Here are some pieces of（1）i

formation

formation

from an encyclopaedia.
   Protozoa
   Protozoa are the smallest（2）a

nimals

nimals

.We can't see them，but they do a lot for us.Protozoa are useful for fish and other animals.Some protozoa are good for human beings by eating dangerous bacteria （細(xì)菌）.But some parasites make human beings ill.
   Fish
   There are over 20，000 different（3）k

inds

inds

of fish.Some of them are very small，but some are big.For example，the whale shark is over 60 feet long.Most fish breathe through gills （鰓）.Gills make them get（4）o

xygen

xygen

in water.Most fish use fins and tails for swimming.
   Amphibians
   Amphibians have eggs in water，and young amphibians are like small fish.The tadpole lives in water.It has a tail and gills.So it can breathe under water.When it becomes a frog，it doesn't have the gills or the tail，but then it has（5）l

egs

egs

for walking on land.Most amphibians can both walk on land and swim in water.