Protostomes are those bilateral animals in which the mouth forms early
in embryology from the entrance to the archenteron in the gastrula stage.
All protostomes have spiral, determinate cleavage.
Two important evolutionary advances made among protostomes were the evolution
of body cavities and the evolution of a complete digestive tract running from
mouth to anus. Analysis of RNA sequences are used to divide protostomes
into two large groups: Lophotrochozoa, containing the Mollusca,
Annelida, Bryozoa, and many smaller phyla; and Ecdysozoa, contining
the Nematoda, Arthropoda and several smaller phyla.
Protostomes: 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.
"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.
Phylogeny and classification of bilateral animals: Studies of
ribosomal RNA sequences provide evidence that bilateral animals evolved
in three large groups:
- 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.
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