36 FOUR-FOOTED AMERICANS 



6. Mollusks. . . . Shell Fish, such as Oysters, Clams and Mussels, 



Snails, Slugs, Cuttle Fish, etc. 



7. Crustaceans . . Animals covered with a hard shell, having 



many legs and a pair of feelers, or antennae, 

 breathing through gills the air that is dis- 

 solved in the water. Lobsters, Crabs, etc., 

 are Crustaceans. 



8. Spiders and (Called Arachnid?e, from Arachne, the Spinner, 

 Scorpions . . . because they spin webs.) Are a sort of 



cousin to Crabs, but live on the earth instead 

 of in the water. 



"The top branches of this group contain the Insects, 

 with many legs, their bodies being divided into three 

 parts. Insects go through many changes in the course 

 of development. Take the butterfly as an example. 

 First an Qgg is laid by a fully grown butterfly ; second, 

 a caterpillar is hatched from the Qgg ; third, the cater- 

 pillar spins itself into a chrj'Salis, or cocoon, out of 

 which comes the winged butterfly. Ants, mosquitoes, 

 flies, and beetles are all insects. 



" Among the next circle of branches we find the ani- 

 mals having backbones, the Vertebrates. I think you 

 will feel more at home Avith them, and we are more 

 nearly concerned with them now, as our mammals be- 

 long in this order, although there are many things you 

 must some day learn of the many backboneless twigs, 

 especially about the insects with their wonderful wings 

 and stings." 



" I suppose my Rattlesnake is a rather low-down Ver- 

 tebrate, Uncle Roy," said Nat. 



"No, my boy, there are two grades below him and 

 two above. See," — and the Doctor drew a iDranch 

 with five divisions. 



