Phylum Platyhelminthes : General Characters, Classification and Example

Defination 

Platyhelminthes are un segmented , Acoelomate , Triploblastic, bilateral symmetrical invertebrate with  organ level of organisation .

General features 

• They are commonly known as flat worm because they have dorso-ventrally flattened body.

• These are mostly endoparasites found in animals including human beings. 

• They are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organisation. 

• Hooks and suckers are present as the parasitic adaptation inside the host body.

• Some of them absorb nutrients from the host directly through their body surface. 

• They have some Special cells called flame cells  or Protonephredia to help in osmoregulation and excretion. 

• Sexes are not separate. Fertilisation is internal and development is through many larval stages. 

• Some members  possess high regeneration capacity.

• They have a brain with two longitudinal ventral nerve cord.

• They Respire through the body surface.

• Digestive system is incomplete because a single opening is present which serve as anus or mouth both.

Examples of Platyhelminthes  

1. Planaria 

• Planaria are well known for its regeneration power. It is capable to form its body part.

• They are found in both aquatic ( fresh or salt water) and terrestrial habitat

• Their body is unsegmented, bilateral symmetrical and leaf life.

• Planaria move by gliding on mucus which is released by ciliated dermis .

• Eye spot and Auricle are sense organ in Planaria.

• It has a developed Intestine but Respiratory and circulatory system is absent. 

• Planaria is hermaphrodite and posses both male and female reproductive system in same individual.

• Planaria is carnivorous and some species are scavengers.

2. Taenia 

• Taenia is commonly called as Tape Worm.

• The body of Tape worm is elongated and divided into many segment  called Proglottids.

• The tapeworm body is divisible into three distinct parts anterior scolex or head, a short-un segmented neck, and a segmented strobila. 

• Scolex is present at the anterior end of body. It has four cup-like muscular suckers having radial muscles.

• Scolex has Rostellum which has chitinous hooks for the attachment with the intestinal mucosa of host.

• Behind the scolex, well developed neck is present which grows continuously and proliferates Proglottids by fisdion or budding

• The neck is followed by  the strobila and  form main body.

• Strobila consists of 800 to 1,000 segments or proglottids arranged in a linear series in a chain-like fashion.

• A proglottid is a unit  of the body enclosing a complete set of genitalia and surrounding tissue.

3. Fasciola 

• It is commonly called as liver fluke. Fasciola gigantica is longest liver fluke.

• Body of fasciola is also flat with come like and contain two type of sucker as oral and ventral sucker. 

• Alimentary canal of fasciola is incomplete.

• It is endoparasite in liver and bile duct of host.

• It requires two inter mediate host to complete its life cycle. These hosts are Snail and Fish.

• The life cycle include various larval stage like Miracidium, sporocyst, Redia, cercariae and Meta cercariae.

• It is hermaphrodite and has male and female sex organ in one individuals.