26.1 Introduction to the Animal Kingdom

Invertebrate

• Animal that does not have a backbone, or vertebral column.

vertebrate

• Animal that has a vertebral column, or backbone

Feedback inhibition

• Process in which the product or result stops or limits the process.

blastula

• Hollow ball of cells formed when a zygote undergoes a series of divisions.

protostome

• Animal whose mouth is formed from its blastopore.

deuterostome

• Animal whose anus is formed from the blastopore of a blastula.

anus

• Opening through which wastes leave the digestive tract.

endoderm

• Innermost germ layer of most animals; develops into the linings of the digestive tract and much of the respiratory system.

mesoderm

• Middle germ layer of most animals; gives rise to muscles and much of the circulatory, reproductive, and excretory systems.

ectoderm

• Outermost germ layer of most animals; gives rise to outer layer of the skin, sense organs, and nerves.

Radial symmetry

• Body plan in which body parts repeat around the center of the body; characteristic of sea anemones and sea stars.

Bilateral symmetry

• Body plan in which only a single, imaginary line can divide the body into two equal halves; characteristics of worms, arthropods, and chordates.

cephalization

• Concentration of sense organs and nerve cells at the front of an animal’s body.

Key Concept

• Animals, members of the kingdom Animalia, are multicellular, eukaryotic heterotrophs whose cells lack cell walls.

Key Concept

• Animals carry out the following essential functions: feeding, respiration, circulation, excretion, response, movement, and reproduction.

Key Concept

• Complex animals tend to have high levels of cell specialization and internal body organization, bilateral body symmetry, a front end or head with sense organs, and a body cavity.