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Flatworms: Read passage and answer questions Name___________________________ The flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments. Most are free-living forms, but many are parasitic on other animals. There are four classes: Trematoda (Flukes), Cestoda (Tapeworms), Monogenea, and Turbellaria. Description The flatworm’s cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs. This means their organ systems form out of three germ layers: an outer ectoderm and an inner endoderm with a mesoderm between them. Turbellarians generally have a ciliated epidermis, while cestodes and trematodes are covered with a cuticle. Body functions There is also no true body cavity (coelom) except the gut and hence they are acoelomates. The interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue. There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells to help distribute nutrients, gases, and waste products. Flatworms respire at their integument; gasses diffuse directly across their moist outer surface. This type of system is called integumentary exchange. However, flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots. Usually the digestive tract has one opening, so the animal can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts can. This blindended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below). Despite the simplicity of the digestive chamber, they are significantly more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level or organization. Mesoderm allows for the development of these organs, and true muscle. Major sense organs are concentrated in the front end of the animals for species who possess these organs. Muscular contraction in the upper end of the gut causes a strong sucking force allowing flatworms to ingest their food and tear it into small bits. The gut is branched and extends throughout the body, functioning in both digestion and transport of food. Flatworms exhibit an undulating form of locomotion. Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. They usually do not fertilize their own eggs. Turbellarians classified as planarians (usually freshwater, non-parasitic) can also reproduce asexually by transverse fission. The body constricts at the midsection, and the posterior end grips a substrate. After a few hours of tugging, the body rips apart at the constriction. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these flatworms is cut in half, each half will regenerate into two separate fully-functioning flatworms. Depending on species and age, individuals can range in size from almost microscopic to over 20 m long (some tapeworms can attain this length). Questions: 1. Are flatworms vertebrates or invertebrates? ______________________ 2. Flatworms can be free-living, but many are ______________ on other animals. 3. The four classes of Platyhelminthes are: _______________, __________________, _________________, and ______________________. 4. What kind of symmetry do flatworms have? ____________________ 5. Flatworms have 3 germ layers, which makes them ______________, and these germ layers are: _________________, ________________, and _______________. 6. Turbellarians generally have a _____________ epidermis, while cestodes and trematodes are covered with a _______________. 7. Flatworms have no true body cavity and therefore are _______________. 8. Respiration occurs at the ___________ where gasses diffuse across their moist outer surface. 9. Flatworms are the simplest animals to have a _____________ which branches repeatedly in an array resembling a ladder. 10. The collection of __________ act as a brain to integrate signals from sensory organs. 11. One example of sensory organs would be the ocelli, also known as ____________. 12. The digestive tract has _______ opening(s). 13. Flatworms show _________ level of organization. 14. Organs and muscles develop from which germ layer? _________________. 15. Flatworm reproduction is _____________, meaning each individual produces both eggs and sperm. 16. Planarians (class Turbellaria) can reproduce asexually by __________________.