General Biology/Classification of Living Things/Eukaryotes/Animals

Key Terms
synapomorphy

Introduction
What makes an animal an animal?

If animals are a monophyletic taxon, then animals should be able to be defined by synapomorphies, (shared, derived characteristics). Ideally, we would NOT define this or any taxon using symplesiomorphies (shared ancestral or primitive characteristics) or homoplastic characters (the independent evolution of similarity, or "convergent evolution"). See pages 654 - 656 and Fig. 32.6 in your text to review these concepts. As you consider the characteristics listed below, ask yourself whether or not each is a synapomorphy.

Characteristics of an Animal

 * There is no one universally accepted definition of an animal. The following treatment follows your text, beginning on page 876.
 * Animals:
 * Are multicellular, heterotrophic eukaryotes …
 * Lack the distinctive cell walls of plants & fungi
 * Share unique characteristics …
 * Share certain reproductive characteristics …
 * Other commonly used definitions …

Animals are multicellular heterotrophic eukaryotes
 * Unfortunately, none of these traits is exclusive to animals:
 * Plants, fungi, and some algae are multicellular.
 * Many bacteria, protists, and all fungi are heterotrophic.
 * Everything other than bacteria and archaea are eukaryotic.
 * Moreover, all three of these characteristics also apply to fungi.
 * However, there is a difference here between animals and fungi. Animals generally take in their food through ingestion, or eating and swallowing something. Fungi are absorptive heterotrophs; they secrete their digestive enzymes onto their food, and then absorb the resulting nutrients.

Animals share unique characteristics
 * Only animals have muscle tissue and nervous tissue.
 * Only animals have collagen, a structural protein
 * Only animals have the following types of intercellular junctions: (See pages 135 - 139, Figure 7.15 in your text for more information on these junctions.)
 * Tight junctions (sealing function)
 * Desmosomes (anchoring function)
 * Gap junctions (communication function)

Animals share certain reproductive characteristics
 * Most animals reproduce sexually, with the diploid stage dominating.
 * In most animals, a small, haploid, flagellated, motile sperm fertilizes a larger, haploid, nonmotile egg to form a diploid zygote.
 * Mitotic division of the zygote yields a blastula stage, followed by a gastrula stage. A synapomorphy? This feature could be another "unique characteristic" shared by animals.
 * Development may be direct to adult form, or there may be a sexually immature stage (or stages) that are morphologically & ecologically distinct from the adult called a larva (plural: larvae).

Other commonly used definitions or characterizations
 * It is surprisingly difficult to find two texts that agree on a precise definition of an animal. Here are a few perspectives from some other texts.
 * Animals are multicellular eukaryotes; they are chemosynthetic heterotrophs that ingest their food.
 * Animals are motile, though many are secondarily sessile. Gametes usually are produced in multicellular sex organs, and the zygote passes through embryonic stages that include a blastula.
 * Animals are organisms that are multicellular, with more than one type of cell. They are heterotrophic. They reproduce sexually (at least sometimes), with a zygote formed from two different haploid gametes. They go through a developmental stage called a blastula.
 * Animals are not photosynthetic, have no cell wall, and no hyphae or mycelia. (What would a cladist think of this definition of the taxon Animalia?)

What kinds of animals are there?
 * Kingdom Animalia generally is recognized to have approximately 30 phyla ...
 * There is relatively little dispute over the number of phyla recognized; however, the phylogenetic relationships among the phyla are hotly debated.
 * Molecular techniques for assess similarity based on nucleotide sequences in nucleic acids are providing valuable new perspectives on this question.
 * Remember that two animals in different phyla generally are considered to be more different from each other than are animals within one phylum (e.g., nematodes are more different from annelids than humans are from sharks).

This text is based on notes very generously donated by Ralph Gibson, Ph.D. of the Cleveland State University.