The Wonderful World of Animalia
What is an 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 more primitive animals, this process is external to a parent
organism. In more derived species, this process may take place within an
egg, or within the body of the female parent.
Animalia: An Overview
The simplest way to consider the vast diversity of Animalia is with an
image:
Note the emergence of each new character, and how it separates the
monophyletic taxa.
Note also that the tree above may not agree with the most recent phylogeny believed to be correct for Animalia.
As we begin our Tiptoe through the Taxa, keep in mind the following
characteristics as they change across phylogenetic groups. As animal taxa diverge and become more derived, organs and organ systems are added.
- tissues (simple and complex)
- body symmetry (radial, bilateral or none)
- internal body cavity (pseudocoelom, coelom or none)
- embryonic development and morphogenesis
- organs and organ systems:
- integumentary system - protection against mechanical injury,
infection, desiccation
- digestive system - food processing
- mouth
- pharynx (sometimes with a grinding organ, such as a gizzard or mastax)
- esophagus
- stomach/crop
- intestine
- anus
- nervous system - rapid coordination of body activities; response to
environmental stimuli
- muscular system - movement
- reproductive system - the obvious
- excretory system - removal of nitrogenous waste from body fluid
- skeletal system - structural support; (in more derived taxa, muscle attachment)
- circulatory system - internal distribution of materials
- respiratory system - gas exchange (O2 in, CO2
our)
- immune and lymphatic systems - body defense against pathogens and
cancer
- endocrine - slower coordination of body
activities, response to environmental stimuli
(There they are, nested in the clade neighboring the one that houses Fungi.)
Porifera ("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)
- The sponges are most likely polyphyletic, having arisen several times from related ancestral lineages.
Eumetazoa - All Other Animal Taxa (Cnidaria, Ctenophora, Placozoa, Bilateria)
Endoderm, ectoderm and mesoderm are the embryonic germ (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)
(Be sure to link to the pictures at the web site above. Fun!)
- In vertebrates, there are two basic types of connective tissue:
- loose (connective tissue that forms the matrix of organs and soft
tissues)
- 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 may be characterized by the nature of the internal body cavity (located
between the ectoderm and endoderm. From primitive to derived:
- acoelomate (no internal body cavity)
- pseudocoelomate (internal body cavity lined with mesoderm
only on parietal surface, and is known as a pseudocoelom)
- coelomate (internal body cavity lined with mesoderm on both visceral and
parietal surfaces)
Let's look...
If a true coelom is present, the animal can be characterized by the course of its embryonic
development:
- 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
And another 1000 words...
Metamerism
More derived animal lineages exhibit an important anatomical innovation, segmentation
(also known as metamerism, with each segment called a
metamere or somite). Muscles, organs and other
anatomical structures are duplicated in each segment, with segments arranged in serial fashion.
Tagmatization is the developmental fusion of groups of body segments (metameres) into functionally distinct body regions, or tagmata (singular = tagmatum). The classic example is the division of the arthropod body into the head, thorax and abdomen, each of which is developed via the fusion of embryonic metameres.
As evolution proceeded, some animals that had segmented ancestors
secondarily lost their segmentation. Can you see any part of your body
that's a remnant of your segmented ancestral heritage? (Hint: You're more likely to see this at the Wellness Center.)
Cephalization
Bilaterally symmetrical animals have an advantageous anatomical
feature: cephalization. This is the presence of a cephalon (Greek for "head" at the
front of the body, where the sense organs are concentrated.
- The head enters the environment first, and is able to sense
environmental cues and react to them.
- The mouth is located 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 choanocytes (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.
A colonial choanoflagellate might well have developed into a "protoanimal"
that resembled a gastrula:
...but which one might call a gastrea to distinguish it from an embryo,
since this early animal was a "finished product," not a developing one.
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
shell-less molluscs are also present
other related groups, such as "worms" (a non-phylogenetic, catch-all
term for a tubular animal) also present
Almost all the major animal Body Plans show up in the fossil record
by the Cambrian, about 5454 - 525 mya, a result of the so-called "Cambrian
explosion" of animal diversity.
There are between 35-50 animal phyla (depending on which systematist you
talk to), and we'll be seeing representatives of only a handful.
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, it is more than
that:
"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
And for the next few lectures, we will immerse ourselves in animal
Bauplan.
