Flatworms and nearly all other animals from here on are bilaterally
symmetrical (right and left halves are mirror images).
The front
end of such animals usually forms a distinct head.
However,
flatworms still have a single all-purpose cavity with only one opening.
Bilateral symmetry: Right and left halves are mirror
images of one another.
Phylum Platyhelminthes (flatworms): Bilaterally symmetrical animals
with a flat body; dorsal (top) and ventral (bottom) surfaces
differ; no circulatory system needed because every part of body is near a
surface.
Anterior (front) end differs from posterior (hind) end.
Sense organs and brain are concentrated at the front end (cephalization)
to form a head.
A single all-purpose gastrovascular cavity, as in
coelenterates; single opening functions as both mouth and anus.
A simple, ladder-like nervous system, more concentrated at the head end.
Simple excretory tubules (flame cells whose beating cilia resemble a
flickering flame).
Three germ layers: ectoderm (outer epidermis);
endoderm (lining of gut); mesoderm (a loose mesenchyme
in flatworms). Acoelomate (no body cavity).
Many flatworms can regenerate missing parts following injury.
- Class Turbellaria: Mostly free-living; digestive tract and sense organs
still present; mouth often in middle of ventral surface.
- Class Trematoda: Small, parasitic worms (flukes) with small, oval
bodies; digestive tract simple; mouth at anterior end.
- Class Cestoda: Highly degenerate internal parasites (tapeworms)
with greatly reduced digestive tract, nervous system, and sense organs.
Flatworms, etc.
Related phyla, also without body cavities:
- Phylum Mesozoa: Small, marine parasites with very few cells.
- Phylum Acoela: Small, simple, bilateral animals similar to
flatworms, but with no gut or digestive tract. Formerly included
in the Platyhelminthes, but many zoologists now regard them as the most
primitive of bilateral animals.
- Phylum Rhynchocoela (Nemertea): "Proboscis worms," with a long, barb-tipped proboscis
(or "evert") that can be protruded as a weapon or withdrawn (inverted, like the finger of a glove)
when not in use.
- Phylum Gnathostomulida: Small worms; outer epidermal cells each have
a single cilium; mouth with paired, cuticle-hardened jaws.
EVOLUTION OF BILATERAL ANIMALS
Bilateral animals above the flatworm stage evolved a complete "assembly-line"
digestive tract running from
mouth to anus. Most also evolved body cavities.
Analysis of RNA sequences allows scientists to divide bilateral animals into:
Protostomes—
bilateral animals in which early cleavages are spiral and
determinate, and in which the mouth forms early
from the blastopore.
Protostomes are further divided into:
Lophotrochozoa, containing the Mollusca,
Annelida, Bryozoa, etc.; and
Ecdysozoa, contining
the Nematoda, Arthropoda and several smaller phyla.
Deuterostomes—
bilateral animals whose early cleavages are radial and
indeterminate, and whose mouth forms
at the other end from the blastopore.
FROM THIS POINT ON, all remaining phyla share several important derived features:
- A complete "assembly line" digestive tract (mouth to anus).
- Some type of body cavity, either a pseudocoel (a persistent blastocoel) or a
true coelom (surrounded with mesoderm throughout).
"Assembly line" digestion: Nearly all animals above the flatworm level
have a complete digestive tract, with a separate entrance (mouth) and
exit (anus). This allows food to be processed in stages, in the manner
of an assmbly line, with different regions or organs specialized for different
sequential steps or for different nutrients.
Evolution of body cavities: Fluid-filled body cavities, whatever their
origin, are useful:
- in support, as a hydrostatic skeleton
- in burrowing, where inflation of the body cavity can swell and
anchor part of the body, or else wedge forward and push sediment aside.
Because of their usefulness, body cavities have evolved many times,
independently, and are often constructed differently in different phyla:
- Some animals have a pseudocoel, lined with both endoderm and
mesoderm, derived from persistence of the blastocoel cavity.
- Other animals have a true coelom, lined with mesoderm throughout.
This may be either an enterocoel, derived from outpouching of the
gut (as in starfish), or a schizocoel, arising within the mesoderm
by splitting (as in mammals).
Differences in the structure of the coelom are useful in distinguishing many
phyla, but are a poor guide to relationships among phyla because body cavities
have evolved repeatedly and independently.
Animal family tree
Evolution of Bilateria
Protostomes are bilateral animals sharing the following traits:
- The opening to the embryonic archenteron becomes the mouth
(protostome means "first mouth").
- Spiral cleavage, introducing an asymmetry in the 8-celled
stage; the top 4 cells are rotated clockwise or counterclockwise
with respect to the lower 4 cells.
- Determinate cleavage, meaning that the cells destined to
form the front left portion of the animal lose the ability to form
structres on the right or the rear.
Deuterostome animals (considered later) have the opposite traits.
Phylogeny and classification of bilateral animals: Studies of
ribosomal RNA sequences show evidence that bilateral animals evolved
in three large groups (the first two are protostomes):
- Lophotrochozoa: A large group that includes annelid worms,
mollusks, and bryozoa, characterized in some cases by a ciliated
feeding organ called a lophophore and in other cases by a
ciliated larval stage called a trochophore.
- Ecdysozoa: A group that includes the two largest phyla,
Arthropoda and Nematoda, characterized by a hard outer covering
that must be shed periodically during growth, using steroid hormones
(ecdysones) to control the molting process.
- Deuterostomes, including the chordates and echinoderms.
REVIEW:
Study guide and vocabulary
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Index
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