An Introduction to Animal Diversity
A. What Is an Animal?
1. Structure, nutrition, and life history define animals.
• However, five criteria, taken together, comprise a reasonable definition.
1. Animals are multicellular, ingestive heterotrophs.
° Animals take in preformed organic molecules through ingestion, eating other organisms or
organic material that is decomposing.
2. Animal cells lack cell walls that provide structural support for plants and fungi.
° The multicellular bodies of animals are held together by extracellular structural proteins,
° Animals have other unique types of intercellular junctions, including tight junctions,
desmosomes, and gap junctions, which hold tissues together.
3. Animals have two unique types of cells: nerve cells for impulse conduction and muscle cells
4. Most animals reproduce sexually, with the diploid stage usually dominating the life cycle.
In most species, a small flagellated sperm fertilizes a larger, nonmotile egg.
° The zygote undergoes cleavage, a succession of mitotic cell divisions, leading to the
formation of a multicellular, hollow ball of cells called the blastula(囊胚期).
° During gastrulation(原腸胚形成), part of the embryo folds inward, forming layers of
embryonic tissues that will develop into adult body parts.
The resulting development stage is called a gastrula(原腸胚).
° A larva(幼蟲) is a sexually immature stage that is morphologically distinct from the adult,
usually eats different foods, and may live in a different habitat from the adult.
° Animal larvae eventually undergo metamorphosis(變態), transforming the animal into an
5. Animals share a unique homeobox-containing family of genes known as Hox genes.
° All eukaryotes have genes that regulate the expression of other genes.
Many of these regulatory genes contain common modules of DNA sequences called
All animals share the unique family of Hox genes, suggesting that this gene family arose