WEB TUTORIAL 22.1
The Architecture of Animals
Text Sections
Section 22.3 Lessons from the Animal Family Tree, p. 372
Introduction
Biologists classify animals according to various traits of body architecture—their
morphology. The major animal groups can be arranged on a phylogenetic tree to
illustrate what biologists believe is each group's evolutionary sequence. This tutorial reviews one such tree and shows the changes that are thought to mark the divisions of the major groups of animals.
Learning Objectives
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Understand the purpose of a phylogenetic tree.
Know the criteria that scientists use to classify animals into specific phyla.
Know the morphological characteristics that developed during the evolution of animals.
Understand how the coelom evolved in the animal phyla.
Narration
The Animal Tree of Life
The animal kingdom's broadest categories are known as phyla, with each phylum
informally defined as a group of organisms that shares a basic body structure.
Phyla can be arranged on a phylogenetic tree to illustrate what biologists believe is
the sequence of events that took place during each group's evolution.
There are between 36-41 animal phyla (only 9 are shown here). In this tutorial,
we'll look at some common features of animals and note the degree to which these
features are shared across the phyla.
One way to conceptualize this tree is that, over time, animals became more complex through a series of additions to the characteristics found in more primitive animals. The twist is that only some varieties of animals evolved to get these additions, while others retained the primitive, “ancestral” condition.
Tissue
A tissue is a group of cells that performs a common function. Looking down at the
trunk of the tree, you can see that all animals have a common ancestor—probably
a primitive protist. Now notice that the tree splits above this common ancestor and
yields, over to the left, the phylum Porifera, which is made up of the sponges.
Porifera are truly the outliers of the animal world in that they are the only phylum
that lacks tissues.
Sponges are more like a collection of cells that has come together to form an organism. In a given sponge, each cell acquires its own oxygen and eliminates its own
wastes, and there are no collections of cells that perform common functions.
Tissues are a marker of organization, and in the animal kingdom, only sponges lack
this organization.
If you look again at the split above the common ancestor, this time going to the
right, you can see that all the animals on this branch did develop tissues. Note also
that this branch leads to all the other animal phyla. Thus, all the animals that stem
from this branch will have tissues. For example, members of the Cnidarian phylum
all have nervous tissue, as well as tissue that functions like our own muscle tissue.
Tissues are a marker of organization, and in the animal kingdom, only sponges lack
this organization.
Body Symmetry
Biologists also use body symmetry as a trait to distinguish among the animals. An
animal is symmetrical if its body can be sliced into at least two matching components. Sponges have an asymmetrical body plan because they lack this distinction.
Animals with symmetry are further divided into two subgroups. Those that can be
sliced several different ways, all yielding matching sections, are said to have radial symmetry. Jellyfish are an example of an animal with radial symmetry.
Animals that can be divided only into a medial slice, resulting in matching right
and left parts, are said to have bilateral symmetry.
Body Cavity
The animal family tree has another lesson for us regarding unity and diversity
among animals. Your stomach expands when you've just had a big meal, but then
contracts when you're busy for a few hours. What allows you to do this? The
answer is an internal space you have; a centrally placed, fluid-filled body cavity
called a coelom. There are only three animal phyla in this phylogenetic tree that
don't have one: the sponges, the cnidarians, and the members of phylum
Platyhelminthes (the flatworms).
Phylum Platyhelminthes have structures that evolved beyond the tissues of the
cnidarians—they have organs, meaning highly organized structures formed of several kinds of tissues. From its gut to its exterior, however, the flatworm is composed of uninterrupted tissue—there is no space, or coelom.
In most instances the coelom surrounds another physical structure, the digestive
tract—meaning the tube, functioning in digestion, that runs from the mouth to the
anus. We can therefore think of the coelom as one tube that encircles another; the
coelom is generally tube-shaped, and it surrounds the tube that is the digestive
tract. The image shown here is that of an earthworm, which has a true coelom. You
may wonder why there are no red lines the phylogenetic tree showing where the
coelom evolved. So valuable is this internal space that it seems to have evolved
independently several times among animals.
You should now be able to…
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List two characteristics that make the phylum Porifera different from all
other animal phyla.
Describe three morphological traits that scientists use to understand the
evolution of the animal phyla and discuss when they evolved on the phylogenetic tree of animals.
Explain the function of a coelom.
Give an example of an animal with radial symmetry.