Aditya Puttigampala
Platyhelminthes (Kingdom Animalia)

Figure 1. Planarian anatomy (RK)
The marine flatworms (polycladids) are the largest of the free-living flatworms, sometimes reaching lengths of 15 centimeters. Polycladids get their name from their highly branched digestive cavity. (RW)

Diagnostic Characteristics Platyhelminthes is another name for flatworms, many of which contain species that are parasitic. Their name comes from the fact that they are so thin; their sizes can range from being microscopic to 20 meters long.

The Platyhelmithes phylum consists of three classes, two of which are entirely parasitic. They are Bilateria which means that they are symmetrical in a single way giving the Platyhelmithes definite front, back, right and left sides. Radial symmetry which entails a uniform distribution of body parts and shape may exist but is always of secondary origin. (ZJ- source 8)
Platyhelminthes are triploblastic meaning that they have 3 primary germ layers: the ectoderm, mesoderm, and endoderm. In comparison to cnidarians and ctenophores, Platyhelminthes are considered more complex organisms because their middle layer, mesoderm, contributes to more complex organ and organ system development.Platyhelminthes are also acoelomates, (no coelom) meaning that they lack a cavity between the body wall and the digestive tract. They also lack organs required for gas exchange and circulation.
Platyhelminthes have soft epidermis, often covered with cuticles, and external suckers/hooks to connect with the host. This characteristic is common in Trematodes and Cestodes. They have well developed muscle layers, but no body cavity; The space between the internal organs is filled with loose parenchyma. They have a nervous system: usually a pair of anterios ganglia or nerve ring connected to pairs of longtitudintal nerve chords with transverse commissures.(AR)
Reproduction: The reproductive systems in each sex has gonads, ducts, and accessory organs; Fertilization occurs internally. The microscopic eggs are enclosed in yolk cells within a shell. The life cycle of the Platyhelminthes can be direct like in some Turbellaria and Trematoda, have larval stages like in Cestoda, or asexual. (AR)
Unique feature: Hydrostatic Skeleton- closed body compartment with fluid held under pressure. The pressure of the fluid and action of the surrounding muscles in a flatworm are used to change its shape and produce movement.
Another unique feature of platyhelminthes is the fact that it doesn't have a body cavity and it has no circulatory/respiration system. Most animals, whether they be mammals or amphibians etc, have a way to breathe with a circulatory system that works in their environment. These flatworms get their oxygen and nutrients via diffusion, which means they flow freely in and out of their skin. (MM) This lack of body cavity is what makes the flat shape of the Platyhelmithes a necessity. Because flatworms respire via diffusion, no cell can be too far away from the environment (ZJ- source 8)
Platyhelminthes also have receptors in their skin for touch and chemicals. They are often clumped on the sides of the heads and provide senses. Much of the navigation and interaction with the environment is through these receptors along the platyhelminthes' entire bodies. (JS)

The Reproductive anatome of a planarian (PS

Acquring and Digesting Food: There is no singular way in which flatworms digest food. Some have digestive tracts, others do not. Certain flatworms have no mouth, and will obtain food and nutrients by absorbing it through its outer covering. Even the flatworms that do contain a gut do not have an anus. Any digested food (and waste) is expelled through the same opening that it originally entered through: the mouth.
Anyway, a muscular pharynx, behind the mouth, sucks food into the digestive tract. A digestive tract can be a simple sac, or highly branched, depending on the species. To digest food, flatworms use extracellular digestion; that is, cells secrete enzymes onto the food while it's in the tract to break it down, and then cells will perform phagocytosis. Phagocytosis is where a cell encloses a material by surrounding it with its cell membrane, and absorbing the nutrient into itself. (MB, Source #2)

They have an incomplete digestive system with a mouth, yet no anus in a branched fashion. There is no digestive system in Acoela and Cestoda. (AR)

When it comes to excreting food after digestion, the platyhelminthes have an organ to excrete excess water, since it enters the body through osmosis and its nutrients are then absorbed. This organ consists of interconnected canals that run along the length of the body and the movement of the cilia keeps the water moving toward excretory pores. (JP- Source 7)

Major Types Flatworms are divided into 4 classes: Turbellaria, Monogenea, Trematoda, and Cestoidea
Otherwise known as Planarians, Aspidogastrea, Flukes and Tapeworms, these unsegmented flatworms include both free-living and parasitic species. Common characterictics include bilateral symmetry, absorption of nutrients through skin, and either sexual or asexual reproduction. (MB)
This aquatic flatworm is not parasitic, but still not the best thing to think about while dining on sushi (LPE)

urbellarians are the free-living flatworms which do not parasitize other animals. Other classes have the obligate parasites, most of which live in or on the bodies of vertebrates(MP).
Cestoidea (tapeworms) have a primary and a secondary host. Nearly every species of vertebrate is susceptible to tapeworms. Adult cestodes usually live in the intestine, where they can easily get food that is already digested, as they’re missing a digestive system. Also, they have no means of locomotion, as it is unnecessary for them to move. They’re little more than a head with hooks to hang onto the intestine, continuously producing more reproductive segments and growing. (LPE)

The Turbellaria class is made up of a non-parasitic marine creature called Planarians-->carnivores of live and dead animals. They have a simple excretory system that uses osmosis (diffusion) to keep in balance. Wastes are excreted using flame cells which waft fluid outside using open ducts. An example is ammonia waste being diffused from the cells to the water outside.

Class Trematoda -All trematodes are parasitic, and most adult trematodes parasitize vertebrates. Their body is covered with a tegument, a peculiar kind of epidermal arrangement in which the main cell bodies are deep, separated from the cytoplasm that lies next to the exterior by a layer of muscle. Trematodes are characterized by one or two suckers or hooks on their anterior ends by which they fasten onto their victims. (RW)

Transport- Their flat shape allows them to distribute food to the whole body because they have a very efficient gastrovascular cavity.
Movement- Planarians contains cilia on their underbellies which help them glide along a film of mucus that they secrete.
Reproduction- asexually through regeneration when parent folds itself in middle until 2 separate pieces are formed, or sexually using cross fertilization (fusion of egg and sperm from 2 different individuals).

Anatomy of Turbellaria/Planarian Pharynx: contains the mouth at the tip (mouth extends from middle of ventral side) Mouth: undigested wastes exit through the mouth Gastrovascular cavity: food is sucked in here by pharynx (after digestive juices are spilled onto the prey), then digestion happens inside the cells lining the cavity Ganglia: near the eye-spots (visual senses), at the head of the flatworm are a dense cluster of nerve cells Ventral Nerve Cords: nerve chords that can be found along the entire underside of the body starting at the ganglia
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More specifically, eyespots are “clusters of light-sensitive cells.” (MR; Source 14)

The Monogenea and Trematoda classes are parasitic. Anatomy- contain suckers that hook up internal organs of parasite to outer surfaces of host. A tough covering on the outside of the parasite helps protect them when attached to host. Reproduction- Trematodes have an alternation of sexual and asexual reproduction where they temporarily have an intermediate host where the larvae develops. Then they infect permanent host where the adults lives and developes. Monogeneans on the other hand have free-swimming larvae that infect a host. The Cestoidea class is made up of parasitic tapeworms that generally live in vertebrates. Anatomy- scolex (head) has suckers that attach to intestinal lining of host. At the opposite end of the scolex is the proglottis which are sacs of sex organs. Transport- Tapeworms absorb food already digested by the host because they lack a digestive tract. One way humans can get tapeworms is by eating undercooked meat containing cysts

Environmental Adaptations Since Tapeworms don’t have digestive systems, they obtain food through absorption and excrete wastes through diffusion. The suckers on their heads attach to the hosts intestinal lining and the tapeworms then feed off of the already digested food of the host. Tapeworms can be land and marine creatures; they contain cilia on their underbellies which when in water helps them glide along a film of mucus that they secrete.

There are over 25.000 different species of platyhelminthes known. (CM)

Reproductive System

Flatworms have a very complex reproductive system. All flatworms have both male and female genitalia and a complex barrier between the two sets of genitals to prevent self fertilization. During mating two individuals fertilize the others egg. The fertilized eggs are then laid in a cocoon to be protected until hatching. (MS 11)

Fossil Record
There is practically no fossil record for platyhelminths. A few trace fossils have been reported as to be made by platyhelminths though. Fossil trematode eggs have been found in Egyptian mummies and dried dung of a ground sloth. These trematode larva that parasitize molluscs can be recognized on fossil sheels because they leave pits or thin spots on the inside of the shell. Irritated molluscs may be able to actually surround trematode larvae with layers of shelly material, making parasites natural pearls! (IL - 13)

Review Questions
1.) Does a platyhelminthes' body shape/function benefit it being a parasite? (AM)
2) What subclasses of flatworms are considered parasitic and which ones are not? (TB)
3) what does a hydrostatic skeleton give a flatworm in terms of evolutionary success?(RJ)
4) How do Platyhelminthes carry out diffusion? (VN)
5) Explain Phagocytosis and its importance to platyhelminthes' digestion. (CP)

Campbell, Neil A., and Jane B. Reece. "Invertebrates." Biology. San Francisco: Benjamin Cummings, 2002. 652+. Print

3) (MB)
4) (LPE)

4) "Flatworms" 30 Oct 2011 (MM)
5)Figure 1. (RK)

6) (AR) (JS)


8. Barnes, Robert. Invertebrate Zoology. 2nd. Philadelphia, PA: W.B. Saunders Company, 1969. Print.

9. (RW)
10. (LPE)

"Flatworm." The Worlds of David Darling. Web. 13 Nov. 2011.
12. (RW)

13. "Introduction to the Platyhelminthes." University of California Museum of Paleontology. Web. 14 Nov. 2011. <>. (IL)
14. "Phylum Platyhelminthes." Sea Studios Foundation. PBS, 2002. Web. 29 Nov. 2011. <>.