Chapter 27: Introduction to Animals

 Heterotrophy  Animals are heterotrophs – that is, they can not make their own food.  Most animals move from place to place searching for food.  Once food is located, it is eaten and then digested in a cavity inside the animal’s body.

 Mobility  Animals are unique among living things in being able to perform rapid, complex movements.  Animals move by means of muscle cells, specialized cells that are able to contract with considerable force.

 Animals can swim, crawl, walk, run, and even fly. In fact, flight has evolved four times among animals, in insects, pterosaurs, birds and bats.

Multicellularity All animals are multicellular. In spite of differences in body size, there is little difference in the size of most cells that make up these animals

 The cells on the skin of your hand are roughly the same size as the cells in the heart of a whale or in the wing of a hummingbird.

 Diploidy  With few exceptions, animals are diploid, meaning adults have two copies of each chromosome, one inherited from their father and one from their mother.

 Only their gametes (egg and sperm) are haploid.  A great advantage of diploidy is that it permits an animal to exchange genes between the two copies of a set of chromosomes, creating new combinations of genes.

 Sexual Reproduction  Almost all animals reproduce sexually by producing gametes, as do many plants, fungi, and protists.  The females’ egg cells are much larger than the males’ sperm cells.  Unlike the egg cells, the sperm cells of animals have a flagella and are highly mobile.

 Absence of a Cell Wall  Among the cells of multicellular organisms, only animal cell lack rigid cell walls.

 The absence of a rigid cell wall has allowed animals mobility that other multicellular organisms do not have.  You may not realize this, but there are cells moving in your body at all time.  Cells called macrophages, for example, act as mobile garbage collectors, crawling over tissues and removing debris.

 Blastula Formation  In all animals except sponges, the zygote (fertilized egg cell) undergoes cell division that form a hollow ball of cells called a blastula.

 Cells within the blastula eventually develop into three distinct layers of cells – ectoderm, endoderm, and mesoderm.  These layers are called the primary tissue layers because they give rise to all of the tissues and organs of the adult body.

Primary TissueGives rise to: Ectoderm Outer layer of skin; nervous system; sense organs, such as eyes Endoderm Lining of the digestive tract; respiratory system; urinary bladder; digestive organs; liver; many glands Mesoderm Most of the skeleton; muscles; circulatory system; reproductive organs; excretory organs

 Tissues  The cells of all animals except sponges are organized into structural and functional units called tissues.  Tissues are group of cells with a common structure that works together to perform a specific function

 All animals have their own particular body plan, a term used to describe an animal’s shape, symmetry, and internal organization.  An animal’s body plan results from a pattern of development programmed into the animal’s genes by natural selection

 Sponges have the simplest body plan of all animals.  Sponges are asymmetrical – irregular in shape and sometimes their shape depends on where they are growing.

 Animals with radial symmetry have body parts arranged around a central axis – like the spokes on a wheel.  Most are aquatic organisms.

 The bodies of all other animals show bilateral symmetry – body design with distinct right and left halves that are mirror images.

 Most bilaterally symmetrical animals have evolved an anterior concentration of sensory structures and nerves, a process called cephalization.

 Bilaterally symmetrical animals have one of three basic kinds of internal body plans:  Coelomates: body plan that includes a body cavity – a fluid filled space found between the body wall and the digestive tract.

 Acoelomates – animals with no body cavity.

 Pseudocoelomates – Animals that have a body cavity located between the mesoderm and endoderm.

 Segmented animals are composed of a series of repeating, similar units called segments.  Segmentation underlies the organization of all advanced animals and is easy to observe in some animals, such as ants and earthworms.

 In vertebrates, segments are not visible externally, but there is evidence of segmentation in a vertebrate embryo.

 Kingdom Animalia contains about 35 major divisions called phyla depending on how certain organisms are classified.

 To visually represent the relationships among various groups of animals, scientists often use a type of branching diagram called a phylogenetic tree.  A phylogenetic tree shows how animals are related through evolution.

 The animal kingdom is divided into two groups of organisms: 1. Vertebrates – animals with backbones – humans, dogs, sharks 2. Invertebrates – animals without backbones – slugs, spiders, jellyfish