What are Platyhelminthes?

Platyhelminthes are also known as "flatworms" because as they are dorsoventrally flattened from head to tail. They are invertebrate, soft-bodied, acoelomate, unsegmented and have bilateral symmetry.

Characteristics of Platyhelminthes

• Platyhelminthes are a dorso-ventrally flattened invertebrate animal.
• They are mainly free-living animals, but some are parasites and commensal too.
• Platyhelminthes are hermaphrodite animals. They move with the help of longitudinal, circular, and oblique layers of muscle.
• They do not have any respiration organs. 
• They respire by simple diffusion through the body surface.
• The digestive tract is absent.
• Platyhelminthes have regeneration abilities.
• Flame cells are present in these animals for excretion.
• The nervous system in this animal consists of the brain and two longitudinal nerve cords.
• These worms are of two types: parasitic and non-parasitic.

Exclusive Characteristics of Platyhelminthes

• Presence of flame cells: these are the specialized excretory cells in flatworms. These cells function like a kidney to excrete unwanted material from the body. Flame cells have cilia which assist in propelling waste material from the body.
• Ladder-like nervous system: This type of nervous system is only found in Platyhelminthes and it names so because of the nerves connecting the nerve cords. The Platyhelminthes have both the central nervous system that contains the brain and the peripheral nervous system that has pairs of nerves.
• Presence of parenchyma in the body cavity: one of the most distinguished features of Platyhelminthes is the presence of parenchyma that separates internal organs from their body wall.
• Self-fertilization: These worms are able to self-fertilize as they have both male and female sex organs in the same individual. This mode of reproduction allows them to produce many offspring at a single time.

Platyhelminthes Classification

The Platyhelminthes are divided into three main classes which are given below:

• Turbellaria
• Trematoda
• Cestoda

Turbellaria Characteristics

• This class of flatworms is all free-living and therefore suckers and hooks are not present.
• Turbellaria worms live majorly in freshwater.
• The ciliated epidermis is present in these worms.
• This flatworm life cycle is simple.
• Reproduction mainly sexual but sometimes asexual and regeneration take place. 
• The excretory organs are flame cells.
• The worms of the turbellaria class have sense organs like tango, chemo, and photoreceptor.
• The example of this class of worms includes Convoluta, Microstomum. 

Trematoda Characteristics

• This class mainly consists of parasitic flatworms.
• Liver fluke is the collective name of a polyphyletic group of parasitic flatworms.
• The fluke mainly lives in the liver of mammals.
• Parasitic flatworms complete their life inside the host.
• Suckers are present but no hooks are present.
• These worms are digenetic. It means they require two hosts to complete their life cycle.
• Their structure varies from leaflike to cylindrical.
• The body is unsegmented.
• The body is covered with cuticles in fluke.
• Have thick teguments without cilia and rhabdites.
• The digestive tract is incomplete in fluke.
• Flame cells are present for excretion.
• These worms are majorly hermaphrodites.
• Three pairs of longitudinal nerve cords.
• Examples includesPolystoma, Fasciola, and Schistosoma.

Cestoda Characteristics

• These class worms are commonly known as tapeworms.
• Have worms that are exclusively parasites.
• These worms are parasitic as they live inside multiple hosts.
• Both suckers and hooks are present in these parasites.
• The body is divided into proglottids of Cestoda worms.
• Have teguments without microvilli.
• The body is covered with cuticles of these parasites.
• These worms' bodies lack epidermis and cilia.
• Tapeworms lack a digestive system.
• Body shape is mainly ribbon-shaped.
• The life cycle could require more than 2 hosts to complete their life.
• Absence of mouth and digestive system.
• The pair of ganglia and 2 lateral longitudinal nerve cords comprise the nervous system of these worms.
• This Cestoda class flatworms are parasites in the digestive tract of all vertebrates. 
• Examples includeGyrocotyle, Myzophyllobothrium, Echinobothrium.

Life Cycle of Taenia Saginata and Taenia Solium

• Taenia saginata worm is also known as beef tapeworm and Taenia solium is known as the pork tapeworm.
• It is the most common type of tapeworm infecting humans.
• Infection in humans occurs because of eating undercooked beef and undercooked pork.
• This worm belongs to the order Cyclophyllidea.
• This worm lives in the intestine of humans.
• Taenia saginata also infect cattle and Taenia solium infects pigs.
• The disease caused by these worms is known as taeniasis.
• These worms are hermaphrodites.
• Each proglottid of Taenia saginata has a complete set of both male and female reproductive organs.
• Reproduction is by self-fertilization in case of these worms.
• These worms require two hosts to complete their life cycle.
• Cattle are the intermediate host of T.saginata worm and pig is an intermediate host of T.solium whereas the human is the definitive host of both the worms.
• Humans get infected when they eat undercooked beef.
• Eggs and proglottids of this worm passed with feces of infected humans to the external environment.
• These worms remain in the environment for some days to months.
• When cattle eat vegetation infected with these feces the worm enters the body of cattle.
• In the cattle intestine, the eggs hatched into the oncospheres which invade the intestinal wall and move to the striated muscles, where they develop into cysticerci.
• A cysticercus could live for many years in the cattle.
• When humans eat undercooked beef, this cysticercus passed into the human intestine and develops into an adult tapeworm.
• This adult tapeworm attaches to the small- intestine of humans with the help of their scolex.
• This adult Taenia saginata produce proglottids that further develop into gravid which move to the anus and passed through stool to the external environment.
• The larval forms of these worms survive in cattle, pig and adult form survives in humans.

Marine Flatworms

Flatworms or Platyhelminthes that live in marine water are also known as “marine flatworms or polycladida”. These flatworms are found in the different zones of water bodies from the littoral zone to the sublittoral zone. The marine flatworms belong to the class Rhabditophora. These worms are distinguished from other Platyhelminthes having folded pharynx, many ocelli, and an elongated intestine. Many of these worms hide away from the light but few of them prefer to be active during the day. These worms remain associated with other invertebrates like sponges and corals for their food and protection.

Acoela

This is an order of small and simple invertebrates and because of their simple organization, these animals are considered to be belonging to the flatworms. But according to the research conducted by Ruiz-Trillo and his fellow researchers, sequencing data of 18S ribosomal DNA genes of acoels disclose that this group does not fall under the Platyhelminthes phylum but belongs to the extant members of the earliest divergent bilateria.

Context and Application

• For a B.sc student: the study of invertebrates is very useful to the evolution of animals for biology students. As invertebrates were the simplest animal, the gradual evolutions of these animals help students to understand the evolution of various parts and organs in modern animals.
• For M.Sc student: the study of flatworms helps the post-graduate students to study the development of exclusive features in them which allow them to live in diverse environmental conditions.