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.
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.
The phoronids, bryozoans, and brachiopods are
all filter-feeders that share a ciliated feeding organ called a lophophore.
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long, stick-like proboscis or "evert" that can be withdrawn by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.
Animal family tree
Lophophore-bearing phyla:
A Lophophore is a crown of ciliated tentacles that help gather suspended food particles.
The cilia trap these particles and bring them to the mouth, a form of filter-feeding.
The animal can withdraw its lophophore if conditions are muddy or if predators threaten.
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 is 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 is probably close to Phoronida.
Phylum Brachiopoda: Probably derived from phoronid ancestors by
addition of a two-part shell as an aid in burrowing. Shell has two unequal
valves; 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 brachiopods 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.
Evolution of Lophotrochozoa
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.
In the 1930s, a comparative anatomist drew a diagram to show the body plan that he thought was primitive
for a mollusk, similar to the diagram shown here as the first image for the phylum Mollusca. At the time, no such animal was
known; the drawing was based only on comparative anatomy and on the fact that Pilina, a very early fossil mollusk,
had a very low, domed shell. About 30 years later, an animal was dredged up from deep Pacific waters that had almost exactly
the anatomy that had been predicted. The animal was named Neopilina and was placed in its own new class, the Monoplacophora
(which means "bearing a single plate").
- 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 (slowly, at a "snail's pace").
Radular scraping organ well-developed.
The radula is used for feeding. It consists of a tongue-like cartilage
surrounded by a connective tissue strap that wraps around the front. Many small, sharp teeth are embedded
in the strap. The snail sticks out its tongue against a surface like a rock, then pulls back and forth on the strap,
first pulling on the top, then on the bottom, then on the top again. With each pull, the teeth scrape off
encrusted algae and other forms of life that cling to the surface. Look at the illustration of the primitive mollusk—
the inset shows the structure of the radula, which is very well-developed in snails.
Garden slugs are basically snails that have lost their shell and scrape into plant tissues with their radulas.
- 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, etc.):
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, like a wedge), often used in burrowing. (The old name "Pelecypoda"
means "hatchet-foot".)
- Interesting behavior: "Wedge and pull" (or "reach and pull") burrowing--
Clam narrows foot and wedges it down, then widens it and pulls body down a few millimeters.
Process repeats until clam is buried just beneath top of sediment, with siphons extending to surface.
- 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.
- Chambered Nautilus (and many extinct Nautiloids and Ammonoids) have gas-filled
chambers connected by a tube called the siphuncle. The gas is about 98% N2 and the animal
can regulate its quantity to control its buoyancy (and thus the depth at which it swims).
- Interesting behavior #1: Changing color and texture--
Most cephalopods have surface cells called chromatophores that come in several different colors. They can quickly
change size in such a way that the skin changes its color and also its pattern.
- Interesting behavior #2: Escape response--
When threatned, cephalopods can squirt dark inky fluid into mantle cavity (hyponome) and out narrow nozzle.
Ink disperses into dark cloud that holds predator's attention while squirting action propels animal backward
(in an unexpected direction), allowing it to escape and leaving the predator confused as the black cloud dissipates.
Watch this octopus film clip.
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.
All annelids exhibit Metamerism, a division of the body into numerous similar segments.
-
Class Polychaeta: Largest group, mostly marine. Sense organs and nervous
system highly developed; several setae (bristles) 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.
REVIEW:
Study guide and vocabulary
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