THE ECONOMIC VALUE OF ANIMALS. 829 



while the sale of the polished shells as curiosities gives employ- 

 ment to many persons. From the whelks a dye is obtained ; 

 sepia comes from the cuttlefish or squid ; while one species pro- 

 vides the cuttlefish bone of commerce. The smallest worm, which 

 Shakespeare tells us, " will turn, being trodden on," has its place 

 in the economy of life. The phosphorescent worms add to the 

 splendors of the festivals of the nights along our shores, and are 

 among the illuminators of the ocean ; as bait they have a relation 

 to the fisheries ; leeches have a medicinal value, while the com- 

 mon earthworm is a valuable ally of the farmer. Darwin esti- 

 mates that there are one hundred thousand earthworms in the 

 upper six feet of every acre of ground in favorable localities, and 

 in New Zealand three hundred thousand. They are continually 

 turning over the soil, dragging down seeds and leaves, and bring- 

 ing to the surface in some places ten tons of mold to the acre 

 every year, thus performing a valuable service to the agriculturist. 



The crustaceans are among the important scavengers of the 

 sea and are also valuable as food for fishes. The collection 

 of crabs, shrimps, and lobsters forms large industries all over the 

 world, contributing directly to the support of man. In Delaware 

 the horseshoe crab is used as guano, while the collection of fossil 

 crabs, as trilobites, is a peculiar industry. The fresh- water cray- 

 fish produces a concretion used as an antacid, well known to chem- 

 ists. We owe many of the beauties of our summer fields to in- 

 sects, all of which have their special functions and use. Even the 

 persecuted flea may render man a service by keeping the drowsy 

 watch dog awake, while the mosquito in tropical countries may 

 aid in preventing the human inhabitants from living a continual 

 siesta. 



The flies are among the most valuable insect scavengers. The 

 spiders prey upon flies, holding them in check. The silk of the 

 spider is used as a cross line in astronomical instruments, and 

 that of a Bermuda species as sewing silk. Bridge makers have 

 obtained valuable suggestions from these silent workers, from 

 whose web one of the kings of France is said to have made a coat. 

 Grasshoppers and locusts are enemies of civilized man, but are 

 eaten by the Indians, while in the Malay country the dragon fly 

 is considered a delicacy. 



The cochineal produces the famous commercial dye, a valuable 

 industry. Manna comes from the puncture of an allied insect, 

 while another insect produces wax. A popular eye powder 

 among the Chinese is made from a fly. The silk industry in the 

 United States, the product of the silkworm, represents a capital 

 of twenty-seven million dollars, and the moths and butterflies 

 perform an equally valuable work in fertilizing flowers, thus in- 

 suring the crojjs. A decoction of oak and other galls of the gall 



