Characteristics of flat worms1/2/2024 Platyhelminthes are mostly worm-like creatures that are dorsoventrally flattened, meaning they look like a ribbon. Scientists believe that the first turbellarians evolved around 550 MYA (million years ago). Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.Platyhelminthes are an ancient phylum, but practically nothing is known of their evolutionary history because they have very soft bodies which do not preserve well as fossils. Class Turbellaria (turbellarians, flatworms).Development may be direct (eggs hatch into tiny worms that resemble the adults) or indirect (with a ciliated larval form).įlatworms include a large number of parasitic forms, some of which are extremely damaging to human populations.Ĭlick on the name of a Class below to learn more: Most of these have developed ways of avoiding self-fertilization. Most flatworms can reproduce sexually or asexually. Other sense organs found in at least some members of this group (not necessarily on the head) include chemoreceptors, balance receptors (statocysts), and receptors that sense water movement (rheoreceptors). In some flatworms, the process of cephalization has included the development in the head region of light-sensitive organs called ocelli. The development of directional movement is correlated with cephalization. Others move along slime trails by the beating of epidermal cilia. Movement in some flatworms is controlled by longitudinal, circular, and oblique layers of muscle. Nonparasitic forms have a simple, incomplete gut even this is lacking in many parasitic species. Flatworms lack a respiratory or circulatory system these functions take place by absorption through the body wall. Excretion and osmoregulation by flatworms is controlled by "flame cells" located in protonephridia (these are absent in some forms). Flatworms have a cephalized nervous system that consists of head ganglion, usually attached to longitudinal nerve cords that are interconnected across the body by transverse branches. Some forms are free living but many are parasitic. Flatworms are unsegmented, bilaterally symmetrical worms that lack a coelom (acoelomate) but that do have three germ layers.
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |