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.
The Lophotrochozoa are a diverse group of phyla with similar ribosomal
RNA sequences. The annelid worms and mollusks are two large phyla in this
group; primitive members of both these phyla share a ciliated larval stage
called a trochophore.
The phoronids, bryozoans, and brachiopods are
all filter-feeders that share a ciliated feeding organ called a
lophophore.
Mollusks are a large and diverse group of animals; familiar
mollusks include snails, clams, squids, and octopuses.
The Annelida are worms with thorough segmentation of the body.
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.
Animal family tree
Evolution of Bilateria
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.
The Lophotrochozoa are a diverse group of phyla. Most have a true
coelom of the schizocoel type, but a few have only a pseudocoel.
Ribosomal RNA sequences show these various phyla to be related.
Phyla placed in this group include:
- Phylum Rhynchocoela or Nemertea: "Proboscis worms" with a
long, stick-like proboscis or "evert" that can be withdrawn by
turning it inside out.
- Phylum Rotifera: Rotifers, with a wheel-like crown of cilia at one end.
- Phylum Acanthocephala: Spiny-headed, parasitic worms.
- Phylum Annelida: Segmented worms, described below.
- Several smaller phyla related to the Annelida:
- Sipunculida: Marine worms with tentacles around the mouth.
- Echiurida: Sausage-shaped worms with a tube-like extension
in front of the mouth.
- Chaetognatha: Arrow worms, with dart-shaped bodies, abundant
in marine plankton.
- Gastrotricha: Gastrotrichs.
- Pogonophora: Deep-sea tube-dwelling worms.
- Phylum Mollusca: Snails, clams, squids, etc., described below.
- Four phyla (Entoprocta, Phoronida, Bryozoa, Brachiopoda) characterized
by a ciliated feeding organ called a lophophore.
Lophophore: A crown of ciliated tentacles that help gather suspended food
particles. The cilia trap these particles and bring them to the mouth. Animal
can withdraw lophophore if conditions are muddy or if predators threaten.
Animal family tree
Evolution of Bilateria
Similarities of phoronids, bryozoans, and brachiopods:
- All have lophophores.
- True coelom, used as a hydrostatic skeleton.
- Simple, U-shaped digestive tube, complete with mouth and anus.
- Benthonic (bottom-dwelling), either mobile or sessile (attached).
Phylum Phoronida (phoronid worms): Tube-dwelling worms with a lophophore
surrounding the mouth. Coelom used in burrowing: muscle contraction builds up
pressure in the coelom, which swells sideways and pushes sediment aside.
Probably related to ancestry of Bryozoa and Brachiopoda.
Phylum Bryozoa ("moss animals"): Largest and most successful lophophorate
group. Members quite varied. All are small, aquatic animals living in colonies.
Many colonies are polymorphic, containing several dissimilar types of
individuals. Ancestry uncertain, but probably close to Phoronida.
Phylum Brachiopoda: Probably derived from phoronid-like ancestors by
addition of a two-part shell as an aid in burrowing. Shell has two unequal
valved; axis of symmetry bisects center of each valve. Valves are connected
by muscles only (class Inarticulata) or by a hinge (class
Articulata). Muscular stalk (pedicle) attaches animal to the bottom.
Fossil record shows that these animals were more abundant and more diverse
during the Paleozoic Era (530 million to 300 million years ago).
Phylum Entoprocta: A small group of sessile (attached) animals with a
lophophore and a simple, U-shaped digestive tract. They differ from the previous
3 phyla in two ways: they have a pseudocoel instead of a true coelom, and their
lophophore surrounds the anus as well as the mouth.
Phylum Mollusca (mollusks): Animals with a true coelom of the
schizocoel type, usually bearing a shell composed mostly of calcium
carbonate and secreted by a mantle. The mantle is always
withdrawn at the rear to form a mantle cavity, which contains
anus and gills. Primitive mollusks and gastropods use a tongue-like
radula with embedded teeth to scrape encrusted algae
from rock surfaces.
-
Class Monoplacophora (primitive mollusks): Mollusks with a simple
dome-shaped or low conical shell. Muscles, blood vessels, and other structures
segmentally arranged. Digestive tract simple.
-
Class Gastropoda (snails and slugs): Body usually undergoes asymmetrical
torsion (twisting and coiling). One-piece (univalve) shell, usually
coiled. Most species herbivorous. Well-developed head, sense organs, and
nervous system. Locomotion typically by creeping on a muscular foot.
- Class Polyplacophora (chitons): Simple, flattened body, with shell
divided into several overlapping plates that permit some flexibility.
Head small but radula well-developed and used in feeding.
- Class Bivalvia or Pelecypoda (clams and other bivalves):
Body usually symmetrical, narrowly compressed from side to side. Two-piece
(bivalve) shell; left and right halves are often mirror images (except at hinge).
Many species filter-feed, straining small particles from the water.
Head and sense organs poorly developed. Muscular foot hatchet-shaped
(flattened side-to-side), often used in burrowing.
- Class Scaphopoda (tusk-shells): Small mollusks with tusk-like shells.
Mantle cavity runs for entire length of shell along posterior margin;
water passes through mantle cavity, exiting through hole at the top.
- Class Cephalopoda (octopuses, squids, nautiloids, etc.):
Body usually symmetrical. One-piece shell is symmetrically curved or coiled
in median plane, or often lost. Most species are actively swimming predators.
Head very well-developed, with sense organs (especially eyes), brain, and
beak. Muscular foot subdivided into numerous tentacles.
Body doubled over, with mantle cavity (originally rear) tucked beneath head
and opening forward. Frequent "ink glands" that secrete dark, inky fluid
to confuse predators.
Metamerism is division of the body into numerous similar segments.
Phylum Annelida: Segmented worms. Complete digestive tract (with both
mouth and anus) runs nearly the entire length of the body. Outer covering
of chitin is thin, flexible, and prevents fluid loss.
The true coelom and most internal organs are segmentally arranged. Blood
cicrulates in closed vessels only. Advanced excretory organs (nephridia)
are present. Some ability to regenerate missing parts after injury.
-
Class Polychaeta: Largest group, mostly marine. Sense organs and nervous
system highly developed; several setae per segment.
- Class Oligochaeta (earthworms): Poorly developed head; only one pair of
setae per segment. Important to soil because their digestive wastes leave
behind soil nutrients and their tunnels let air reach plant roots.
- Class Hirudinea (leeches): Mostly parasitic, live in fresh water,
attach to the outside of animals and suck blood. Leeches have degenerate
anatomy: fewer sense organs, fewer segments, etc.
Locomotion in annelids (controlled separately in each segment):
- Each segment contains a walled-off portion of the body cavity.
- Muscles parallel to the body axis can shorten segments; these segments
swell and anchor into the surrounding sand or soil.
- Muscles perpendicular to the body axis will lengthen body segments
and cause them to push forward.
- Nervous system produces rhythmic waves of shortening and waves of
lengthening among the segments.
- Small bristles (setae) may help anchor the shortened segments.
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