XVII. SKELETONS OF THE LOWER 

 ANIMALS.^ 



The most striking point of difference between the higher 

 and the lower animals is the difference in their skeleton or 

 supporting framework. In fact, it is customary to sub- 

 divide all animals into two groups (vertebrates and inver- 

 tebrates) on the basis of the presence or absence of a 

 backbone. 



Invertebrate animals are characterized either by the 

 entire absence of skeletal structures, as in the slugs, worms, 

 jellyfishes, etc., or by the presence of an external protec- 

 tive layer called an external skeleton. A few, such as 



Fig. 101 — Protozoans (foraminifera) with external skeleton. 



the cuttle fishes, have an internal framework, but it is 

 not in anjr way similar to the internal skeleton of the 

 vertebrate animals. Vertebrates all possess an internal 



' See Footnote, p. 146, Chapter X. 

 252 



