General characteristics of phylum platyhelminthes (or flatworm) are discussed below:

1) Platyhelminthes are commonly called as flatworm.

2) Their body is bilaterally symmetrical and dorso-ventrally flattened.

3) Body colour of majority of flatworms are white colourless, while the free-living forms are brown, grey, black or brilliantly coloured.

4) They are small to moderate in size varying from microscopic to extremely elongated forms measuring upto 10-15 metres.

5) Anterior end of their body is differentiated into head.

6) Ventral surface of their body contains mouth and genital pores, which is well marked in turbellarians and less marked in trematodes and cestodes.

7) Hooks and suckers are present in them as an organ of attachment and adhesion.

8) They are acoelomate.

9) They are either free-living or ectocommensal or endocommensal or parasitic.

10) The body of some of the platyhelminthes (such as: turbellarians) is covered is with a ciliated epidermis, while in trematodes and cestodes the body is covered with cuticle.

11) Their body lacks skeleton and so their body is soft.

12) Spines, thorns, teeth, etc. are present in them.

13) Body space between various organs of their body is filled with mesenchyme usually called as parenchyma.

14) Their digestive system consists of mouth, pharynx and intestine.

15) Respiratory and circulatory systems are absent in them.

16) Excretory system in them consists of protonephridia with flame cells.

17) Their nervous system is primitive.

18) They are mostly hermaphrodite (or bisexual) with very few exceptions.

19) Their reproductive system is highly evolved and complex.

20) They can reproduce in both sexual and asexual methods.

21) Fertilization in them is internal.

22) Their life cycle is complicated and involves one or more hosts.

Classifications of platyhelminthes:

Class- 1: Turbellaria

1. They are mostly free-living, but some are either ectocommensal or endocommensal or parasitic.
2. Their body is covered with ciliated epidermis.
3. Their body is unsegmented.
4. They have large number of adhesive organs, which are of two types: glandula epidermal adhesive organs and glandula muscular adhesive organs.
5. Their digestive system consists of mouth, pharynx and intestine except in Acoela where intestine is absent.
6. Their excretory system consists of protonephridia with terminal flame cells.
7. Sense organs in them consist of tangoreceptors and chemoreceptors.
8. They are mostly hermaphrodite with very few exceptions.
9. Reproduction in them is sexual, asexual and by regeneration.
10. Their life cycle is simple.

Examples: Canvoluta, catenula, Mesostoma, etc.

Some orders of class turbellaria are given below:

Order- 1: Acoela

Order- 2: Rhabdocoela

Order- 3: Alloecoela

Order- 4: Tricladida

Order- 5: Polycladida

Class- 2: Trematoda

1. They are either ectoparasitic or endoparasitic.
2. Their body shape is usually leaf-like.
3. Their body is unsegmented.
4. Their body is covered with cuticle.
5. They have well developed sucker.
6. Their digestive system is complete and consists of mouth, pharynx and intestine.
7. Their excretory organs are protonephridia.
8. They are mostly hermaphrodite.
9. They have single ovary but their testes are two to many in numbers.
10. They life cycle may be simple or complicated.

Examples: Gyrodactylus, Aspidogaster, Fasciola, etc.

Some orders of class trematoda are given below:

 Order- 1: Monogenea

 Order- 2: Aspidobothria

 Order- 3: Digenea

Class- 3: Cestoda

1. There are endoparasitic.
2. They are commonly called as tapeworms.
3. Their body is covered with a cuticle.
4. Their body is segmented.
5. Anterior end of their body contains adhesive structures.
6. Digestive system is completely absent in them.
7. Their excretory system consists of protonephridia with terminal flame cells.
8. Their nervous system consists of a pair of nerve ganglia and two lateral longitudinal nerve cords.
9. They are exclusively hermaphrodite.
10. Their life cycle is complicated.

Examples: Amphilina, Gyrocotyle, Taenia solium, etc.

Some orders of class cestode are given below:

Subclass- 1: Cestodaria

    Order- 1: Amphilinidea

    Order- 2: Gyrocotylidea

Subclass- 2: Eucestoda

    Order- 1: Tetraphyllidea

    Order- 2: Diphyllidea

    Order- 3: Trypanorhyncha

    Order- 4: Pseudophyllidea

    Order- 5: Cyclophyllidea

Also Read

• Characteristics and classification of Phylum Porifera with examples
• General characteristics and classification of phylum Nematodes/ Nemathelminthes/ Aschelminthes