Phylum Platyhelminthes

1. They are dorsoventrally flattened and some are unsegmented worms. Most of them have mouth and gut with no anus.
2. They have flame cells in the mesoderm for excretion and osmoregulation.
3. They are hermaphrodites with a complex reproductive system, which prevents self-fertilisation, but favours cross-fertilization.
4. They are triploblastic acoelomate animals, which have three body layers (ectoderm, endoderm, and mesoderm) without a body cavity or coelom.
5. Some have cilia on their outer surface for locomotion, for example Planaria, others have cilia in larval stage, for example miracidium larva in flukes, but lack cilia in adult stage.
6. They exhibit bilateral symmetry.
7. They have a very simple nervous system, with two nerve cords which run down on either side of the body. They have two simple brains called ganglia, which are simple bundles of nerves.
8. Some have two eyespots that help in sensing the presence of light.

Common Platyhelminthes fall into three classes, namely class Cestoda, Trematoda and Turbellaria. Class Cestoda consists of endoparasite animals such as the tapeworms, including Taenia solium (the pork tapeworm), Dipylidium caninum (the dog tapeworm) and Taenia saginata (beef tapeworm). Class Trematoda comprises the flukes, for example Fasciola hepatica (liver fluke) and the blood flukes Schistosoma haematobium and Schistosoma mansoni. Class Turbellaria consists of the free-living worms such as Planaria. They are flat worms.

An adult tapeworm, for example Taenia, consists of a knob-like head or scolex, equipped with hooks and suckers for attachment to the intestinal wall of the host; a neck region; and a series of flat, rectangular body segments (or proglottids), generated by the neck. The chain of proglottids may reach up to 15 or 20 ft (4.6–6.1 m) long. Terminal proglottids break off and are egested in the faeces of the host. Nevertheless, the new ones are constantly formed at the neck of the worm. As long as the scolex and the neck are intact, the worm is alive and capable of growing. Rudimentary nervous and excretory systems run the length of the worm, through the proglottids. However, there is no digestive tract; the worm absorbs the host's digested food through its cuticle or outer covering.

Taenia exhibits various special adaptations to its parasitic mode of life as follows:

1. It has scolex (head) with hooks and suckers for fixation and attachment on the host's gut wall.
2. It lacks alimentary canal because it absorbs digested food materials from its hosts.
3. It has a thin and flattened body, which provides a large surface area for gaseous exchange and absorption of digested food.
4. Its body is covered by a living epidermal layer called the tegument which produces antienzymes to protect it from hosts' digestive enzymes.
5. It has a large number of proglottids, which ensures production of a large number of eggs, hence high chance of survival.
6. It can respire anaerobically; therefore, it is able to live under low oxygen concentration, for example in the host's gut.

Morphologically, an adult liver fluke, for example Fasciola, has a flattened leaf-shaped appearance. In the anterior part, there is a triangular projection with a mouth surrounded by oral suckers at its apex. Ventrally, at the base of the projection, there are ventral suckers; and between the two types of suckers, there is a genital pore. Posteriorly, there is a minute excretory pore. The body is enclosed in a tough cuticle, which is extended into backward-directed spines. Their muscle fibres have small glands with minute ducts. The mouth runs into the oesophagus which branches into two blind parts.

1. It has suckers that provide a means of attachment to the host.
2. It has tegument with spines that prevent it from being washed away by bile; and also help the parasite to erode liver cells.
3. The tough tegument and secretions from the glands prevent the worm from the effects of the host's toxins.
4. The parasite secretes enzymes that help it to penetrate the liver cells at various stages of its life cycle.
5. It has mechanisms and chemicals that suppress the actions of the host's digestive enzymes.
6. It has a high reproductive potential and different multiplication phases that balance its high mortality rate. They have two hosts, namely primary hosts (for example: sheep and humans) and secondary host (for example: fresh water snail called Lymnea sp.). They have several larval stages (miracidium, sporocyst, redia, cercaria and metacercaria) which increase the chances of survival and more perpetuation.