Platyhelminthes

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Flatworm, common name for soft-bodied, usually parasitic animals, the simplest of animals possessing heads. They are bilaterally symmetrical and somewhat flattened, and most are elongated. Three main classes are included in the flatworm phylum: tapeworms, which in the adult stage are parasitic in the digestive tracts of animals; flukes, which are parasitic in various parts of different animals; and planarians, which are free-living and nonparasitic. Some authorities include a group of unsegmented marine worms. Other authorities consider them a separate phylum.

The ectoderm (outer surface) of the free-living flatworms is usually covered with cilia; in the parasitic forms the ectoderm usually secretes a hardened material called cuticle. A well-developed musculature, found directly under the epidermis (skin layer), allows the body to expand and contract, thus changing the body shape to a remarkable degree. Vivid pigmentation is sometimes present in the free-living forms, but the parasitic forms are usually unpigmented. Flatworms have no true body cavity; the spaces between the organs are filled with a compact connective tissue called parenchyma. Except in the simplest forms, one end of the body is more specialized for sensory perception, and locomotion takes place in the direction of specialization. The oral and genital openings are on the ventral (under) side. When present, the digestive tract is either saclike or branched and has only one opening. This opening may be equipped with a sucker, as in the flukes, or, as in most planarians, it may have a well-developed pharynx. The nervous system consists of a network with a large ganglion (brain) and various longitudinal nerve cords forming the principal parts. Sensory cilia and “eye spots” may be present in the free-living forms and in the larvae of the parasitic forms. The flatworm has no blood or vascular system. Specialized cells possessing cilia, called flame cells, lead from the interior to one or more openings in the exterior by means of a network of tubes. Together these structures form the excretory system. The reproductive system is highly complex and occupies a large portion of the interior of the animal. Although flatworms are almost all hermaphroditic (both male and female reproductive organs are present in each individual), the eggs and sperm are formed separately. These germ cells either leave the body by separate openings or enter a common chamber, called the genital atrium. Flatworms also are able to reproduce asexually both by binary fission—that is, by pinching themselves apart to become two—and by regeneration, producing an entire new worm from a piece that has been cut off.

Free-living flatworms are found in almost every kind of environment, on land and in fresh and salt water. These forms feed mainly on plankton. The parasitic flatworms often display a complicated life cycle, which may require development in four or five hosts before completion.

Scientific classification: Flatworms constitute the phylum Platyhelminthes. Tapeworms constitute the class Cestoda. Flukes constitute the class Trematoda. Planarians constitute the class Turbellaria.

Types of Invertebrates

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Ctenophora -> Jellyfish-like marine animals distinguished by eight rows of cilia that propel the body in swimming. They feed on other invertebrates using two retractable sticky tentacles to capture prey. All ctenophores are hermaphroditic and reproduce sexually. Many are luminescent. – Examples: Sea walnuts, comb jellies

Platyhelminthes -> Structurally simple worms with no anus or circulatory system. Known as flatworms, their flattened bodies enable internal tissues to be near the skin surface, permitting gas and nutrient exchange with the environment. A muscular layer just beneath the skin layer aids in locomotion. Flatworms are hermaphrodites and reproduce sexually. They typically have a life cycle involving a parasitic larval stage. Many types of flatworms are parasites of vertebrates. – Examples: Flatworms, flukes, tapeworms

Mesozoa -> Minute parasitic animals of invertebrates, particularly squids and octopuses. These worms have a simple, elongated, ciliated body. They reproduce sexually and have a complex life cycle involving more than one larval form. – Exmaple: Rhopalura granosa

Nemertea or Rhynchocoela -> Animals characterized by a proboscis, a long, muscular tube used in capturing invertebrate food. Nemerteans have elongated, flattened bodies and they lack an internal body cavity. They have a mouth opening for food ingestion and an anal opening for wastes. The blood of some nermerteans contains oxygen-carrying hemoglobin. Nemerteans reproduce sexually. – Examples: Nemertine worms, ribbon worms

Gastrotricha -> Microscopic multicellular animals that inhabit both freshwater and marine water. An external layer, the cuticle, encasing these animals contains cilia that aid in locomotion. Adhesive tubes found on the sides or at the posterior end aid in surface attachment. Freshwater species have a forked tail. These animals feed on dead or living bacteria, diatoms, or small protozoa. Marine species are hermaphroditic, while most fresh water species are female, reproducing by parthenogenesis. – Examples: Turbanella cornuta, Chaetonotus anomalus

Rotifera or Rotatoria -> Microscopic aquatic animals characterized by a corona, a wheel-shaped organ on the head used in feeding and swimming. The rapid beating of the cilia on the corona draws nutrient-containing water into the mouth. A protective cuticle covers the elongated, cylindrical body. Reproduction is sexual during brief periods of the year and throughout the rest of the year females reproduce via parthenogenesis. – Examples: Synchaeta oblonga, Phylodina roseola

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