194 



COLLEGE ZOOLOGY 



leeches is the most obvious. Earthworms are 

 widely used as bait for fishing; various 

 methods have been used to drive them out 

 of their burrows so that they can be collected 

 in large numbers. These include use of an 

 electric current, jarring the soil by beating 

 a stick driven into it, and pouring a solution 

 of chemicals such as mercuric chloride 

 (poison) on the ground. Raising earthworms 

 as bait for fishing has become quite profit- 

 able in some resort districts. 



Figure 108. Diagram showing the burrow and 

 castings of an earthworm. 



Charles Darwin demonstrated, by careful 

 observations extending over a period of 40 

 years, the great economic importance of 

 earthworms. One acre of ground may con- 

 tain over 50,000 earthworms. The feces of 

 these worms are the little heaps of black 

 earth called castings (Fig. 108) which strew 

 the ground; they are especially noticeable 

 early in the morning. Darwin estimated that 

 more than 18 tons of earthly castings may 

 be carried to the surface in a single year on 

 one acre of ground; and in 20 years, a layer 

 three inches thick would be transferred from 

 the subsoil to the surface. By this means 

 objects are covered up in the course of a few 

 years. The continuous honeycombing of the 

 soil by earthworms makes the land more 

 porous and insures better penetration of air 

 and moisture. The mixing of soil and organic 

 matter in the digestive tract of the earth- 

 worm should contribute something to in- 

 creasing humus; however, the claim that the 

 addition of earthworms to an unproductive 



soil will greatly increase its fertility is false. 

 Earthworms may also be harmful. They 

 disfigure lawns and golf courses with their 

 castings and may serve as intermediate hosts 

 of parasitic worms. For example, they are 

 intermediate hosts in the life cycle of a 

 cestode of chickens, Amoebotaenia, and in 

 that of a pig lungworm of the nematode 

 genus Metastrongylus; and they are passive 

 carriers of the nematode worm Sy7igamus 

 trachea, which causes gapes in fowls. 



As a transporter of soil, the lugworm, a 

 species of Arenicola (Fig. 90), a polychaete, 

 is even more effective than the earthworm. 

 The amount of sand brought to the surface 

 on 19 measured areas was 82,423 castings to 

 an acre; the average amount of sand brought 

 up to the surface each year on these areas 

 was about 1911 tons to the acre, which, if 

 spread evenly, would form a layer about 13 

 inches deep. Other observations made at 

 different places showed about 34 to 38 cast- 

 ings to the square yard; the amount brought 

 up was estimated to be about 3700 tons to 

 the acre in a year, or equivalent to a layer 

 about 24 inches thick. Species of lugworms 

 aire widely used as bait in all places where 

 they are found. A bed where fishermen con- 

 stantly dig may contain about three million 

 worms; removal of a few thousand a day 

 produces no noticeable effect. In certain 

 bays of New England, it is estimated that 

 12^2 million worms (species of Neanthes 

 and Glycera) are picked up by diggers in 

 one year. At one time, a digger may collect 

 about 350 worms. 



Oyster pests include polychaete worms of 

 the genus Polydora; they cause mud blisters 

 in the nacreous layers of the shells and ren- 

 der the oysters unsalable; or the oyster may 

 be weakened, if not actually killed. Oyster 

 growers call it "worm disease." In some re- 

 gions where oyster culture once flourished, it 

 had to be discontinued, or different methods 

 had to be introduced, such as rearing the 

 spat (young oysters) on elevated or only 

 partially submerged surfaces. Not only 



