The Wonderful World of Kingdom Animalia
Recall the characteristics that make an animal and animal...
- multicellular, with true tissues
- ingestive heterotroph
- energy storage: long term as fat; short term as glycogen ("animal
starch")
- unlike plants and fungi, cells lack walls external to plasma membrane.
- unique to animals: The Nervous System
- unique to animals: The Muscular System
- primitively, all animals reproduce sexually, though a few are secondarily
parthenogenic.
- animals have a characteristic embryonic development sequence
- the zygote undergoes a series of cell divisions known as CLEAVAGES
- Multiple cleavages result in a (primitively) hollow ball of cells
known as a BLASTULA. (In more derived animals, the blastula may not be
hollow or ball-shaped)
- The blastula undergoes a process known as invagination
to produce a GASTRULA
- The gastrula undergoes MORPHOGENESIS (literally "origin
of form") to become a LARVA.
- The larva undergoes a process known as METAMORPHOSIS
to become an adult.
- In its most primitive condition, this process is external to a parent
organism. In more derived condition, this process may take place within an
egg, or within the body of the female parent.
Kingdom Animalia: An Overview
Let's look at our old pal, the (incomplete) phylogenetic tree of Animalia:
Note the emergence of each new character, and how it separates the
monophyletic taxa.
Subkingdom Parazoa: The Sponges
These are the simplest animals, consisting only of specialized cells
exhibiting a division of labor, but no true tissues. Most have no defined
planes of symmetry (i.e., the "body" is asymmetrical)
Subkingdom Eumetazoa - All Other Animal Phyla
In the Eumetazoans...
Endoderm, ectoderm and mesoderm are the EMBRYONIC tissue layers.
These develop into the complex tissues of the adult animal, which may
include:
- epithelial tissue - a sheet of cells that covers an internal or
external surface
examples: simple (all metazoans) or stratified (mostly vertebrates)
epithelium of the skin (epidermis)
- connective tissue - diverse tissues that serve various binding
and structural functions. Composed of:
- cells (relatively few)
- connective fibers (large amounts)
- matrix (fluid or gel in which the above are embedded)
In vertebrates, there are two basic types of connective tissue:
- loose (connective tissue that forms the matrix of organs and soft
tissues) (blood is considered a connective tissue!)
- dense (connective tissue such as tendons, ligaments, bone,
cartilage)
- muscle tissue
- skeletal striated (under voluntary control)
- cardiac striated (under mostly involuntary control)
- smooth (under involuntary control)
- nervous tissue
- neurons (cells that conduct electrical/nervous impulses)
- glial cells (insulation and support of neurons)
animals can be classified by the anatomy of their internal body
cavity (located between the ectoderm and endoderm). From primitive to
derived, the three possible animal body plans are:
- acoelomate (no internal body cavity)
- pseudocoelomate (internal body cavity lined with mesoderm
only on visceral surface)
- coelomate (internal body cavity lined with mesoderm on both visceral and
parietal surfaces)
Let's look...
If a true coelom is present, the animal is further classified (by
the characteristics of its
embryonic development) into one of TWO MAJOR LINEAGES:
- Protostomes
- blastopore becomes the mouth
- second opening becomes the anus
- coelom formed via schizocoely
- spiral, determinate cleavage at 4 --> 8 cell division
- circulatory system primitively dorsal
- nervous system primitively ventral
- Phyla Mollusca, Annelida, Arthropoda and others
- Deuterostomes
- blastopore becomes the anus
- second opening becomes the mouth
- coelom formed via enterocoely
- radial, indeterminate cleavage at 4 --> 8 cell division
- circulatory system primitively ventral
- nervous system primitively dorsal
- Phyla Echinodermata, Hemichordata, Chordata and others
In something said to be worth 1000 words...
Metamerism
Some animal phyla exhibit an important anatomical innovation:
SEGMENTATION (also known as "metamerism", with each segment called a
"metamere" or "somite"). In each segment, muscles, organs and other
anatomical structures are repeated in a serial fashion.
As evolution proceeded, some animals that had segmented ancestors
secondarilyi lost their segmentation. Can you see any part of your body
that's a remnant of your segmented ancestral heritage?
Cephalization
Bilaterally symmetrical animals have an advantageous anatomical
feature: CEPHALIZATION. This is the presence of a "head end" at the
front of the body, where the sense organs are concentrated.
- Head end enters the environment first, and is able to sense
environmental cues and react to them.
- Mouth is almost always on the head, which makes food gathering
much more efficient.
- Polarization along an anteroposterior (i.e., head to tail) axis is
shown as a gradient of various activities along the length of the
body. (e.g., sensing at the head end; reproduction closer to the tail
end, etc.)
Where did Animals come from?
Animals are believed to share a most recent common ancestor with the
primitive protists known as CHOANOFLAGELLATES.
Sponges actually have "collar cells" that are extremely similar to these
choanoflagellates, and since they are the first type of cell to develop in
sponges, there is strong evidence that other types of animal cells may be
been derived from choanoflagellate cells.
The earliest animal fossils appear during the late PreCambrian - early
Cambrian (about 545
mya), and a rapid diversification lasting about 40 million years followed.
Most of these early fossils are Cnidarians (The phylum containing
jellyfish, sea anemones and corals)
shell-less molluscs also present
other related groups, such as "worms" (which is sort of a catch-all
term)
Almost all the major animal Body Plans show up in the fossil record
by the Cambrian, about 545-525 mya, a result of the so-called "Cambrian
explosion" of animal diversity.
There are 32 animal phyla (sometimes more, depending on which systematist you
talk to...), and we'll be visiting representatives of only a handful of
these phyla.
Animals within each phylum exhibit a series of distinctive characters that
set them apart from other taxa, and the overall body form seen in a given
taxon is sometimes called the BAUPLAN of that group.
The literal translation of the German bauplan is "a structural plan
or design." But when the word is applied to animal groups...
"An animal's Bauplan is, in part, it's "body plan"--but it is more
than that. The concept of a Bauplan really captures in a single word
the essence of both structural range and architectural limits, as well
as the functional aspects of a design. If an organism is to "work,"
all of its body components must be both structurally and functionally
compatible. The entire organism encompasses a definable Bauplan, and
the specific organ systems themselves also encompass describable
Bauplane; in both cases the structural and functional components
of the
particular plan establish its capabilites and limits. Thus
Bauplane
determine the major constraints that operate at both the organismic
and the organ system levels."
-- From Invertebrates by Brusca and Brusca
