Flatworms

• The most obvious characteristic that separates this phylum from the other worm pula is that they are flat.
• Some are free-living, but most are parasitic and live on or within other animals.

Free-living Flatworms: The Planarians

• These flatworms usually live in freshwater lakes and streams. They may be black, brown or white. 
• Their body is essentially a strip of flat tissues about a centimeter long that ends in a triangular point.
• This point is the head. There are eyespots on the dorsal side. They do not focus on objects, but are sensitive to the presence and direction of light.
• There are three layers of cells: 
• Epidermis: protection from environment
• Mesoderm: it is similar to the mesenchyme and mesoglea, but this time instead of being an acellular jelly, it is composed of cells. Many of the different organs develop from the mesoderm.
• Gastroderm: lines the digestive tract.
• Movement: 
• Planarians must seek and capture food.
• Able to do locomotion
• Special cells on the ventral surface enable it to move by secreting a layer of slime. Then ciliated cells propel the flatworm over the slime.
• Support: 
• The three layers of cells provide support
• Protective body covering:
• Epidermis
• Nutrients: 
• Usually feed by scavenging pieces of decayed animals or plant matter and eating small organisms when they can catch them
• Their mouth is located on the ventral surface (only one opening for ingestion and excretion)
• A muscular, tubelike pharynx is extended through the mouth where it sucks up food into the intestine. There it is broken down with enzymes. It is then diffused through the body. 
• Excretion/Circulation:
• Any indigestible food that remains in the intestine is egested through the pharynx.
• The cells also excrete waste by diffusion. 
• A system of tubules that extends throughout the planarian’s mesoderm, connected to flame cells, or hollow bulbs containing cilia that beat vigorously to maintain a current, carry waste to excretory pores to be released into the environment.
• Respiration:
• Flatworms have a thin body and therefore do respiration on a cellular level through diffusion.
• Response:
• Flatworms have an elaborate nervous system. 
• They have a mass of nerve tissue that coordinates responses to the environment, control the eyespots and sensory glands that allow them to taste, smell, and touch. 
• There are two longitudinal nerves that extend from the brain down the length of the animal. 
• A number of transverse nerves unite the longitudinal nerves in a ladder-like pattern.
• Reproduction:

• Asexual: regeneration
• Sexual:
• All planarians are hermaphroditic, or have both sets of sexual organs. But they always cross-fertilize. After mating, each will release several fertilized eggs enclosed in a capsule. The capsule then attaches to a rock or plant in the water.
• Larval stages

Parasitic Flatworms

• A parasite feeds off its host, often harming the host. 
• There are some big differences between the free-living flatworms and the parasitic type because they do not need to same things for survival. Parasitic flatworms have:
• Few sensory organs in adults
• No external cilia in adults
• A thick tegument (protective body covering)
• Suckers or hooks or both for attachment to the host. 
• There are two main groups: the flukes and the tapeworms

The Flukes

• The sheep liver fluke is a great example. 
• As an adult it inhabits the liver of sheep or other grazing animals, causing a disease known as “liver rot.” 
• The anatomy is very similar.
• The fluke feeds by sucking the tissue and fluids of the host into its mouth through the pharynx. 
• The adult produces eggs, which pass through the she eps feces. The eggs hatch in water, releasing ciliated larvae, which enter particular species of snail. The snail becomes the intermediary. 
• While in the snail, the fluke undergoes a series of changes while reproducing asexually. 
• Eventually the fluke leaves the snail and attaches to vegetation. The sheep eats the plants and then the fluke is in the sheep again. 

Tapeworm 

• Bears little resemblance to the flukes.
• The pork tapeworm is a human parasite, living in the intestines of humans. 
• The tapeworm has a ribbon-like segmented body with suckers and hooks. It has no mouth or digestive organs. It simply absorbs the already digested nutrients in the host. 
• The tapeworm’s tegument protects it from the digestive juices.
• Most of the tapeworm’s energy is dedicated to reproduction. Each segment of the tapeworm contains both female and male reproductive organs. This means that each segment produces, fertilizes and stores the eggs. Once mature, the segment, containing 1000s of eggs, detaches and is egested with the feces. 
• If a pig eats one of these segments, the tapeworm will burrow into its muscle and wait for an unsuspecting human to eat the pig. If the pork is not completely cooked, the human will ingest the tapeworm and the cycle starts all over again.