Invertebrate Zoology: A Textbook - T. A. Dauda 2014
Metazoa
Flatworms
There are approximately 13,000 known species of Flatworms. While some are free-living forms inhabiting seas, oceans, freshwaters, and damp tropical leaf litter (such as the white planarian and black planarian, accounting for about 3,000 species), the overwhelming majority of flatworms are parasites of animals and humans.
The Water/140.html">Anatomical Structure of flatworms is more complex than that of Cnidarians. This evolutionary advancement is associated with The Emergence of bilateral Symmetry and triploblasty. Only a single plane can divide a flatworm's body into two mirror-image halves: right and left. During embryonic development, a third Cell layer—the mesoderm—appears between the ectoderm and endoderm, later giving rise to many of the animal's Internal Organs.
An essential structural feature of flatworms is the presence of a body wall, or Skin-muscular sac. This structure consists of an epithelium overlying a complex system of Muscle fibers. These fibers are often arranged in distinct layers (circular and longitudinal), forming a continuous muscular sheath beneath the epithelium that encloses the entire body. The contraction of these muscular elements enables the characteristic worm-like locomotion of flatworms.
All flatworms, both free-living and parasitic, share common organizational features: in most species, the body is dorsoventrally flattened and elongated; there is no true body cavity, and the spaces between organs are filled with a loose tissue known as parenchyma. The Digestive System comprises two sections: the foregut and a blind-ending midgut (which is entirely absent in tapeworms). A rudimentary excretory system consisting of protonephridia appears for the first time, featuring terminal tubules capped by flame Cells with beating cilia. Circulatory and respiratory systems are absent. Most flatworms are hermaphrodites. The Reproductive System is characterized not only by Gonads, but also by genital ducts and accessory structures involved in egg nourishment, shell formation, and cocoon production. Parasitic species typically exhibit complex life cycles involving multiple hosts.
The phylum Platyhelminthes comprises several classes. From a practical standpoint, the parasitic Representatives of the classes Trematoda (flukes) and Cestoda (tapeworms) are of the greatest significance to humans.
Last update: 13/08/2026
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