Muscles surrounding the tube contract, squeezing the food and pushing it along in a process called peristalsis. Indigestible wastes pass out through the anus. Unlike flatworms, nematodes are slender, and they are covered by a protective cuticle.
A cuticle is a waxy covering secreted by the epidermis, or outermost cellular tissue. Because of this covering, gas exchange cannot occur directly across the skin as in flatworms.
Rather, gas exchange and waste excretion in nematodes occurs by diffusion across the wall of the gut. Although nematodes do have a space in the body between the digestive tract and the body wall, it is not lined with tissue and is not considered to be a true coelom. Thus, nematodes are sometimes referred to as pseudocoelomates Fig.
Most worms have two bands of muscles: longitudinal muscles that run the length of the body and circular muscles that form circular bands around the body. Unlike other worms that have two bands of muscles, nematodes only have longitudinal muscles. This explains their characteristic thrashing movement, as they can move only by contracting the long muscles on either side of their body and wriggling forward. The nervous system of nematodes consists of a set of nerves that run the length of the body and connect to anterior ganglia.
Free-living nematodes are capable of sensing light with ocelli, and most nematodes have fairly complex chemosensory abilities. Most nematodes are not hermaphrodites , with both sexes in one individual, but are known as dioecious —having individuals of separate sexes. Their chemosensory abilities are very helpful, as they rely on pheromones to locate potential mates.
The worms in the phylum Annelida from the Latin root word annelus meaning ring typically have complex segmented bodies Fig. The body of an annelid is divided into repeating sections called segments with many internal organs repeated in each segment. Earthworms class Oligochaeta are familiar terrestrial members of this phylum and leeches class Hirudinea are well-known parasitic members of the phylum, most commonly found in freshwater.
They occur mostly in marine and brackish water habitats. Polychaete from the Greek root words poly meaning many and chaeta meaning bristle annelid worms are so named because most of their segments have bristles called chatae or setae. The free-moving not sessile polychaetes have muscular flaps called parapodia from the Greek para meaning near and podia meaning feet on their sides, and the setae on these parapodia dig into the sand for locomotion.
Fireworms are a type of polychaete that have earned their name from stinging bristles on each parapodium Fig. These bristles can penetrate human skin, causing irritation, pain and swelling, similar to the irritation caused by exposure to fiberglass.
Tubeworms are sessile polychaetes that live in tubes that they build by secreting the tube material. The tubes, attached to rocks or embedded in sand or mud, may be leathery, calcareous, or sand-covered depending on the worm species Fig. Tubeworms feed by extending tentacles from the tube. Bits of food move along grooves in the tentacles to the mouth.
Some tubeworms retract their tentacles when food lands on them. Tubeworms use their parapodia to create currents of water that flow through the tubes to aid in respiration and help clean the tubes. By contrast, the free-living or mobile polychaete worms have a proboscis that can extend from their mouths to catch prey.
This is a feeding organ that is often armed with small teeth or jaws on its tip. With their active lifestyle and good defenses, free-moving polychaetes can make their living in a variety of habitats such as mud, sand, sponges, live corals, and algae.
Like flatworms, annelids have a mesoderm with muscle, a central nervous system, and an excretory system. Each of these systems is more complex in the annelid than in flatworms or nematodes. In addition to a more specialized complete digestive system, annelid worms have also evolved body features not found in flatworms or nematodes.
These features appear in some form in all larger, more complex animals:. Recall that the coelom is a fluid-filled cavity lying between the digestive tube and the outer body tube and surrounded by mesodermal tissue.
The digestive tube lies inside the outer body tube. The fluid in the coelom supports the soft tissues of the body wall much as it does in the hydrostatic skeleton of cnidarians.
Mesodermal muscles in the wall of the body tube and digestive tube can put pressure on the fluid to aid in movement. In the body wall of the annelids are two types of muscles: circular and longitudinal. Development may be direct eggs hatch into tiny worms that resemble the adults or indirect with a ciliated larval form. Flatworms include a large number of parasitic forms, some of which are extremely damaging to human populations.
Brusca, R. Sinauer Associates, Sunderland, MA. To cite this page: Myers, P. Disclaimer: The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common.
The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and one or more secondary hosts in which asexual reproduction occurs.
The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke. The disease infects an estimated million people in the tropics, leading to organ damage and chronic symptoms like fatigue.
Infection occurs when the human enters the water and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing.
Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the primary snail host. The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates Figure 3. Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body.
The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but contain reproductive structures, both male and female. Figure 3. Tapeworm Taenia spp. Most reproduction occurs by cross-fertilization. The proglottid detaches from the body of the worm and is released into the feces of the organism. The eggs are eaten by an intermediate host.
The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. Invasive flatworms can cause major disruptions to ecosystems : read about an invasive flatworm on the Shape of Life website.
Researchers at the University of New Hampshire found that flatworms in estuaries can indicate the health of the ecosystem. Flatworms play a role in many food chains. Read about some of them. Parasitic flatworms can disrupt ecosystems. Some flatworms, planaria, can be used as models for toxicology Platyhelminthes from the Florida Museum of Natural History.
The Marine Flatworm Galleries. Skip to main content. Videos Planaria illustrates bilateral symmetry. A predatory flatworm hunts a snail, on land Flatworm swimming A flatworm lifting off from a coral reef and swimming A song, with video about flatworms. General information about flatworms Read about the advantages and disadvantages of being hermaphroditic A primer on marine flatworms for K Students From the Museum Victoria Victoria, Australia more about marine flatworms Read about a cave-dwelling freshwater flatworm A large group of flatworms are parasites that can infect humans causing diseases, read the article.
Images Download platyhelminthes.
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