Biology: Characteristics of Animals

Key Characteristics of Animals

• All animals are heterotrophs, obtaining energy and organic molecules by consuming other organisms.
• All animals are multicellular, with many having complex bodies.
• Animals lack rigid cell walls, and their cells are held together by extracellular frames of structural proteins like collagen.

Features of Animal Movement

• Animals move more rapidly and in more complex ways, related to the flexibility of their cells and the evolution of nerve and muscle tissues.
• Not all animals can move from place to place; some are sessile or sedentary.

Diversity of Form and Habitat

• Animals vary greatly in form, ranging in size from microscopic to enormous.
• Almost all animals lack a backbone and are therefore called invertebrates.
• Animals are grouped into 35-40 phyla, with most occurring in the sea, but many also in fresh water and on land.

Reproduction and Development

• Most animals reproduce sexually, with non-mobile eggs being much larger than small, usually flagellated sperm.
• Embryonic development begins with a series of mitotic divisions producing a ball of cells (the blastula).

Tissues and Body Structure

• Cells of most animals are organized into structural and functional units called tissues.
• Development from a zygote begins with a series of divisions, called cleavage, which produces a multicellular structure called a blastula.

Evolution of the Animal Body Plan

• Five key innovations in animal evolution: symmetry, tissues, body cavity, patterns of development, and segmentation.

Symmetry in Animals

• Virtually all animals have a symmetry defined along an imaginary axis drawn through the animal's body (exception: sponges).
• There are two main types of symmetry: radial and bilateral.

Radial Symmetry

• Exhibited by members of the phylum Cnidaria (jellyfish, sea anemones, and corals).
• Body parts are arranged around a central axis.
• Can be divided into two equal halves by any plane that passes through the center.

Bilateral Symmetry

• Body has right and left halves that are mirror images.
• Only the sagittal plane divides the animal into two equal halves (left and right).
• Has dorsal and ventral portions divided by the frontal plane, and anterior and posterior portions divided by the transverse plane.

Advantages of Bilateral Symmetry

• Advantages not specified in the text.