26.1 Amniotes

KEY CONCEPT Reptiles, birds, and mammals are amniotes.

26.1 Amniotes

Amniote embryos develop in a fluid-filled sac.

• The amniotic sac contains everything an embryo needs to grow.– some develop inside mother’s body– some develop inside a tough, semipermeable shell

26.1 Amniotes

• The amniotic egg allowed vertebrates to reproduce on land.

Embryo

AllantoisHolds waste materialsas the embryo grows

Yolk sac Contains the nutrient supply for the growingembryo

AmnionProtects andsurrounds theembryo

Chorion Allows gas exchangewith outside environment

26.1 Amniotes

Anatomy and circulation differ among amniotes.

• Other amniotes, including dinosaurs, evolved a more upright stance.

• The first animals walked in a sprawl.

26.1 Amniotes

• All amniotes have two circuits of blood vessels.

– pulmonary circuit moves blood from the heart to the lungs

– systemic circuit moves blood from the heart to the rest of the body

26.1 Amniotes

• Amniotes have a three- or four-chambered heart.– reptiles hearts have three chambers

THREE-CHAMBERED HEART

– birds and mammals hearts have four chambers

FOUR-CHAMBERED HEART

26.1 Amniotes

Amniotes can be ectothermic or endothermic.

• Amniotes manage body heat in different ways.– Ectotherms have body temperatures determined by

the surrounding environment.– Endotherms use metabolic heat to keep tissues

warm.• Endotherms can live in a wider range of climates than

ectotherms.