Phylum: Platyhelminthes

PHYLUM : PLATYHELMINTHES (Flatworms) 

Phylum Platyhelminthes contains about 13,000 species, mostly parasites that live in other animals including man.

The important characters of this phylum are:

1. The flat worms are mostly parasites but some are free living e.g., Planaria.

2. They are acoelomate, triploblastic, bilaterally symmetrical and dorsoventrally flattened animals.

3. They have organ system level of organisation.

4. Body is not segmented except in class Cestoda (false segmentation).

5. Body is covered with a cellular, syncytial, one layered, partly ciliated epidermis; while in parasitic trematodes and cestodes, epidermis is lacking and the body is covered with cuticle.

6. Exoskeleton and Endoskeleton are completely absent. However hooks, spines, suckers (in parasitic form), teeth or thorns may be present which act as adhesive organs.

7. The space between the body wall, alimentary canal and other organs is filled with a peculiar connective tissue called the parenchyma. It helps in transportation of food materials.

8. Digestive system is totally absent in tapeworms, so they absorb food directly through body surface. In Trematoda and Turbellaria, it consists of mouth, pharynx and blind intestine (anus absent).

9. Respiratory and circulatory systems are absent.

10. Excretory system consists of single or paired protonephridium with flame cells.

11. Nervous system is primitive. The main nervous system consists of a pair of cerebral ganglia or brain and one to three pairs of longitudinal nerve cords connected to each other by transverse commissures. This type of nervous system is called ladder like nervous system e.g., Planaria (Ougesia).

 

12.    Sense organs are of common occurrence in Turbellaria but these are greatly reduced in parasitic forms. 

13.    Sexes are united, i.e., hermaphrodite with very few exceptions like Schistosoma (Blood fluke). 

14.    Asexual reproduction by fission occurs in many fresh water forms like Turbellarians. 

15.    In majority of forms, eggs are devoid of yolk but provided with special yolk cells and are covered by egg shell. 

16.    Cross fertilization in trematodes and turbelleria and self-fertilization in cestodes is very common. Fertilization is internal. Development indirect with many larval stages. 

17.    Life cycle complicated, involves one or more hosts. 

18.    Regeneration. It is well marked in some flat worms like Planaria.

CLASSIFICATION

• Phylum platyhelminthes is divided into three classes.