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  1. Systematic position type worm flat, round and Annelida
  2. Classification

    Kingdom: Animalia
    Sub - Kingdom: Metazoa
    Phylum: Plathyhelminthes
    Phylum: Nemathelminthes
    Phylum: Annelida

    What are flatworms?

    Flatworms are soft, flattened worms that have tissues and internal organs. They are the simplest animals to have:

    1. three germ layers – in other words they are triploblastic,
    2. bilateral symmetry, and
    3. cephalization.

    Flatworms are also known as acoelomates, meaning without a coelom. A coelom is a fluid filled body cavity that is lined with tissue derived from the mesoderm. No coelom forms between the tissues of flatworms.

    Groups of Flatworms

    The three main groups of flatworms include the:

    1. Turbellaria (freeliving),
    2. Cestoda (parasitic tapeworms), and
    3. Trematoda (parasitic flukes).


    The phylum was created by Gegenbaur in 1859 for unsegmented round worms but now phylum Nematoda is commonly used instead. The phylum includes bilaterally symmetrical triploblastic and pseudocoelomate animals with organ system of organization. Excretion involves a giant excretory cell called Renette cell. They have tube-within-tube type of body plan.

    Class NEMATODA

    It includes round worms that are both free living and parasitic, containing about 28,000 species, of which 16,000 are parasitic. They are slender and unsegmented worms having bilateral symmetry and their skin consists of a syncytium covered by a thick layer of cuticle. They possess only longitudinal muscles. Most of the species are dioecious and lay shelled eggs.

    Phylum Annelida

    The Annelida are a medium sized phylum of more than 9,000 species of worms. Most species prefer aquatic environments, but there are also a number of well know terrestrial species. Only a few species of annelids are commonly known to human beings, these include the delightful Rain, Dew or Earthworms that work so hard to make our soils healthy, the Ragworms and Lugworms used by marine fishermen and the much smaller Tubifex or Red worms used by aquarists to feed their fish. In many countries people are still familiar with Medicinal leeches, and people who live closer to nature are naturally more familiar with a much wider range of Annelids than those who live in cities.

    The Phylum Annelida is divided into 3 classes, one of which the Clitellata could really be called a Superclass, it contains three subclasses, the Oligochaeta, the Branchiobdella and the Hirundinea. The other two classes are the Polychaeta which contains the largest number of species and the Aelosomatida which contains very few.

  3. The external morphology of flat, round and annelids
  4. Flatworm, also called platyhelminth, any of the phylum Platyhelminthes, a group of soft-bodied, usually much flattened invertebrates. A number of flatworm species are free-living, but about 80 percent of all flatworms are parasitic—i.e., living on or in another organism and securing nourishment from it. They are bilaterally symmetrical (i.e., the right and left sides are similar) and lack specialized respiratory, skeletal, and circulatory systems; no body cavity (coelom) is present. The body is not segmented; spongy connective tissue (mesenchyme) constitutes the so-called parenchyma and fills the space between organs. Flatworms are generally hermaphroditic—functional reproductive organs of both sexes occurring in one individual. Like other advanced multicellular animals, they possess three embryonic layers—endoderm, mesoderm, and ectoderm—and have a head region that contains concentrated sense organs and nervous tissue (brain). Most evidence, however, indicates that flatworms are very primitive compared with other invertebrates (such as the arthropods and annelids). Some modern evidence suggests that at least some flatworm species may be secondarily simplified from more complex ancestors.

    Nematodes are the most speciose phylum after the arthropods, they occur in nearly every habitat including as parasites in all sorts of plants and animals, (they don't like dry places however).

    Characteristics of Nematoda:- Bilaterally symmetrical, and vermiform; body has more than two cell layers, tissues and organs; body cavity is a pseudocoel, body fluid under high pressure; body possesses a through gut with a subterminal anus; body covered in a complex cuticle; has a nervous system with pharyngeal nerve ring; has no circulatory system (no blood system); reproduction normally sexual and gonochoristic; feed on just about everything; live just about everywhere, many species are endoparasites.

    Annelida [Lat., anellus = a ring], phylum of soft-bodied, bilaterally symmetrical (see symmetry, biological), segmented animals, known as the segmented, or annelid, worms. Over 12,000 known species are grouped in three classes: the earthworms and freshwater worms (oligochaetes), the leeches (hirudineans), and the marine worms (polychaetes). Annelids are found throughout the world, from deep ocean bottoms to high mountain glaciers. They live in protected habitats such as mud, sand, and rock crevices, and in and among other invertebrate animals, such as sponges. Many live in tubes they secrete around themselves. The fundamental characteristic of the phylum is the division of the body into a linear series of cylindrical segments, or metameres. Each metamere consists of a section of the body wall and a compartment of the body cavity with its internal organs. The external divisions, which may be seen in the common earthworm, correspond to the internal divisions. The annelid body consists of a head region; a trunk, made up of metameres; and an unsegmented terminal region called the pygidium. In some primitive members of the phylum the metameres are identical, or very similar to one another, each containing the same structures; in more advanced forms there is a tendency toward a consolidation of some segments and a restriction of certain organs to particular segments. Because of the soft nature of the annelid body, fossils are not common. Fossils of tube-dwelling polychaetes have been found, but there is scarcely any fossil record for earthworms and none for leeches.

  5. Distinctive features of flat sheets, and round worms


The body of flukes is oval to elongate, usually not more than a few cm long, and the mouth is typically at the anterior end. Adhesive suckers are usu-ally present around the mouth and may also be present midventrally. The monogenetic flukes possess large posterior attachment organs, called opisthaptors, provided with various structures, such as suckers and hooks (Fig. 7-25).In contrast to the epidermis of the turbellarian, the body of a trematode is covered a nonciliated cytoplasmic syncytium, the tegument, overlying consecutive layers of circular, longitudinal, and diagonal muscle. The syncytium represents extensions of cells that are located in the parenchyma (Fig. 7-26).

The mouth leads into a muscular pharynx that pumps into the digestive tract the cells and cell fragments, mucus, tissue fluids, or blood of the host on which the parasite feeds. The pharynx passes into a short esophagus and one or, more commonly, two blind intestinal ceca that extend posteriorly along the length of the body (Fig. 7-27A). The physiology of nutrition is still incom-pletely understood, but secretive and absorptive cells have been reported, so digestion is apparently extracellular in part.

The tegument plays a vital role in the physiology of flukes. It provides protection, especially against the host's enzymes in gut-inhabiting species. Nitrogenous wastes are passed to the exterior through the tegument, and it is the site of gas exchange. In endoparasites the tegument absorbs some amino acids. The protein synthesis involved in fluke egg production and in larval reproduction places especially heavy demands on the amino acid supply.


The nematode cuticle is considerably more complex than that of other aschelminths. It contains collagen, as well as other compounds, and it is or-ganized within three main layers (Fig. 9-14). The outer cortical layer is bounded externally by a thin epicuticle, which may exhibit quinone tanning. It is typically annulated (ringed). The median layer varies from a uniform granular structure in some species to the occurrence of struts, skeletal rods, fi-brils, or canals in others. The basal layer may be striated or laminated or contain spiral fibers.

Growth in nematodes is accompanied by four molts of the cuticle. Beginning at the anterior end, the old cuticle separates from the underlying epidermis and a new cuticle is secreted, at least in part. The old cuticle is shed in fragments or intact. Molting does not occur after the worm becomes adult, but the cuticle continues to grow.

The epidermis, also called hypodermis, is usually cellular but may be syncytial in some species. A striking feature of the nematode epidermis is the expansion of the cytoplasm into the pseudocoel along the middorsal, midventral, and midlateral lines of the body (Fig. 9-15). The bulging epidermis thus forms longitudinal cords that extend the length of the body. The epidermal nuclei are commonly restricted to these cords and are typically ar-ranged in rows.The muscle layer of the body wall is composed entirely of longitudinal, obliquely striated fibers arranged in bands, each strip occupying the space between two longitudinal cords. The fibers may be relatively broad and flat, with the contractile filaments limited to the base of the fiber, or they may be relatively tall and narrow, with filaments at the base and sides. In both types the base of the cell containing the contractile fiber is located against the hypodermis, and the side of the cell with the nucleus is directed toward the pseudocoel (Fig. 9-16). Each nematode muscle fiber possesses a slender arm that extends from the fiber to either the dorsal or the ventral longitudinal nerve cord, where innervation occurs (Fig. 9-15). In most animals a nerve process extends from the nerve cord to the main body of the muscle. The nematode pseudocoel is spacious and filled with fluid. No free cells are present, but fixed cells, located either against the inner side of the mm layers or against the wall of the gut and the inte organs, are characteristic of many nematodes.


Body Wall of Polychaeta.

The polychaete epidermis, or integument, is composed of a single layer of cuboidal or columnar epithelium, which is covered by a thin collagen cuticle (see Fig. 10-9 for evolution of cuticle). Mucus-secreting gland cells are a common component of the epithelium.

Beneath the epithelium lie, in order, a layer of circular muscle fibers, a much thicker layer of longitudinal muscle fibers, and a thin layer of perito-neum. Although the muscles of the body wall essentially comprise two sheaths, the longitudinal fibers typically are broken up into four bundles—two dorsolateral and two ventrolateral.

Within the spacious coelom, the gut is sus¬pended by septa and mesenteries. Thus, each coelomic compartment is divided into right and left halves, at least primitively. However, the septa have partially or completely disappeared in many polychaetes.

Body Wall and Coelom of Oligochaeta.

The structure and histology of the oligochaete body wall, especially in terrestrial species, is essen¬tially like that of burrowing polychaetes. A thin cu-ticle overlies an epidermal layer, which contains mucus-secreting gland cells. Circular muscles are well developed, and the septa partitioning the coelom are relatively complete. Earthworms, which have the best developed septa, may possess sphincters around septal perforations to control the flow of coelomic fluid from one segment to another.

In most earthworms each coelomic compartment, except at the extremities, is connected to the outside by a middorsal pore located in the intersegmental furrows and provided with a sphincter. These pores exude coelomic fluid, which aids in keeping the integument moist. When disturbed, some giant earthworms squirt fluid several centimeters.

Body Wall and Coelom of Hirudinea.

The body wall contains a more distinct connective tissue dermis than is present in other annelids, and some of the unicellular gland cells of the integu-ment are very large and sunken into the connective tissue layer (Fig. 10-63). The longitudinal muscle layer of the body wall is powerfully developed, but there are also circular, oblique, and dorsoventral muscle fibers.