Most flatworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather than a complete digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body.
The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.
Figure 1. The planarian is a flatworm that has a gastrovascular cavity with one opening that serves as both mouth and anus. The excretory system is made up of tubules connected to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end.
There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions.
Most flatworm species are monoecious, and fertilization is typically internal. Asexual reproduction is common in some groups. Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda Figure 2.
As discussed above, the relationships among members of these classes is being reassessed, with the turbellarians in particular now viewed as a paraphyletic group, a group that does not have a single common ancestor. Figure 2. Phylum Platyhelminthes is divided into four classes. Dactylogyrus , commonly called a gill fluke, is about 0. The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments.
The ventral epidermis of turbellarians is ciliated and facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment. The monogeneans are ectoparasites, mostly of fish, with simple lifecycles that consist of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form.
The parasite has only one host and that host is usually only one species. Since flatworms are small distribution of food by diffusion is possible. 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. To carry out aerobic respiration, the planarian must get oxygen to the cells of its body. Oxygen enters the planarian by diffusion. The special features of the Phylum Platyhelminthes includes the protonephridia. These structures can be considered primitive kidneys. They are unique to the phylum. Planarians are placed in the Class Turbellaria, and these were discussed extensively above.
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 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.
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. Each proglottid contains reproductive structures and eggs are produced and become fertilized within each segment.
The phylum Platyhelminthes consists of simple worm-like animals called flatworms Fig. Flatworms live on land, in fresh water, in the ocean, and in or on other animals as parasites e. 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 centimeters 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 colored Fig. 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 Fig. 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 we will subsequently study have a mesoderm and muscular system.
The cells of the ectoderm and endoderm are also more organized than similar cells of cnidarians. For the first time, we see groups of tissues that have evolved to form organs, such as the ones in the digestive, nervous, and excretory systems. Like the cnidarians, 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 Fig.
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 Fig. 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. 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 Fig. 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.
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 Fig. 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. 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. 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 Fig.
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 Fig. 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.
Species in the phylum Nematoda from the Greek root word nema meaning thread are better known as the roundworms Fig. There are about 25, species of nematodes formally described by scientists. Nematodes are found in almost every habitat on Earth. One species was first discovered living inside felt beer coasters in German alehouses. Studies of farmlands have found as many as 10, nematodes in cubic centimeters cm 3 of soil.
Nematodes are similarly abundant in marine and freshwater sediments where they serve as important predators, decomposers, and prey for other species like crabs and snails. Like flatworms, roundworm species adopt either a free-living or a parasitic lifestyle. Parasitic nematodes Fig. Many nematodes that are parasitic on plants can devastate crops.
Some nematodes are cryptobiotic and have demonstrated a remarkable ability to remain dormant for decades until environmental conditions become favorable. Like the flatworms, nematodes are bilaterally symmetrical. They take their name from their round body cross-sectional shape. Unlike the flatworms in which food and waste enter and exit from the same opening, nematodes have a complete digestive system.
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