PROBLEM I. The Kinds of An'wials of the Earth 



55 



Fig. 77 Although the slug is a molUtsk, belong- 

 ing to the same group as the snail, it has no 

 shell. This one has just laid its eggs on a leaf. 



(MARY C. DICKERSON) 



Fig. 78 The clain has a double shell and a 

 hatchet foot. The claifis above a7-e using the 

 foot to ploiv through the sand, (ward's natu- 

 ral SCIENCE ESTABLISHiMENT) 



Mollusks and man. A great many spe- 

 cies of mollusks are eaten by man. Snails 

 are considered a delicacy by some people, 

 and the octopus and squid are eaten in 

 many parts of the world. Oysters, the 

 many species of clams, the scallops, and 

 the mussels are commonly eaten. Oysters 

 are valuable also as the source of mother- 

 of-pearl, from which buttons are made. 

 Precious pearls are found only in certain 

 tropical species and then rarely. 



There are comparatively few kinds 

 of pests among mollusks. One of the 

 worst is the "shipworm." It bores into 

 timber which is under water, riddling 

 it with tunnels until the \\'ood collapses. 

 Now that ships are made of steel the 

 damage done by shipworms is confined 

 to wharves. 



PHYLUM -ECHINODERAIS 



Invertebrates ivith Spiny Skins 



A different body plan. The inverte- 

 brates with spiny skins are called 

 Echinoderms (eh-kine'o-derms). They 

 are built on a plan different from that 

 found in the more complex animals. 

 Most animals have bilateral syimnetry. 

 This means that if they were cut down 

 the middle the two halves would be 

 about the same in appearance. But the 

 invertebrates with spiny skins have 

 radial synmtetry, like a wheel. Just as 

 the spokes of a wheel radiate from the 

 hub, so the parts of these animals radi- 

 ate from a central point. Besides this, 

 these animals have a spiny skin and a 

 complicated system of water vessels that 



