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Internal Organs
Crayfish, snails, and humans all have a heart to pump
blood. They all have a stomach to digest food and organ
systems to excrete wastes.
Humans and land snails have lungs to breathe air. Crayfish
usually live in the water. Instead of using lungs to breathe,
they take their oxygen out of the water through gills. The
gills are tucked up under the carapace where the legs attach
to the body.
Limbs
Crayfish have five pairs of legs. They can walk quickly in any direction
on four of these pairs. This helps them move along the bottoms of ponds
or streams as they look for food or avoid predators. The fifth pair of legs,
located near the head, has large pincers. These pincers are more like arms
than legs. The crayfish uses them to pick up food and defend itself.
Snails have no arms or legs. A snail moves with a muscular foot on
the bottom of its body. This foot allows the snail to glide over almost
any surface.
Humans have two legs to
walk, run, and climb. They
have two arms to pick up and
carry things.
A model of a
human heart
How do snails and humans move?
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