Phylum Platyhelmintes-Nematodes and Rotifera, Biology tutorial


Animals under phylum Platyhelminthes are soft bodied bilaterally symmetrical animals commonly called as flat worms that number over 15,000 species. Gelatinous mesoglea of cnidarians is replaced by mesodermal, cellular, parenchyma, kind of packing tissue comprising of more cells and fibres than mesoglea. Another characteristic in Platyhelminthes is evolution of more elaborate and clear 'organ systems' for first time - protonepluidial feeding comprising reproductive, nervous, sensory etc.

Features of Phylum Platyhelminthes:

i) Bilaterally symmetrical, having anterior and posterior ends.

ii) Body dorsoventrally flattened.

iii) Triploblastic - having three germ layers.

iv) Acoelomate - no internal body cavity. Space between organs is filled with form of connective tissue known as parenchyma derived from mesenchyme.

v) Digestive system absent in some; if present contains only mouth but no anus.

vi) Nervous system ladder-like, having simple sense organs.

vii) Do not have respiratory, circulatory or skeletal systems.

viii) Hermaphrodites, with complex reproductive system

ix) Eggs show spiral cleavage, which may be highly modified.

x) Developments usually direct in free living forms; in some there is a freeswimming larva (Muller's larva or Gotte's larva). In certain parasites development may be much complicated involving many larval stages in the life cycle.

Categorization of Phylum Platyhelminthes:

Phylum Platyhelminthes is split into 4 classes: Turbellaria, Monogenea, Trematoda and Cestoda.

1) Class Turbellaria: These are generally free living and aquatic. Few are terrestrial, confined to humid areas. Body of these animals is enclosed with ciliated epidermal cells having rhabdoids. Mouth is on ventral side. These animals range from few millimeter to few centimeter. They move through cilia covering body. Undulations of body also assist in locomotion of larger forms. Cilia may be missing dorsally. If you disturb animal rhabdoids are ejected. So they are defensive in function. The loose kind of connective tissue, parenchyma-composed of aggregation of irregular cells, packs space between internal organs. Apart from having well-developed reproductive system, turbellarians possess great capacity for asexual reproduction particularly by transverse binary fission.

2) Class Monogenea: Body of the animals is enclosed with non-ciliated syncythrm tegument. They are leaf similar to cylindrical. They are parasitic, generally on skin or gills of fish. For this they contain posterior attachment organs in form of hooks, suckers; clamps etc. Their growth is direct, with only one host. They have generally a free-swimming ciliated larva. These are monogenetic flukes. Adult does not have sense organs. Examples Gyrodactylus, Polystoma, a parasite in urinary bladder of frogs.

3) Class Trematoda: Body of the animals is also leaf-like to cylindrical enclosed with non-ciliated syncitium tegument. Though, they contain oral sucker and ventral sucker, but do not have hooks. Development is indirect. Definitive host is vertebrate. First host or intermediate host, in which asexual reproduction takes place, is mollusk. These are digenetic flukes, endoparastic, several of them causing diseases in humans and domestic animals.

4) Class Cestoda: These are tape worms, parasitic in digestive various vertebrates. In these animals also, body is enclosed with not syncytial tegument, but body is tape-like, with the anterior scolex carrying suckers and hooks for attachment to host tissues. Body is also separated into number of proglottids. Example is Taenia. As these are found in digested food mater host, although these animals contain no mouth or gut, food can directly be absorbed through highly altered tegument.

Pseudocoelomata - phylum nematoda:

Body cavity of Pseudocoelomata is pseudocoel. It is original blastocoel of embryo enduring between alimentary canal and body wall. It isn't lined by mesodernal peritoneal lining. This lining is characteristic of true coelom body cavity of coelomates. Pseudocoelonmates include the given phyla: Gastrotricha, Kinorhyncha, Loricifera, Priapulida, Nematomorpha, Acanthocephala, Entoprocta, Nematoda and Rotifera. Phylum Nematoda and Phylum Rotifera are explained below:

Phylum Nematoda:

Phylum Nematoda are commonly called as round worms. The very successful group animals with approx 12,000 species known, but unknown species are expected to far outnumber these (about 5, (00,000!).


i) Vermiform body bilaterally symmetrical, but with the tendency for radial symmetry along longitudinal axis. Cross sectional area circular: no segmentation or appendages.

ii) A complex cuticle present.

iii) Body contains more than 2 cell-layers: tissues and organs present.

iv) Circular muscles absent in body wall.

v) Body cavity is pseudocoel with body fluid at high pressure.

vi) Gut extending from mouth at anterior end to anus which is subtennina Muscular pharynx.

vii) No circulatory system. No flame cells or nephridia. No cilia or flagella. Excretory tubules in one or restricted number of renette cells.

viii) Highly determinate kind of cleavage; Development direct.

Categorization of Phylum Platyhelminthes:

Categorization of Phylum Nematoda in classes is based on characters that are difficult for non-specialists to differentiate. Phylum is divided into 2 classes: I) Class Phasmidia (Secementea), 2) Class Aphasmidia (Adenophorea).


Ascaris lumbricoides is human intestinal round worm. Ascaris megalocephala is found in intestine of horses. Ascaris female may lay approx 2, 00,000 eggs per day that pass out along stools of host. Eggs stay alive for years, in soil. They gain entry in host alimentary canal through contaminated food and there small juveniles come out of eggs. Then they burrow through intestinal wall and enter veins and lymph vessels, from where they are carried to heart and from there to lungs. There they penetrate alveoli and enter trachea.

Pseudocoelomata - phylum rotifer:

These are small animals bearing ciliated crown. When cilia beat, crown has appearance of rotating wheel. They generally range from 100-500 mm, and are cosmopolitan in distribution, although only approx 1800 species are known. Most of them are fresh water forms. Few are marine, few are terrestrial epizoic living on body of other animals, or even parasitic.

Some characteristics are:

i) A pre oral and post oral band of cilia in form of crown at anterior part of body. Crown provides form of rotating wheel when cilia beat, from which animal derives its name.

ii) Alimentary canal with mouth, jaw apparatus, muscular pharynx. The posterior anus, opens into Cloaca.

iii) Epidermis contains intracellular cuticle. This is frequently thickened to form lorica.

iv) protonephridia present

v) Body cavity pseudocoel.

vi) No circulatory or respiratory system.

vii) Development direct, with modified spiral cleavage.

Categorization of Phylum Platyhelminthes:

Phylum Rotifera is split in three classes.

1. Class Seisonida. Marine, lengthened, having vestigial corona, sexes identical in size and form, female with the pair of ovaries and no vitellaria. E.g. Seison, epizoic on gills of crustacean Nebalia.

2. Class Bdelloidea. Swimming or creeping, retractile interior end, Corona having two trochal discs, males unknown. Parthenogenetic. Two ovaries and vitellaria. E.g.: Philodina

3. Class Monogonata. Swimming or sessile, sole ovary and vitellarium. Males smaller, Eggs 3 kinds: diploid, haploid and dormant. E.g. Asplanchna.

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