18 ANIMAL RESOURCES OF THE UNITED STATES 
53. ARACHNIDA—Continued. : 
Useful traits: Destroy noxious insects, (spiders.) 
Injurious traits : 
Venomous, (scorpions.) 
Parasites, (1nites.) 
XJ. ARTHROPODS. 
54. CRUSTACEA. (Crabs, lobsters, shrimps, prawns, crawfish, lim- 
noria, fish-lice, lernzans, sand and water 
fleas, barnacles, horseshoe crabs, &e.) 
. Useful products: 
Foods, (fresh and canned crabs, lobsters, shrimps, prawns, 
crawfish, lobsters.) 
Baits, (erabs, lobsters, shrimps, prawns, &e.) 
Manures, (horseshoe crabs.) 
Useful traits: Skeleton cleaners, (beach fleas, &c.) 
Injurious traits: 
Parasites on fishes and marine mammals, (barnacles, fish- 
lice, &c.) 
Destroy earthworks, dams, &c., (crawfish.) 
Destroy submerged timbers, (limnoria, &c.) 
Modes of protection against injurious species. E. 4. 
Methods of capture. D. 7, 31, 32. 
XU. WORMS. 
55. ANNELIDA. (Sipunculoids, leeches, earth-worms, serpule, sea- 
worms, &c.) 
Useful products: 
Food of aborigines, (earth-worms.) D. 1,2. 
Baits, (earth-worms, sea-worms.) D.5. 
Useful traits: 
Used in surgery, (leeches.) 
Used as barometers, (leeches.) 
Injurious traits: External parasites of animals, (leeches.) 
Methods of culture, (leeches.) E,. 11. 
56. SCOLECIDA. (Tape-worms and flukes, planarians, nemerieans, 
trichinz, thread-worms, rotifers, &c.) 
Injurious traits: lnternal parasites, (numerous species.) 
