PHYLUM PLATYHELMINTHES (FLATWORMS)

There are over 20,000 species of Platyhelminthes, of which most are parasitic or free-living in nature. They are also commonly known as flatworms (unsegmented) and have ribbon-like structures. They are divided into four different classes known as Turbellaria (free-living flatworms), Monogenea (Monogenetic flukes), Trematoda (Digenetic & Aspidogasterean flukes) and Cestoda (Tapeworms).

General Characteristics

All flatworms share similar general characteristics. They are triploblastic (3 cell layers; outer ectoderm, inner endoderm and mesoderm) and they have specialized tissues to perform various functions. They are also bilaterally symmetrical (flattened dorsoventrally) meaning that the left and right side of their bodies are mirror images of each other and they have distinctive head and tail ends.

Flatworms are also acoelomate meaning that they do not have any body cavity. Thus the organs within the body are in direct contact with the epithelium.

Another general characteristic is that these flatworms lack special circulatory or gas exchange system. How then, is it able to survive with such a simple body plan? By having a flat body (small diffusion distance), it allows easy gaseous exchange through diffusion across body walls between tissues and the environment.

However, there are also limitations to this great body adaptation. Due to its flat bodies, it increases the body surface area for gaseous exchange, but it also creates many problems such as desiccation (drying). Therefore, platyhelminthes are typically found only in moist areas.

Organ Systems

A) Nervous System

The nervous system of the platyhelminthese is made up of nerve tissues and nerve chords. The cerebral ganglions are located at the head of platyhelminthes that can be called ‘brain’. From this nerve tissue, the nerve chords are then separated into two where it sends projection to the tip of the worm. At the same time, there are also other nerve branches that branched out from the main body and these branches are connected to sensory organs which detect sight, touch and movement. To illustrate, the simple diagram on the right shows the nervous system of platyhelminthes.

B) Excretory System

Platyhelminthes do not have any anus for excretion. How are they then able to excrete the waste from their body?

Although they do not an anus, they do have flame cells (enlarged cells) which have tube-like structures called protonephridia (excretory pores) to excrete out the wastes in the body.

Watch this video on flatworms to get a general view of this phylum! Phylum Platyhelminthes .Generally, platyhelminthes are divided into 4 classes namely Turbellaria, Monogenea, Trematoda and Cestoda.

Class Turbellaria

These classes of unsegmented flatworms are mostly aquatic and are free-living (ie, they are not parasitic). Beneath their epidermis (“skin layer”), there is a thick basement membrane (circled in red in diagram below) that lends structural support to the body. Within these basement membranes are smooth muscle cells that facilitate movement.

Within the epidermal structures, there is also a rod-shaped structure called Rhabdoids (circled green). Rhabdoids is used to produce mucus that prevents the flatworms from drying and protect it from predators.

Digestive Tract

These flatworms from this class Turbellaria have an incomplete digestive tract, where the mouth leads to pharynx, then to blind intestines (enteron) or simply put, a temporary space that contain cells which ingest food particles by phagocytosis (engulfing the food). There are various types of pharynx, which is often used for classification. The pharynx ranges from simple to complex structures and shapes. For simple pharynx, the tubellarians feed on the food by simply sweeping the food particles and tiny prey by ciliary action. For the more complex pharynx, the tubellarians will fold its body around the prey and surround it with mucus after which the pharynx is everted over the prey. Most tubellarians would swallow their food by using their powerful pharyngeal muscles.

Locomotion

For the Turbellarian to move, it secretes mucus to provide lubrication as it moves along and serves as a medium against which cilia act. The turbellarians then glide along the slimy trails and the ciliated epidermis facilitate the locomotion. You can watch a video of the Turbellarian moving here Turbellarian Locomotion

Reproduction

There are two types of reproduction; asexual and sexual reproduction.

Asexual Reproduction
Asexual reproduction is common among the freshwater and terrestrial turbellarians such as triclads, Dugesia. They reproduce asexually by transverse fission (split into two) and have great regenerative capabilities. This means that if a part of their body is cut into half, they are able to grow the lost body part wit

hin a period of time.

You can view these video on the regeneration of planaria here Regeneration of Planaria .

Sexual Reproduction

Most Turbellarians are hermaphroditic, meaning that they contain the male and the female sex organs. However, they are unable to self-fertilize and thus need the sperm of other turbellarians to fertilize the egg. Now, let us take a look at the male and female systems of the Turbellarians.

Male System

The male part consists of single, paired or multiple testes, sperm ducts, seminal vesicle and penis.

Female System

For the female system, the Turbellarians may choose to either produce an entolecithal (both egg and yolk produced) or ectolecithal (only egg produced, yolk produced in yolk gland transported through vitelline duct) ova. Eventually, the eggs will move through an oviduct towards the female antrum.

So, during sexual reproduction, the penis will penetrate e caused by muscles contraction and insert into the female antrum, depositing the sperms there. Fertilization will then take place when the egg passes through the oviduct into the female antrum.

Once the egg is fertilized, it is enclosed in a protective cocoon. The embryos of these fertilised eggs will then consumed the yolk stored in the egg. When the egg hatches, the larvae which resembles the adult, continue to be a free-living flatworm in the aquatic and continue the life cycle.