skeletal system - structural support; (in more derived taxa, muscle attachment)
circulatory system - internal distribution of materials
respiratory system - gas exchange (O2 in, CO2
immune and lymphatic systems - body defense against pathogens and
endocrine - slower coordination of body
activities, response to environmental stimuli
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.
The mechanism of transition from unicellular to multicellular organisms is unknown, but several gene families have been implicated in the origin of multicellularity, which may have evolved several times, independently.
Among the most important is the cadherin gene family. Cadherins ("calcium-dependent adhesion" proteins; they require calcium (Ca2+) to function) are trans-membrane proteins involved in both cell adhesion and intercellular signaling.
Extant colonial choanoflagellates have cadherin genes, and their ancestors may have developed into a protoanimal resembling a gastrula:
...but which one might call a gastrea to distinguish it from an embryo,
since this early animal was a mature, reproductive organism.
A gastrea-like creature was most likely the ancestor of all Eumetazoans.
It shared these characters with modern animals:
simple gut (mouth, but no anus)
diploblasty (i.e., ectoderm and endoderm)
One lineage of gastreas may have evolved into flattened, wormlike creatures only slightly more complex than a gastrea.
These may have looked something like members of the extant taxon "Acoela":
These extant marine flatworms are among simplest of all eumetazoans, and are considered basal to them.
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
"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
particular plan establish its capabilites and limits. Thus
determine the major constraints that operate at both the organismic
and the organ system levels."
-- From Invertebrates by Brusca and Brusca
As we tour the Metazoans, note and anticipate the increasing in complexity of animal body plans.
The name is from the Greek por - "small hole" and fer - "to bear".
Porifera is a form taxon, a grouping based on superficial similarity. The evolutionary relationships among the sponges are still being determined. They share a common cellular division of labor that may have evolved several times.
General Anatomy of a Sponge.
There are no true tissues, hence no true plane of symmetry, even in forms that superficially appear radial or bilateral.
Sponges are composed of four types of cooperative cells
choanocytes - "collar cells" - set up the water current via flagellum and engulf food particles
pinacocytes - form the surface covering (not a true "skin" or epithelium)
porocytes - barrel-shaped cells form the incurrent pores
archaeocytes/amoebocytes - roving scavenger cells that participate in digestion and feeding other cells
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 think of any part of your body
that's a remnant of your segmented ancestral heritage?
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
Body plane terminology:
Now go examine yourself in the mirror and be able to identify all those planes and points.