PROBLEM I. The Kinds oj AnmiaJs 0^ the Earth 



53 



Fig. 73 Lobster catchmg a [^ 

 crab. Both are crustaceans. 

 How can you distinguish 

 lobsters jrorn crabs? (Ameri- 

 can MUSEUM OF NATURAL 



history) 



Fig. 74 The scorpion is 



grouped with spiders, al- 

 though it looks quite differ- 

 ent. It carries its young on 

 its back. The sting at the 

 end of its abdoinen can be 

 waved over its head. (Ameri- 

 can MUSEUM OF NATURAL 



history) 



For strangeness, the barnacles take the 

 prize. It was a long time before thev 

 were recognized as relatives of lobsters 

 and crabs. A famous English biologist, 

 T. H. Huxley, has given this striking 

 description: "A barnacle may be said 

 to be a crustacean fixed by its head and 

 kicking the food into its mouth with its 

 legs." 



Exercises 6 and 7 would be good re- 

 view exercises before you leave the 

 group of jointed-legged invertebrates. 

 You will next examine briefly eight other 

 invertebrate phyla. We shall proceed 

 from the more complex to the more 

 simple forms. 



PHYLUM - MOLLUSKS 



The Soft-bodied Invertebrates 



What are mollusks? If you examine 

 Figs. 75-78 you will see examples of 

 three different groups (classes) of mol- 

 lusks: those that have a foot which is 

 used in creeping, like the snail; those 

 that have feet used in seizing prey, like 

 the octopus; and those that have a 

 hatchet foot used in plowing through 

 wet sand or mud, like the clams. Most 

 have a shell of lime which protects 

 the soft body. The shell takes very dif- 

 ferent forms; it may be single or double 

 and may even be carried internally. 



