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BIO190: The Class: Acoelomate Animals: Lesson 15

The Acoelomate Animals

Review Coelom Chapter 7

The three phyla considered in this chapter are the Platyhelminthes or flatworms; the Nemertea or ribbon worms; and the Gnathostomulida or jaw worms.

The following is a list of Acoelomate animal characteristics.

Acoelomate Characteristics

Phylum Platyhelminthes (4 classes)

Browse this site for more information:

Flatworms (fig. 15-2). The word worm is loosely applied to elongated, bilateral invertebrate animals without appendages.

  • "flatworms" (fig. 15-2)

  • body flattened in dorsoventral plane

  • epidermis
    1. can be ciliated in some
    2. either
      1. cellular
      2. syncytial

    3. rhabdites (Class Turbellaria)
    4. tegument (Classes Monogenea, Trematoda, Cestoda)

  • nervous system
    1. anterior ganglia
    2. longitudinal nerve cords with transverse connections

  • eyespots (some members)

  • excretory system
    1. lateral canals
    2. flame cells
      1. protonephridia

  • following lacking
    1. respiratory
    2. circulatory
    3. skeletal



  • Class Turbellaria
  • Turbellarians are mostly free-living worms that range in lengh from 5mn or less to 50cm. Usually covered with ciliated epidermis, these are mostly creeping worms that combine muscular with ciliary movements to achieve locomotion. (fig. 15-3 & 15-5; 15-7)

  • Class Trematoda

    Trematodes are all parasitic flukes, and as adults they are almost all found as endoparsites of vertebrates.
    Broswe this website: Platyhelminthes: Trematodes

    1. flukes
    2. all parasitic
    3. examine typical body form of fluke (fig 15-14)

  • Subclass Digenea
    1. indirect life cycle (LC)
      1. one to several host
        1. intermediate host
        2. final (definitive) host

    2. Human liver fluke (Clonorchis sinensis)

        -LC = eggs out with human feces -->
        eggs contain a miracidium (ciliated larva) eaten by snail -->
        eggs hatch and release miracidium which enters snail tissues -->
        each miracidia forms a sporocyst (egg containing structure) -->
        each sporocyst produces a rediae -->
        each rediae produces a cercariae (forked tailed swimming larva which exits the snail -->
        cercariae penetrate fish of the minnow family and encysts forming metacercariae in the muscles of the fish -->

        if not cooked or improperly cooked and eaten by human then infection continues: (fig. 15-11).

  • Schistosomiasis (Read pp. 287-288; fig. 15-12)

    Infection with blood flukes of the genus Schistosoma ranks as one of the major infectious diseases in the world, with 200 million people infected.
    Visit this site for more information: Schistosomiasis
    Click on the gallery photo for a view of swimmer's itch.

  • Class Cestoda

    Cestoda, or tapeworms have long flat bodies in which there is a linear series of sets of reproductive organs.

    For more information browse these sites:
    Cestodes: Tapeworms
    Platyhelminthes: Cestodes

    Cestodes are all parasitic.

    1. tapeworm terminology
      1. scolex (fig. 15-19)
      2. hooks/suckers
      3. proglottid (fig. 15-19)
        1. gravid
        2. polyzoic

      4. strobila

    2. Taeniarhynchus sp.
      1. beef tapeworm

          LC = eggs out with human feces -->
          cow injest eggs from contaminated grass -->
          eggs hatch into oncospheres -->
          oncospheres burrow through intestinal walls and enter blood stream -->
          enter muscles and form cysts (bladder worms or cysticerci) -->
          human eats uncooked or improperly cooked infected beef and infection results

    3. Tab. 15-1 and pp. 292-293 for additional cestodes

    For more information on tapeworms browse this site:

    Phylum Nemertea

    Nemerteans are often called the ribbonworms (fig. 15-22)

    Phylum Gnathostomulida (fig. 15-26)

    The first species of the Gnathostomulida was observed in 1928 in the Baltic, but its decription was not published until 1956.

    Phylogeny/adaptive radiation of the three phyla: pp. 297- 298

    Once you have completed the lesson, you should go to Assignment 15-1.

    E-mail Sylvester Allred at


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