Phylum PlatyhelminthesLecturer

Zoology DepartmentMay 31, 2011

contentIntroductionWhere do they liveDo they have any advancement in their

body structure?General characteristicsClassificationParasitic adaptationsReferences

Introduction

ETYMOLOGYPlatyhelminthes also known as ‘Flat worms

"are a phylum of triploblastic , acoelomate invertebrate animals.

The word platyhelminthes comes from the Greek words platy meaning “flat” :and helminthes meaning “worm”.

Probolitrema ). Flukes,

polycladids marine flatworms

Planaria

An ancient phylumPlatyhelminthes are an ancient phylum.Nothing is known of their evolutionary history. Scientists believe that the first turbellarians

evolved around 550 mya (million years ago).The platyhelminthes are a successful phylum

with around 25,000 known species .

Dugesia

Where do they live

Free living

Parasitic

Aquatic terrestrial ecto endo

Marine Fresh water

Platyhelminthes

Do they have any advancement in their body structure?Although platyhelminthes are morphologically very simple but

the show many advancement over the other lower phyla.Congregation of Sensory organ.They also have distinct upper and lower (dorsal and

ventral) body surfaces. The evolution of connective tissue, called parenchyma, the

cells of which serve as storage reservoirs as well as protecting the internal organs, is a major step forward toward the more complex body plans of higher animals, such as humans.

However they still no anus, instead they have only a blind ending gut, or no gut at all. Those species with a gut must therefore excrete there digestive waste products through their mouths.

General CharacteristicsUnsegmented worm like The animals of Phylum Platyhelminthes are worm like animals ,

dorsoventrally flattened with soft and unsegmented bodies, resemble ribbons.

Dorsoventrally flattened Their flattened shape helps them to obtain oxygen and nutrients by diffusion

through their body membranes. Some of these species are carnivorous, while the rest are parasitic.

Bilateral symmetryThese worm like animals are bilaterally symmetrical, which means that their right and left sides are the mirror images of each other. This also indicates that these animals have distinct head and tail ends.

Triploblastic They are triploblastic, that is

they possess three main cell layers: outer ectoderm, middle mesoderm and inner endoderm just like the other bilateral animals, with organs and organelles. The middle layer in these animals is made up of spongy mesoderm cells and is also called parenchyma.

AcoelomateUnlike the other bilateral animals, these animals possess no internal body cavity. Coelomate means body cavity present and since body cavity or coelom is absent, they are termed as acoelomate.

Acoelomate

Epidermis: Epidermis soft and ciliated(Turbellaria),

or covered with cuticle and with external suckers or hooks, or both for connection to host (Trematoda, Cestoda).

some spp. epidermis is syncitial

Having one/few epidermal cells having many nucleus. Usually elaborating cilia in many spp. Aid in locomotion and protection.

Rhabdites - specialized epidermal cells and extracellular structures of many flukes, which aid in locomotion.

Syncytial tegument - extra epidermal

layer of one/few multinucleated cell(s) w/ protective covering. In many flukes.

Same cellular extra epidermal structures and epidermis. One nucleus/cell. Multicellular arrangement of epidermis and sometimes ciliated structures.

Epidermis

Absence of circulatory and respiratory organsThe animals of platyhelminthes lack circulatory and respiratory organs, which is why their bodies are so flat. The flat body shape permits the flow of oxygen and nutrients to reach all parts of the body by simple diffusion process. Carbon dioxide leaves the body by the same diffusion process. The gut of these animals is branched profusely in order to facilitate adequate diffusion of nutrients to all parts of the body.

Restricted to wet environmentsRespiration occurs throughout the surface of the body, thereby making the Platyhelminthes animals susceptible to loss of precious body fluids. This can result into dehydration. Thus these animals are restricted to living in wet environments such as in freshwater, sea or in moist terrestrial environments. The different moist terrestrial environments include grains between the soil, in leaf litter or living as parasites in the bodies of other insects.

SkeletonMesenchyme, a connective tissue is seen to fill the space between the skin and gut in these animals. It consists of two kinds of cells: fixed cells and stem cells. Fixed cells have fluid filled vacuoles and stem cells are the cells capable of transforming into any other type of cell, thereby useful during the tissue regeneration and asexual reproduction. This mesenchyme is reinforced by collagen fibers that provide attachment points for the muscles, thereby acting like a skeleton. Mesenchyme comprises internal organs and permits the passage of nutrients, oxygen and waste products. DigestionThe digestive system is incomplete OR BLIND GUT with a single opening serving as the mouth and anus. A single layer of endodermal cells line the gut, whose role is to absorb and digest the food materials. Some species also feature the secretion of enzymes in the gut or pharynx to soften and break up the food. The undigested food material is regurgitated through the mouth as an anal opening is absent.

However, there are exceptions to this. Large species do possess an anal opening and some with exceptionally profusely branched guts possess more than one anal opening. This is because solely excreting from the mouth would prove difficult for them.

None in Acoela or Cestoda.

BLIND GUT

Excretory systemThe excretory system consist of a protonephredia.

Protonephredia comprises a network of tubules within the animal's body tissues. One end of the tubule extends into an exterior pore on the surface of the body, while the other end leads into spherical structure called flame cells.

These flame cells possess long cilia which carry out a beating function. When the cilia beats, it gives the flame cells an appearance of a flickering candle and this is where the cells get their name. Excess water and body wastes enter the flame cells, are pushed into the tubules by the movement of cilia and thrown out of the body from the pore on the surface. Excretory

system

OsmoregulationThe control or regulation of water balance and ion levels in the body is termed as Osmoregulation. Platyhelminthes animals live in the environments with high concentrations of dissolved materials. All animals have to maintain the concentration of dissolved substances at a constant level. These animals let their body tissues have the same concentration level as that of the environment with the help of protonephredia network.

protonephredia

Nervous systemNervous System marked at the head region is of the primitive type. It comprises pair of anterior ganglia or a nerve ring connected to 1-3 pairs of longitudinal nerve chords with transverse commissures. These ganglia serve as the brain.

Movement In some flatworms is controlled by

longitudinal, circular, and oblique layers of muscle. Others move along slime trails by the beating of epidermal cilia. The development of directional movement is correlated with cephalization.

In some flatworms, the process of cephalization has included the development in the head region of light-sensitive organs called ocelli.

Other sense organs found in at least some members of this group (not necessarily on the head) include chemoreceptors, balance receptors (statocysts), and receptors that sense water movement (rheoreceptors )

Nervous system

Reproductive systemThese animals can reproduce asexually and as well as sexually. They can reproduce asexually by transversal bipartition due to their wonderful ability to regenerate their tissues. They attach themselves to a substrate and induce a constriction in the body's mid region and get divided into two parts. The two divided body parts can regenerate into two different animal individuals.

Sexes These animals are mostly

hermaphrodite, that is both female and male reproductive organs are situated in one animal itself. Rarely animals with separate sex are found. Reproductive organs are highly developed.

Reproductive system

Fertilization Self fertilization (fusion of male and

female gametes from the same animal) as well as cross fertilization (fusion of male and female gametes from different animals of the same species) is seen.

Development The development is mostly direct,

which means that the development does not include larval stage formation (some Turbellaria and monogenetic Trematoda) or with one or more larval stages (digenetic Trematoda and some Turbellaria and Cestoda .

Eggs The eggs are microscopic, each

enclosed with several yolk cells in a shell.

An Egg

Some of the terrestrial and free living marine animals of this phylum are beautiful; however, most of them are unattractive to humans. A large number of these parasitic species infect man and cattle and some can even be fatal if not treated. The main diseases caused by the species of this phylum are Schistosomiasis, tapeworm disease (cestodiasis) and cysticercosis. However, the good news is that mostly all diseases caused by these worms can be treated by modern medicines.

Classification Phylum Platyhelminthes comprises 34,000

known species which are categorized under four classes:

Turbellaria Monogea Trematoda Cestoda

CLASSIFICATIONClass Turbellaria - common name Planaria - scientific name Dugesia sp. 1) Free-living 2) Mostly freshwater spp.   Class Monogenea 1) Mostly parasites of fish in skin/gills. 2) Free-living larval stages 3) Ciliated, mobile larvae 4) Some parasitic flukes in class   Class Trematoda - genus Fasciola spp. - Clonorchis spp. 1) Major parasitic flukes 2) Includes blood flukes and liver flukes.   Class Cestoda - tapeworms - Taenia spp. - pork tapeworm 1) All spp. are parasitic, which lack any type of digestive tract.  

PARASITIC ADAPTATIONS Have suckers & hooks so that they can attach to the

host Have a tough tegument to prevent being digested by

the host (tapeworm) or they have aCuticle (flukes) Absent or reduced digestive, circulatory, and muscular

systems as they are parasites living inside a host Produce 10’s to 1000’s of eggs to ensure survival Have a complex lifecycle which involves several hosts

and different environmental conditions which makes prevention difficult.

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