Chapter 27

Introduction to Animals

Section 1: Characteristics of Animals

Section 2: Animal Body Systems

Section 1

Characteristics of Animals

Objectives:•Identify the features that animals have in common.•Distinguish radial symmetry from bilateral symmetry.•Summarize the importance of a body cavity.•Identify how scientists determine evolutionary relationships among animals.

Section 1

Characteristics of Animals

General Features of Animals•Heterotrophy Animals are heterotrophs—that is, they cannot make their own food.•Mobility Animals are unique in being able to perform rapid, complex movements.•Multicellularity All animals are multicellular.•Diploidy With few exceptions, animals are diploid.•Sexual Reproduction Almost all animals reproduce sexually by producing gametes.

Section 1

Characteristics of Animals

General Features of Animals continued•Absence of a Cell Wall Among the cells of multicellular organisms, only animal cells lack rigid cell walls.•Blastula Formation In all animals except sponges, the zygote undergoes cell divisions that form a hollow ball of cells called a blastula.•Tissues The cells of all animals except sponges are organized into structural and functional units called tissues.

Section 1

Characteristics of Animals

Body Symmetry•Radial Symmetry Animals with radial symmetry have body parts arranged around a central axis.•Bilateral Symmetry Animals with bilateral symmetry have a distinct right and left half, and most display cephalization.

Section 1

Characteristics of Animals

Internal Body Cavity•Types of Body Cavities Animals have one of three basic body plans: acoelomate, psuedocoelomate, and coelomate.

Section 1

Characteristics of Animals

Body Segmentation•Segmentation Segmentation in body structure underlies the organization of all advanced animals.

Section 1

Characteristics of Animals

Kinds of Animals•Classification There are about 35 animal phyla, which contain an extraordinary range of body forms and body systems. Scientists classify animals using several different types of data, which include comparing anatomy and physiology, patterns of development, and DNA. The animal kingdom is divided into two groups: vertebrates and invertebrates.

Section 2

Animal Body Systems

Objectives:•Summarize the functions of the digestive, respiratory, circulatory, nervous, skeletal, and excretory systems.•Compare a gastrovascular cavity with a one-way digestive system.•Differentiate open from closed circulatory systems.•Distinguish asexual from sexual reproduction.

Section 2

Animal Body SystemsTissues and Organs•Digestion Simple animals have a gastrovascular cavity with only one opening, while more-complex animals have a one-way gut.•Respiration Simple animals exchange gases directly through their skin. More complex aquatic animals use gills, while terrestrial animals use a variety of respiratory organs, such as lungs.•Circulation In an open circulatory system, circulatory fluid leaves the vessels and enters the body cavity. In a closed circulatory system, blood remains in the vessels.

Section 2

Animal Body SystemsTissues and Organs continued•Conduction of Nerve Impulses While simple animals have little coordination among their nerve cells, complex animals have nerve cords and a brain with associated sensory structures.•Support While simple animals have a hydrostatic skeleton, complex animals have either an external skeleton, called an exoskeleton, or internal skeleton, called an endoskeleton. •Excretion For most animals, eliminating wastes is linked to maintaining the correct water balance in their body.

Section 2

Animal Body Systems

Reproductive Strategies•Asexual Reproduction Asexual reproductive methods include fragmentation, splitting in two, and parthenogenesis.•Sexual Reproduction In sexual reproduction, male and female gametes combine to form a new individual.