Phylum Platyhelminthes

1.                  The Phylum Platyhelminthes (flatworms) includes planarians (Class Turbellaria), tapeworms (Class Cestoda), and flukes (Class Trematoda). The planarians are harmless, free-living flatworms. They live in an aquatic environment (freshwater or marine) or on moist soil. Both tapeworms and flukes are internal parasites that inhabit a host’s tissues, cavities in body organs, or blood vessels.

2.                  The flatworms, like the sponges (Phylum Porifera) and Phylum Cnidaria (Hydra, jellyfish, corals, and sea anemones) are multicellular. However, the flatworms are more complex in structure than either of the two other groups. In flatworms, tissues are organized into organs (structures composed of more than one tissue and performing as specific function) and these organs are organized into organ systems (composed of more than one organ and performing a generalized function) such as the digestive system or the reproductive system.

3.                  Unlike, the animals in the Phylum Cnidaria which have radial symmetry, those in the Phylum Platyhelminthes have bilateral symmetry. This means there is only one plane of symmetry (one way you can slice the animal in half and produce two pieces that are mirror images of one another). It also means that in these animals you can identify the anterior and posterior, right and left, and dorsal and ventral portions of the animal. A bilaterally symmetrical animal moves with the anterior forward and crawls on its ventral surface with the dorsal surface upward.

4.                  Members of the Phylum Platyhelminthes (especially true of the planarians, Class Turbellaria) are organized with brain and sense organs at the front of the animal. This is called cephalization. In animals with cephalization, the sense organs come in contact with the environment first.

5.                  Since the planarian was the primary animal that was viewed in lab, most of the information below pertains to this animal. The planarian eats small animals or larger animals that have died. The planarian is a carnivore. To ingest (swallow) food, the planarian extends its pharynx out of its mouth. The pharynx is a muscular tube that is used by the planarian to swallow prey animals whole (in the case of small animals) or to suck in body juices of a larger animal. The ingested food winds up in the animals gastrovascular cavity (GVC). Like the Hydra and other members of the Phylum Cnidaria, the planarian has a cul-de-sac gut, one with only one opening, the mouth.

6.                  To capture prey animals, the planarian moves from place to place. It does this in one of two ways. The planarian has opposing muscles, circular muscles that extend around the flatworm and longitudinal muscles that extend from anterior to posterior of the flatworm. If the circular muscles contract, the flatworm changes shape to become long and thin. If the longitudinal muscles contract, the flatworm changes shape to become short and wide. These opposing muscles push against the solid body of the worm. Unlike other animals with opposing muscles, flatworms lack a body cavity (coelom). The body, instead, has cells called cellular mesoderm surrounding body organs. The cellular mesoderm provides a structure for the muscles to push against. The second way in which the flatworm moves is through the use of cilia that are located on the ventral surface epidermis. The cilia beat in a coordinated fashion, and the flatworm is able to glide through the water.

7.                  The planarian has a nervous system that allows it to respond to the environment. Two clusters of nerve cells called ganglia are located near the anterior of the animals. This pair of ganglia constitutes the brain of the flatworm. Two ventral nerve cords extend from the brain to the posterior of the planarian. The two ventral nerve cords are connected to one another by smaller nerves. Additionally, the planarian has two visible sensory organs. The auricles are lateral flaps near the anterior of the animal. The auricles are chemoreceptors and sense chemicals in the water. Also near the anterior are two eyespots. These allow the flatworm to sense whether it is darkness or light. It does not allow to animal to see images.

8.                  Once a planarian has sensed food, moved to it, and ingested it, the food is digested by both extracellular digestion and intracellular digestion. The lining of the gastrovascular cavity (the gastrodermis) releases digestive enzymes into the GVC. The ingested food is broken down to small food particles by this process. Since the process takes place in a cavity within the animal, rather than within the animal’s cells, this is called extracellular digestion. The small food particles are then taken into the gastrodermis cells by endocytosis. Within food vacuoles formed by endocytosis, the food particles are further broken down to food molecules. These food molecules diffuse or are actively transported into the cytoplasm of the gastrodermis cell. Food is distributed from the gastrodermis to other cells of the body by diffusion primarily. Since flatworms are small distribution of food by diffusion is possible.

9.                  Food digestion produces digestive waste. This is released from the planarian through the mouth. The digestion of proteins produces nitrogen waste (ammonia). The planarian has a system of tubules extending throughout its body from the anterior to posterior on both sided of the body. These tubules are called protonephridia and they collect both excess water and much of the nitrogen waste. The tubules have pores that lead out of the body. Water and ammonia leaves through these pores. Aerobic respiration produces carbon dioxide as a waste product. This is released from the planarian by diffusion.

10.              To carry out aerobic respiration, the planarian must get oxygen to the cells of its body. Oxygen enters the planarian by diffusion.

11.              The special features of the Phylum Platyhelminthes includes the protonephridia. These structures can be considered primitive kidneys. They are unique to the phylum.

12.              The Phylum Platyhelminthes includes three classes: Class Turbellaria (free-living flatworms), Class Cestoda (tapeworms), and Class Trematoda (flukes). Planarians are placed in the Class Turbellaria, and these were discussed extensively above.

13.              The flukes (Class Trematoda) are mostly parasites of vertebrate animals (those with backbones). The animals generally have a sucker around the mouth and one on the ventral surface. These suckers allow the animal to cling to the body organs in which they live. The adult flukes live in hollow organs like the heart, tongue, kidney, and gall bladder (on lower surface of the liver). Many flukes have immature stages that live in other animals. For example, immature stages (larvae) of the human liver fluke live in snails. These are released from snails, and a second immature stage then lives within fish. When humans ingest raw or undercooked fish, they also ingest immature stages of the fluke. The immature stages migrate to the bile ducts of the liver where they mature. The mature stages may live within the bile ducts for 15-20 years. While there they cause damage of the liver and may cause death of the human host. Eggs are produced by the adult within the bile ducts. The eggs travel through the bile ducts to the digestive tract and pass out of the host in feces. The eggs are ingested by snails, and the cycle is repeated. Humans can also become infected with a human blood fluke. Immature stages of this fluke also live within snails. These larvae leave the snail and are present on the ground in areas of poor sanitation. The immature stages enter humans by burrowing through the skin of a human host. The immature flukes enter blood vessels and mature there. They can cause anemia and damage to the liver, bladder, and brain.

14.              Tapeworms (Class Cestoda) are also parasite flatworms. Humans can become infected with tapeworms by ingesting uncooked or undercooked pork or beef. Larval (immature) stages of the tapeworm present in the muscle of pigs or cows are ingested. The tapeworm attaches to the intestine while passing through the digestive tract using hooks or suckers. The tapeworm matures and grows in the intestine, a food-rich environment. The tapeworm grows longer by budding. The buds are called “proglottids”. Each proglottid contains reproductive structures and eggs are produced and become fertilized within each segment. The oldest proglottids break off of the tapeworm and leave the host with feces. In areas of poor sanitation, pigs or cows ingest the zygotes. The immature stages burrow into the muscle tissue of the pig or cow and the cycle repeats.

15.              The impact the Phylum Platyhelminthes has on humans is the fact that several animals within the phylum are human parasites.


Phylum Nematoda

1.                  The Phylum Nematoda (roundworms or nematodes) includes harmless, soil-dwelling roundworms (nematodes) that eat decaying organic material or small soil animals. The phylum also includes plant parasites that infect the roots of plants. These parasitic nematodes decrease the productivity of many human crops. The phylum includes several human parasites (see below).

2.                  Like the Phylum Platyhelminthes, the Phylum Nematoda consists of bilaterally symmetrical animals that have the organ system level of organization.

3.                  The Phylum Nematoda differs from the Phylum Platyhelminthes in two significant ways. First, roundworms have a complete digestive system. This means that there are two opening to the digestive system. The mouth at the anterior ingests or swallows food, and the anus at the posterior releases digestive waste. A complete digestive system is much more efficient than a cul-de-sac gut. The complete digestive system allows continuous processing of food. A roundworm can eat continuously, food digestion can occur continuously, and waste material can be released continuously. Animals with a cul-de-sac gut must wait until a meal has been digested, release digestive waste from the mouth, and only then swallow the next meal.

4.                  The second significant difference between the Phylum Nematoda and the Phylum Platyhelminthes is that the roundworms have a fluid filled body cavity. The presence of this structure allows space and cushioning for organs, provides the roundworm with a hydraulic skeleton, and aids in the distribution of food from the digestive tract to the other cells of the worm.

5.                  Several human parasites are roundworms. Many people in tropical countries are infected with hookworm. Immature stages of this parasitic worm burrow through the skin, travel through the blood vessels to the lungs, enter the air spaces of the lungs and crawl into the esophagus. The immature stage is then swallowed. The worm attaches to the intestine with hooks and matures into an adult. Fertilized eggs are released with feces, and the zygotes develop into immature stages on soil. When people walk barefoot over the soil, they become infected. The mature hookworm drinks blood and lymph juices. They cause anemia due to blood loss.

6.                  The human roundworm is common where human feces is used as plant fertilizer. People ingest eggs when they eat plant material. The immature stages travel through the human body in blood vessels. Mature human roundworms live in the intestine where they produced eggs that are released with feces.

7.                  People can become accidentally infected with the trichina worm by eating undercooked port. The muscle of pork may contain immature stages of trichina worm. When people ingest the larval stage, it matures in the intestine where the adult worms reproduced. Immature stages migrate from the intestine to muscle tissue. There the larva forms a cyst. Since humans aren’t generally eaten, the cysts become coated with calcium carbonate. This causes muscle stiffness. We call this condition trichinosis.