27.2 Features Used to Classify Animals

Animal Body Plans and Development

Animal body plans describe the structural "blueprint" of an organism: its symmetry, internal cavities, and how its parts are organized. These features are some of the most important tools biologists use to classify animals into major groups. Development, the process by which a fertilized egg becomes a complex organism, reveals even more about how animals are related. The way embryos form layers, cavities, and openings tells us a surprising amount about evolutionary history.

Body Plans and Symmetry Types

Symmetry describes how body parts are arranged around a central point or axis. It's one of the first features you'll notice when comparing animal groups.

• Asymmetry means there's no plane of symmetry at all. Sponges are the classic example: their bodies have irregular shapes with no predictable pattern.
• Radial symmetry means the body can be divided into equal halves along multiple planes, all passing through a central axis. Think of a jellyfish or sea anemone: you can slice it several ways and still get roughly mirror-image halves. Radially symmetric animals are often sessile (attached in place) or slow-moving.
• Bilateral symmetry means only one plane divides the body into matching left and right halves. This is the body plan of insects, fish, and mammals. Bilateral symmetry is associated with cephalization, the concentration of sensory organs and nerve tissue at the anterior (front) end, which is a major advantage for directional movement.

Body cavities are fluid-filled spaces between the digestive tract and the outer body wall. The type of body cavity an animal has (or doesn't have) is a key classification feature.

• Acoelomates lack a body cavity entirely. The space between the gut and body wall is packed with tissue. Flatworms are a good example.
• Pseudocoelomates have a body cavity, but it's only partially lined by mesoderm (the middle tissue layer). Roundworms fall into this category.
• Coelomates have a true coelom, a body cavity completely lined by mesoderm. This lining, called the peritoneum, allows organs to move and grow more independently. Annelids (earthworms) and all vertebrates are coelomates.

Segmentation is the division of the body into repeating units.

• Metameric segmentation produces a series of similar segments along the body. Earthworms show this clearly: each segment contains its own set of muscles, nerves, and blood vessels.
• Tagmatization takes segmentation a step further by fusing groups of segments into specialized regions called tagmata. In insects, for example, segments are grouped into the head, thorax, and abdomen, each with distinct functions.

Germ Layers in Animal Development

During early embryonic development, cells organize into germ layers, the primary tissue layers from which all organs and structures will form. Most animals have three germ layers (making them triploblastic), while simpler animals like cnidarians have only two (diploblastic).

• Ectoderm (outer layer) gives rise to the epidermis (skin), the nervous system, and sensory organs like the eyes.
• Mesoderm (middle layer) produces muscles, the skeleton, the circulatory system, and most internal organs such as the heart, bones, and kidneys.
• Endoderm (inner layer) forms the lining of the digestive tract and associated organs like the liver, pancreas, and lungs.

These layers form during gastrulation, a process that reorganizes the hollow ball of cells (the blastula) into a layered embryo (the gastrula). Gastrulation can happen in different ways depending on the organism:

1. Invagination: The wall of the blastula folds inward, creating a pocket that becomes the primitive gut (archenteron). This is common in sea urchins and amphibians.
2. Ingression: Individual cells migrate inward from the surface into the interior cavity (blastocoel). This occurs in birds and mammals.

The germ layers are significant because they establish the body's main axes (dorsal-ventral, anterior-posterior) and determine cell fate. Once a cell is part of a specific germ layer, its developmental path is largely set.

Protostomes vs. Deuterostomes

This is one of the most fundamental divisions in animal classification. Protostomes and deuterostomes are the two major lineages of bilaterally symmetric animals, and they differ in several key aspects of embryonic development.

A few of these deserve extra explanation:

• Cleavage refers to the early cell divisions of the embryo. In determinate cleavage (protostomes), each cell's fate is fixed early on; if you remove a cell, the embryo can't compensate. In indeterminate cleavage (deuterostomes), early cells are flexible, and each one retains the potential to develop into a complete organism. This is why identical twins are possible in humans (deuterostomes).
• The blastopore is the first opening that forms during gastrulation. The word "protostome" literally means "first mouth" because the blastopore becomes the mouth. "Deuterostome" means "second mouth" because the mouth forms as a separate, secondary opening, and the blastopore becomes the anus instead.
• Coelom formation differs too. In protostomes, the coelom forms by schizocoely: the mesoderm splits apart to create the cavity. In deuterostomes, it forms by enterocoely: pouches bud off from the wall of the archenteron (primitive gut) and expand to create the coelom.

Classification and Evolutionary Relationships

Biologists use several approaches to classify animals into groups that reflect their evolutionary history.

• Morphology, the study of form and structure, provides the most visible classification criteria. Features like symmetry, body cavities, and segmentation are all morphological traits.
• Homology refers to structural similarities between organisms that exist because they share a common ancestor. For example, the forelimbs of humans, bats, and whales look different but share the same underlying bone arrangement, pointing to a shared evolutionary origin. Homologous structures are distinct from analogous structures, which look similar but evolved independently (like bird wings and insect wings).
• Phylogeny is the study of evolutionary relationships among groups of organisms, often represented as branching tree diagrams called phylogenetic trees or cladograms.
• Taxonomy is the science of naming and classifying organisms into hierarchical groups (domain, kingdom, phylum, class, order, family, genus, species) based on shared characteristics.

Together, these tools allow biologists to organize the enormous diversity of animal life into a framework that reflects both structural similarities and evolutionary descent.