76 THE STORY OF ANIMAL LIFE 



that live in earth that is damp ; but the earth- 

 worm is in the strictest sense a terrestrial animal. 

 Darwin showed that it not only dwells in the 

 soil, but is in a sense the manufacturer of soil, 

 fcince the fertility of the earth depends greatly 

 upon the work of earthworms. They pass the 

 soil through their bodies, digesting the organic 

 particles they find in it, and thereby loosen the 

 soil, reduce it to a state of fine division, and 

 render it more fit to support the growth of plants. 

 The " worm-casts " formed by the soil that the 

 earthworm has passed through its body may not 

 have been noticed by everybody. More obvious 

 are the worm-casts in sand left by the sand- 

 dwelling marine annelids. These everyone must 

 have seen who has walked on a sandy shore at 

 low tide. 



The worms include many puzzling forms, which 

 have not been alluded to here. Among these 

 must not be forgotten the Rotifers, or wheel- 

 bearing animals. These are of minute size, and 

 when first discovered were therefore placed 

 amongst the Infusoria. They are common in 

 ponds. 



CHAPTER VIII 



ARTHROPODA, THE LOBSTERS, SPIDERS AND 

 INSECTS 



THE above is a very descriptive name for a 

 division which includes the Crabs and Lobsters 

 and the Insects. Formerly they were included, 

 along with the worms, under the name Annulosa, 

 the Ringed Animals. They resemble these as 



