ARACHNIDS 
65 
maturity only after passing through many 
changes. 
51. Other Crustaceans. — Shrimps, 
lobsters, and crabs are crustaceans of 
much economic importance, because of 
their food value. The trade in these 
animals amounts to millions of dollars 
each year. In order that these important 
food animals may not become extermi¬ 
nated by careless and excessive fishing, 
the state and national governments have 
attempted to control the numbers taken 
and have also established hatcheries in 
which the eggs are hatched and the young 
protected during the earliest stages of 
their development. 
Crustaceans of less economic impor¬ 
tance are the barnacles which cling to 
rocks, wharves, and ships; the hermit 
crabs that live in the shells of mollusks (mol'liisks); and the 
smaller fresh-water crustaceans such as the Cyclops (sl'klops), 
Daphnia (daf'm-a), and 
Cypris (sl'pris), which are 
barely visible to the un¬ 
aided eye. 
52. Arachnids. — The 
spiders, scorpions (skor'pi- 
uns), ticks, and mites are 
arthropods that are 
grouped together under the 
name Arachnida (a-rak'- 
m-da: Greek, arachne, 
spider). The spiders and 
scorpions have eight walking appendages. The forward 
pincers of the scorpions are mouth-parts, and not walk- 
Figure 55. — Daddy-long-legs. 
(much enlarged). 
This is one of the 
commonest of the 
small freshwater ani¬ 
mals. The two large 
sacs on the abdomen 
are the egg-sacs. It 
is the chief food of 
Hydra and the White- 
fish. 
