Arthropods. These animals are distinguished by having 

 jointed body and legs. They form two great groups. The higher 

 forms of the Crustacea have only two regions in the body, a fused 

 head and thorax, called the cephalothorax, and an abdominal 

 region. A second group is the Insecta, of which we know some- 

 thing already. Crustacea breathe by means 



of gills, which are structures for taking oxygen 

 out of the water, while adult insects breathe 

 through air tubes called trachea. 



Two smaller groups of arthropods also exist, 

 the Arachnida, consisting of spiders, scorpions, 

 ticks, and mites, and the Myriapoda, examples 

 being the "thousand leggers" found in some 

 city houses. 



VII. Mollusca. Another large group is the 

 Mollusca. This phylum gets its name from 

 the soft, unsegmented body (mollis = soft). 



Mollusks usually have a shell, which may be of one piece, as a 

 snail, or two pieces or valves, as the clam or oyster. 



VIII. The Vertebrates. All of the animals we have studied 

 thus far agree in having whatever skeleton or hard parts they 

 possess on the outside of the body. Collectively, they are called 

 Invertebrates. This exoskeleton differs from the main or axial 



skeleton of the higher 

 animals, the latter be- 

 ing inside of the body. 

 The exoskeleton is 

 dead, being secreted 

 by the cells lining the 

 body, while the endo- 

 skeleton is, in part at 

 least, alive and is 

 capable of growth, e.g. 

 a broken arm or leg 



The skeleton of a dog ; a typical vertebrate. bone will gTOW to- 



