Animal Biology #4
FLATWORMS AND RELATED ANIMALS

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.
  • VIDEO:   Planarian flatworms

    • 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(Nemertea): "Proboscis worms," with a long, barb-tipped proboscis (or "evert") that can be protruded as a weapon or withdrawn (inverted, like the finger of a glove) when not in use.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.

    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).


    Index             Syllabus
    Prev rev. Mar. 2016 Next