Rubrics for Lesson 9
Rubrics for Lesson 9
Reproduction on dry land presented problems for animals who
were used to laying their eggs in the sea water. Over millions
of years the animals developed a number of different reproductive
Animals that live in water release their
eggs into the water, where the eggs are fertilized. The embryos
develop in the eggs. Often the parents swim away, and the multitude
of small creatures in the water provide food for the hatchlings.
However, just as the plants had to find new ways to reproduce
on land, animals, too, had to invent new structures and strategies.
Exothermic (cold-blooded) insects
and spiders lay many small eggs. Some of these eggs hatch in
a few days, some can lie dormant over the winter, allowing new
insects to hatch in the spring when there is plenty of food.
The organisms that hatch from the eggs generally do not look
like their parents: the insect life cycle has four phases, egg,
larva (caterpillar or grub), pupa, and adult.
| Cold-blooded amphibians,
such as frogs, toads, and salamanders, live on land and breathe
air but return to the water to lay their eggs. These eggs are
enclosed in a membrane and contain stored food (the yolk) to
fuel the embryo's development. These eggs start to develop right
away. These animals begin life in the water as tadpoles (or newts
if they will be salamanders) and metamorphosize into air-breathing
|| Cold-blooded reptiles,
such as lizards, crocodiles, snakes, and turtles, developed eggs
that could survive out of water. These eggs had a leathery outer
covering that protected the embryo and the yolk sac from dehydration.
Most of these animals lay many eggs, bury the eggs in warm sand,
and walk away. The embryos develop, hatch, dig their way out
of the sand, and start looking for food. An interesting fact
about reptile eggs is that the gender of the hatchlings is determined
by the temperature in which the embryos develop, not by X and
Warm-blooded birds developed
the hard-shelled egg. This provided better protection from would-be
egg-eaters and allowed the developing embryo to get oxygen from
the air through pores in the shell. However, the embryo in this
egg required higher temperatures in which to develop.
Birds became attentive parents, first
keeping the eggs warm and then feeding the hatchlings. With parental
care, demands on the parents increase and the chances of the
young surviving also increase. Unlike reptiles and amphibians,
birds lay only a few eggs, but they are more successful in raising
their young to maturity than reptiles or amphibians.
Because birds fly, they cannot afford
to carry much extra weight. Having the young birds in a nest
allowed the adults to fly efficiently even as the young birds
attained their mature size and weight in the nest.
mammals and monotremes developed a different strategy.
They shelter the developing embryo inside their own bodies and
then nourish the baby with milk. Monotremes, such as kangaroos
and koala bears, allow a fertilized egg to begin development
inside their bodies. The baby is not attached to the mother,
so it cannot grow very big. It soon emerges, but in a very undeveloped
| The baby clings to the mother's
fur and climbs up into her pouch, where it attaches itself to
a milk-giving nipple. It completes its development in the pouch,
warmed and fed by the mother's body. When it is fully developed
it begins to come out of the pouch and start to explore the world.
mammals have wombs. A womb is a special structure in the
mother's body. Fertilized eggs can implant themselves in the
womb and receive nutrients, oxygen, and antibodies from the mother's
blood-stream through a special structure called a placenta. The
placenta also allows the mother's blood to carry the embryo's
waste products away.
| Being in a womb allows the
young organism to develop in a safe environment over a pre-determined
period of time. It can be born when food is plentiful.
Once born, the youngster is nourished with milk. This leads to
bonding between mother and child, and makes it possible for the
baby to learn from its mother.
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