Bio Review Notes #83
LOPHOTROCHOZOA

The Lophotrochozoa are a diverse group of phyla with similar ribosomal RNA sequences. The annelid worms and molluscs 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.

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  • Phylum Rhynchocoela or Nemertea: "Proboscis worms" with a long, stick-like proboscis or "evert" that can be withdrawn by turning it inside out.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, to be described later.
  • Several smaller phyla related to the Annelida:
    1. Sipunculida: Marine worms with tentacles around the mouth.
    2. Echiurida: Sausage-shaped worms with a tube-like extension in front of the mouth.
    3. Chaetognatha: Arrow worms, with dart-shaped bodies, abundant in marine plankton.
    4. Gastrotricha: Gastrotrichs.
    5. Pogonophora: Deep-sea tube-dwelling worms.
  • Phylum Mollusca: Snails, clams, squids, etc., to be described later.
  • 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.

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.


Index             Syllabus
Prev rev. July 2010 Next