1. Worms
Flat, Round, & Segmented

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Flatworms
Photo Credit: ©Carolina Biological Supply Company/Phototake
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What Is a Flatworm?
Flatworms are soft, flattened worms that have tissues and internal organ systems.
They are the simplest animals to have three embryonic germ layers, bilateral symmetry, and cephalization.
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What Is a Flatworm?
Flatworms are acoelomates, which means they have no coelom.
A coelom is a fluid-filled body cavity that is lined with tissue derived from mesoderm.
The digestive cavity is the only body cavity in a flatworm.
Flatworms have bilateral symmetry.
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5. Form and Function in Flatworms
Most parasitic worms do not need a complex digestive system.
They obtain nutrients from foods that have already been digested by their host.
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Eyespot
Head
Digestive cavity
Digestive Structures of a Planarian
Mouth
Pharynx
All flatworms, including this planarian, have organ systems that perform essential life functions. The digestive cavity is branched throughout the body and opens to the outside through the pharynx. Photo Credit: ©Carolina Biological Supply Company/Phototake
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Groups of Flatworms
The three main groups of flatworms are
turbellarians
flukes
tapeworms
Most turbellarians are free-living.
Most other flatworm species are parasites.
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Turbellarians
Turbellarians are free-living flatworms. Most live in marine or fresh water.
Most species live in the sand or mud under stones and shells.
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Flukes
Flukes are parasitic flatworms. Most flukes infect the internal organs of their host.
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Tapeworms
Tapeworms are long, flat, parasitic worms that are adapted to life inside the intestines of their hosts.
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11. Structures of a Tapeworm
Tapeworms are parasitic flatworms that live in the intestines of their host. A tapeworm attaches to the host using hooks or suckers on its scolex. A single tapeworm is made of many proglottids. The youngest proglottids are at the anterior (head) end, and the largest and most mature proglottids are at the posterior (tail) end. After eggs have been fertilized, proglottids break off and release zygotes that are then passed out of the host in feces.
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Proglottids are the segments that make up most of the worm's body.
Mature proglottids contain both male and female reproductive organs.
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Roundworms
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What Is a Roundworm?
Most species of roundworms are free-living, inhabiting soil, salt flats, aquatic sediments, and water, from polar regions to the tropics.
Others are parasitic and live in hosts.
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Roundworms are unsegmented worms that have pseudocoeloms and digestive systems with two openings—a mouth and an anus.
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What Is a Roundworm?
This cavity is partially lined with tissue derived from the mesoderm and is called a pseudocoelom, meaning, “false coelom.”
Roundworms such as hookworms are unsegmented worms that have a pseudocoelom and a digestive system with a mouth and an anus. Roundworms develop from three germ layers, and a pseudocoelom forms between the endoderm and mesoderm layers.
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Feeding
Many free-living roundworms use grasping mouthparts and spines to catch and eat other small animals.
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Reproduction 
Roundworms reproduce sexually.
Most species have separate sexes.
Parasitic roundworms often have life cycles that involve two or three different hosts or several organs within a single host.
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19. uman Ascarid Reproduction
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Annelids
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What Is an Annelid?
Annelids are worms with segmented bodies. They have a true coelom that is lined with tissue derived from mesoderm.
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22. Three Germ Layers of an Annelid
Annelids are among the simplest animals to have a true coelom that is lined with mesoderm. Annelids are also called segmented worms because the body is divided into many similar segments.
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The body of an annelid is divided into segments.
Each segment is separated by septum, which are internal walls between each segment.
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24. Form and Function in Annelids
Annelids have complex organ systems.
Many of these systems are unique because of the segmented body plan of this group.
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Feeding and Digestion
In carnivorous species, the pharynx usually holds two or more sharp jaws that are used to attack prey. -Leeches
Annelids that feed on decaying vegetation have a pharynx covered with sticky mucus. -Earthworms
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Circulation 
Annelids typically have a closed circulatory system, in which blood is contained within a network of blood vessels.
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Response  
Most annelids have a well-developed nervous system consisting of a brain and several nerve cords.
Brain
Ganglia
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Movement  
Annelids have two groups of body muscles that function as part of a hydrostatic skeleton.
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Reproduction   
Most annelids reproduce sexually.
Earthworms are hermaphrodites. Two worms attach to each other, exchange sperm, and then store the sperm in special sacs.
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