Which flatworm is not parasitic

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Platyhelminthes flatworms Facebook. Animal Diversity. Brown, Dubuque, IA. Read more Classification Kingdom Animalia animals Animalia: information 1 Animalia: pictures Animalia: specimens Animalia: sounds Animalia: maps For the first time, we see groups of tissues that have evolved to form organs, such as the ones in the digestive, nervous, and excretory systems. Like the cnidarians, flatworms have a digestive system with only a single opening into the digestive cavity, but in independently living marine flatworms the cavity branches into all parts of the body Fig.

These flatworms feed through a pharynx. A pharynx is a long, tubular mouthpart that extends from the body, surrounds the food, and tears it into very fine pieces Fig.

Cells lining the digestive cavity finish digesting the food. Then the dissolved nutrients move to other cells of the body. Undigested food passes back out through the mouth, as in the cnidarians. Parasitic tapeworms usually absorb their nutrients directly from the host, while parasitic flukes have retained a digestive system. Like most self-propelling animals, independent-living flatworms have a central nervous system. A central nervous system consists of a mass of nerve cells, called a ganglion , in more complex organisms, the ganglion evolves into a brain in the anterior part of the body, and a nerve cord extending from the brain toward the posterior end of the body Fig.

Sensory cells in the head detect changes in the environment. In free-living flatworms, sensory cells that respond to light are clustered in two eyespots in the head. Sensory cells that detect water currents, solid objects, and chemicals are in two flap-like projections on the head called auricles. In self-propelling animals, these sensory organs in the head are the first part of the animal that encounters new surroundings.

The ganglion receives information from the sensory structures and sends signals to other parts of the body along two strands of nerve cells running toward the tail. The excretory system removes waste products and excess water from tissues of flatworms. Flatworms have a surprisingly elaborate system to rid the body of wastes Fig. This network runs the length of the animal on each side and opens to the outside through small pores in the posterior region of the body.

Connected to the tubes are tiny cells that move wastes and water from the tissues into the tubes. These cells contain flagella that beat back and forth, creating a current of fluid that constantly moves toward the excretory pores.

Under a microscope the flagellar movement looks like a flickering fire, and the structure is called a flame bulb. Flatworms have no circulatory system. Animals without a circulatory system have limited abilities to deliver oxygen and nutrients to their body cells because of the way that molecules behave. As molecules spread through water, they become less concentrated as they move away from their source. This is known as diffusion.

But cnidarians have no problem with diffusion because most cells of their bag-shaped bodies are in direct contact with the water, making the exchange of oxygen and nutrients easy Fig. Flatworms, bag-shaped but flattened, also get oxygen and nutrients to their body cells easily because all their cells are close to either their outer surface or their digestive cavity Fig.

As animals become larger and more complex, diffusion is often no longer an option, and then we begin to see the development of circulatory and respiratory systems.

Species in the phylum Nematoda from the Greek root word nema meaning thread are better known as the roundworms Fig. There are about 25, species of nematodes formally described by scientists. Nematodes are found in almost every habitat on Earth. One species was first discovered living inside felt beer coasters in German alehouses. Studies of farmlands have found as many as 10, nematodes in cubic centimeters cm 3 of soil.

Nematodes are similarly abundant in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species like crabs and snails. Like flatworms, roundworm species adopt either a free-living or a parasitic lifestyle. Parasitic nematodes Fig. Many nematodes that are parasitic on plants can devastate crops.

Some nematodes are cryptobiotic and have demonstrated a remarkable ability to remain dormant for decades until environmental conditions become favorable. Like the flatworms, nematodes are bilaterally symmetrical. They take their name from their round body cross-sectional shape. Unlike the flatworms in which food and waste enter and exit from the same opening, nematodes have a complete digestive system.

They seem to detect their food chemically—like sharks, they hone in on meat juices in their environment. They are sensitive to vibration, to temperature change, to currents, to smell, and to chemicals.

Many rhabdocoels also have a few long, sensory cilia. Most planarians are scavengers and carnivores and cannibals. There are no digestive juices, but large phagocytic cells that line the gastrovascular cavity pick up nutrients which then diffuse through them into other cells in the animal.

Some species of planarians secrete a strand of mucous onto the substrate and then roll it up and eat it, along with all the tiny algae, bacteria and single-celled critters that stick to it. Planarians produce more planarians in two ways, and most species can employ both. Almost all planarians are hermaphroditic, with each individual having both male and female organs, but it still takes two to tango. Young planarians emerge looking like mini-adults. Adults generally live a few months, but some can encyst themselves and survive the drought cycle of an ephemeral pond.

But planarians are famous for their asexual exploits — they can divide their bodies on purpose and can regenerate the missing end of each section.

The eggs are eaten by an intermediate host. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked or poorly cooked pork, beef, and fish. Flatworms are acoelomate, triploblastic animals.

They lack circulatory and respiratory systems, and have a rudimentary excretory system. This digestive system is incomplete in most species.

There are four traditional classes of flatworms, the largely free-living turbellarians, the ectoparasitic monogeneans, and the endoparasitic trematodes and cestodes. Trematodes have complex lifecycles involving a molluscan secondary host and a primary host in which sexual reproduction takes place.

Cestodes, or tapeworms, infect the digestive systems of primary vertebrate hosts. Improve this page Learn More. Skip to main content. Module Invertebrates. Search for:. Phylum Platyhelminthes Learning Outcomes Describe the unique anatomical and morphological features of flatworms. In Summary: Phylum Platyhelminthes Flatworms are acoelomate, triploblastic animals.

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