|Thysanozoon nigropapillosum, a marine
turbellarian (order Polycladida).
For animals that spend their lives sitting and waiting, as do
most members of the two radiate phyla we considered in
the next section, radial symmetry is ideal. One side of
the animal is just as important as any other for snaring prey
coming from any direction. But if an animal is active in
seeking food, shelter, home sites, and reproductive mates, it
requires a different set of strategies and a new body organization.
Active, directed movement requires an elongated
body form with head (anterior) and tail (posterior) ends. In
addition, one side of the body is kept up (dorsal) and the
other side, specialized for locomotion, is kept down (ventral).
What results is a bilaterally symmetrical animal in
which the body can be divided along only one plane of
symmetry to yield two halves that are mirror images of each
other. Furthermore, since it is better to determine where one
is going than where one has been, sense organs and centers
for nervous control have come to be located on the head.
This process is called cephalization. Thus cephalization and
primary bilateral symmetry evolved together.
The three acoelomate phyla considered in this section
are not greatly more complex in organization than radiates
except in symmetry. The evolutionary consequence of that
development alone was enormous, however, for it is the
type of symmetry assumed by all more complex animals.
Acoelomate body plan.
The three phyla considered in this
section have the simplest organization
within the Bilateria, a grouping of
phyla that includes all the rest of the
animal kingdom. These three are Platyhelminthes
, flat, + belmins
worm), or flatworms; Nemertea (Gr. Nemertes
, one of the nereids, unerring
one), or ribbon worms; and Gnathostomulida
, jaw, + stoma
mouth, + L. ulus
, dim.), or jaw worms.
They have only one internal space, the
digestive cavity, with the region
between the ectoderm and endoderm
filled with mesoderm in the form of
muscle fibers and mesenchyme (parenchyma).
Since they lack a coelom
or a pseudocoel, they are termed acoelomate bilateral
14-1), and because they have three
well-defined germ layers, they are triploblastic
. Acoelomate bilateria
show more specialization and division
of labor among their organs than do
radiate animals because the mesoderm
makes more elaborate organs possible;
thus, acoelomate bilateria are said to
have reached the organ-system level of
These phyla belong to the protostome
division of the Bilateria and typically
have spiral cleavage. They have
some centralization of the nervous system,
with a concentration of nerves
anteriorly and a ladder-type arrangement
of trunks and connectives down
the body. They have an excretory
(or osmoregulatory) system, and the
nemerteans also have a circulatory system.
They also have a one-way digestive
system, with an anus as well as a
Position in Animal
- Platyhelminthes, or flatworms,
Nemertea, or ribbon worms, and
Gnathostomulida, or jaw worms, are
the simplest animals to have primary
- These phyla have only one internal
space, a digestive cavity, with the
region between the ectoderm and
endoderm filled with mesoderm in
the form of muscle fibers and mesenchyme
(parenchyma). Since they
lack a coelom or a pseudocoelom,
they are termed acoelomate
Bilateria, and because they have
three well-defined germ layers, they
are termed triploblastic.
- Acoelomate bilateral animals show
more specialization and division of
labor among their organs than do
radiate animals because the mesoderm
makes more elaborate organs
possible. Thus the acoelomates are
said to have reached the organsystem
level of organization.
- They belong to the protostome division
of the Bilateria and have spiral
cleavage, and at least platyhelminths
and nemerteans have determinate
- Acoelomate Bilateria developed the
basic bilateral plan of organization
that has been widely exploited in the
- Mesoderm developed into a welldefined
embryonic germ layer (triploblastic),
making available a great
source of tissues, organs, and systems.
- Along with bilateral symmetry, cephalization was established. Some
centralization of the nervous system
evident in the ladder type of systemfound in flatworms.
- Along with the subepidermal musculature,
there is also a mesenchymal
system of muscle fibers.
- They are the simplest animals with an excretory system.
- Nemerteans are the simplest animals
to have a circulatory system with
blood and a one-way alimentary
canal. Although not stressed by zoologists,
the rhynchocoel cavity in ribbon
worms is technically a true
coelom, but because it is merely a
part of the proboscis mechanism, it is
probably not homologous to the
coelom of eucoelomate animals.
- Unique and specialized structures
occur in all three phyla. The parasitic
habit of many flatworms has led to
many specialized adaptations, such as
organs of adhesion.