Question

The main features of platyhelminthes. How can platyhelminthes be described according to examples of representative species, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction?

Answer 1

The phylum Platyhelminthes consists of simple worm-like animals called flatworms. The name Platyhelminthes is derived from the Greek root word platy meaning flat and the Greek root word helminth meaning worm. Flatworms live on land, in fresh water, in the ocean, and in or on other animals as parasites (e.g., tapeworms). Parasitic flatworms that live on or inside other animals—including humans—can injure or even kill the host organism. Free-living non-parasitic flatworms are typically less than 10 centimetres long. Marine species live buried in the sand or under rocks in shallow water. All free-living flatworms are predators that actively hunt for food. Some live symbiotically with crabs, clams, oysters, shrimp, and barnacles. Some marine flatworms are brilliantly coloured while others are drab and blend into the environment. Examples are free-living marine flatworm Maritigrella fuscopunctata, trematode flukes Schistosoma mansoni, tapeworm Taenia saginata, marine flatworm Pseudobiceros fulgor, freshwater planarian flatworm Dugesia subtentaculata, yellow papillae flatworm (Thysanozoon nigropapillosum) etc. They have a bilateral type of symmetry.

Coelom

Their flat shape comes from the fact that they are acoelomates (having no coelom, or fluid-filled body cavity).

Germ Layers

Flatworms are more complex than cnidarians. Cnidarians have two layers of cells, the ectoderm and the endoderm; flatworms have a middle layer called the mesoderm between the other two layers. This extra layer is important because its cells specialize into a muscular system that enables an animal to move around. Beginning with the flatworms, all the animals subsequent to them have a mesoderm and muscular system. The cells of the ectoderm and endoderm are also more organized than similar cells of cnidarians.

Digestive System

Flatworms have a digestive system with only a single opening into the digestive cavity, but in independently living marine flatworms the cavity branches into all parts of the body. These flatworms feed through a pharynx. A pharynx is a long, tubular mouthpart that extends from the body, surrounds the food, and tears it into very fine pieces. Cells lining the digestive cavity finish digesting the food. Then the dissolved nutrients move to other cells of the body. Undigested food passes back out through the mouth, as in the cnidarians. Parasitic tapeworms usually absorb their nutrients directly from the host, while parasitic flukes have retained a digestive system.

Nervous System

Like most self-propelling animals, independent-living flatworms have a central nervous system. A central nervous system consists of a mass of nerve cells, called a ganglion, (in more complex organisms, the ganglion evolves into a brain) in the anterior part of the body, and a nerve cord extending from the brain toward the posterior end of the body. Sensory cells in the head detect changes in the environment. In free-living flatworms, sensory cells that respond to light are clustered in two eyespots in the head. Sensory cells that detect water currents, solid objects, and chemicals are in two flap-like projections on the head called auricles. In self-propelling animals, these sensory organs in the head are the first part of the animal that encounters new surroundings. The ganglion receives information from the sensory structures and sends signals to other parts of the body along two strands of nerve cells running toward the tail. Because the nerve strands are connected by cross-strands in the shape of a stepladder, this kind of nervous system is often called a “nerve ladder.”

Excretory System

The excretory system removes waste products and excess water from tissues of flatworms. Flatworms have a surprisingly elaborate system to rid the body of wastes. This network runs the length of the animal on each side and opens to the outside through small pores in the posterior region of the body. Connected to the tubes are tiny cells that move wastes and water from the tissues into the tubes. These cells contain flagella that beat back and forth, creating a current of fluid that constantly moves toward the excretory pores. Under a microscope, the flagellar movement looks like a flickering fire, and the structure is called a flame bulb.

Circulatory System

Flatworms have no circulatory system. Animals without a circulatory system have limited abilities to deliver oxygen and nutrients to their body cells because of the way that molecules behave. As molecules spread through water, they become less concentrated as they move away from their source. This is known as diffusion. A ball-shaped marine animal would not get adequate oxygen and nutrients to its innermost cells because the cells are too far from the body’s surface for molecules to move (diffuse) to them. But cnidarians have no problem with diffusion because most cells of their bag-shaped bodies are in direct contact with the water, making the exchange of oxygen and nutrients easy. Flatworms, bag-shaped but flattened, also get oxygen and nutrients to their body cells easily because all their cells are close to either their outer surface or their digestive cavity. As animals become larger and more complex, diffusion is often no longer an option, and then we begin to see the development of circulatory and respiratory systems.

Respiratory System

They take in oxygen but have no formal respiratory system. This is because they are very simple animals. They went through such small evolution that they never developed a complex respiratory system. They breathe through their skin which they have to keep moist which is why most live in water or watery environments.

Reproduction

Just like other bodily systems, the Platyhelminthes reproductive system is very simple to understand.

Asexual Reproduction
The commonly observed Platyhelminthes reproduction is asexual type. Several species of the nonparasitic flatworms undergo transverse fission, in which a single organism splits up into smaller fragments through transverse division. Since flatworms have the ability to regenerate themselves, the lost parts develop and each of the fragment grows into a separate individual. Once, they enter the host organism, they travel to the intestinal tract and start feeding on the partially digested foods. Very soon, they break up into smaller fragments, and each single piece then develops into an individual worm later.

Sexual Reproduction
According to studies conducted on Platyhelminthes breeding, it is found that majority of the species are simultaneous hermaphrodites (or monoecious). In other words, both the male and female reproductive organs are present in a single flatworm. This allows a platyhelminth to undergo sexual reproduction on its own. The actual mode of reproduction may differ slightly from one species to another. For all flatworms, fertilization is of the internal type, and the male and female gametes fuse inside the body. Some Platyhelminthes release cocoons filled with eggs, which later hatch into young worms that are similar to the matured ones.

In other cases of Platyhelminthes (e.g., tapeworm), reproduction is preceded by self-fertilization in a single proglottid (segment containing both male and female reproductive structures) or between two proglottids. On the other hand, there are other flatworms that require cross-fertilization for reproduction. Based on the species, fertilized eggs are released from the host organism through faeces, or the proglottid filled with eggs is shed as a whole from the host body. The eggs encased in an outer shell remain as they are until a prospective intermediate host ingests them. Inside the host body, they hatch to larvae and life cycle continues again.