IIo ANIMAL ACTIVITIES. 
Among the lower crustacea there are many which 
show greater variations from the typical form. The 
cyclops is a small lobster-like crustacean often found 
in drinking-water. Specimens can usually be obtained 
by tying a piece of muslin over the end of a faucet and 
allowing the water to run for a little while and then 
rinsing the muslin in a glass of water. The horseshoe 
crab is an ancient form sometimes classified with the 
spiders because it seems to have more homologies with 
them than with the common forms of crustacea. 
Degeneration. The lower crustacea are called Exto- 
mostraca. Among these are found many forms which 
would not be recog- 
nized as allies of the 
crayfish and crab, but 
for the study of their 
embryology. The 
common barnacle 
(Balanus) found on 
all salt-water shores 
between high and 
low tide bears no re- 
semblance to a cray- 
~ fish, yet soon after 
Fic. 98.—A Barnacle. hatching from the 
egg it is a_ free- 
swimming active crustacean, provided with organs of 
sense and looking much like a young shrimp. After 
a short time it seems to tire of its active life, and, 
looking about for a place of rest, it glues its head to a 
rock and lies feet uppermost kicking food into its mouth 
from the surrounding water. It builds around itself a 
conical shell which it opens and closes at pleasure. 
Thus sitting at ease and catching food as it comes, it 
has no use for organs of sense or locomotion and so 
loses these marks of higher animal life. It finally 
becomes a blind and stupid mass, capable of little 
else than the digestion of food brought to it by the 
