Cnidarians include jellyfishes, hydras and their relatives, and sea anemones and corals.

• soft-bodied, carnivorous animals that have stinging tentacles arranged in circles around their mouths

• the simplest animals to have body symmetry and specialized tissues

Cnidarians get their name from the cnidocytes, or stinging cells, that are located along their tentacles. Within each cnidocyte is a nematocyst, a poison-filled, stinging structure that contains a tightly coiled up dart.

•Cnidarians are radially symmetrical

central mouth surrounded by many tentacles that extend outward from body

• typically have a life that includes two different-looking stages: a polyp and a medusa

•both medusa and polyp have a body wall that surrounds a gastrovascular cavity, the gastroderm is the inner lining of the g.c, where digestion takes place, the epidermis is the outer layer of cells, the mesoglea is the layer that lies between these two tissues

After paralyzing its prey, a cnidarian pulls the prey through its mouth and into its gastrovascular cavity

• food enters and leaves through this opening

• digestion takes place here and is extracellular, meaning it takes places outside of cells

Partially digested food is absorbed by the gastroderm

• digestion is completed intracellularly, meaning within cells in the gastroderm

Any materials that can’t be digested are passed out of body through the mouth

• After digestion, nutrients are usually transported throughout body by diffusion

• cnidarians respire and eliminate wastes of cellular metabolism by diffusion through their bodies

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Diffusion is process by which molecules tend to move from an area where they are more concentrated to an area of low concentration

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• Both polyp and medusa have a nerve net which is a loosely organized network of nerve cells that together allow cnidarians to detect stimuli such (ex.: the touch of a foreign object)

•Nerve net is usually distributed uniformly around body, but in some species it is concentrated around the mouth or in rings around the body

Cnidarians also have:

• statocysts, which are groups of sensory cells that help determine direction of gravity and ocelli, which are eyespots made of cells that detect light

• different cnidarians move in different ways

• sea anemones have a hydrostatic skeleton which is a layer of circular and longitudinal muscles that, with the water in the gastrovascular cavity, enable it to move

• medusas, such as jellyfish, move by jet propulsion

this is when muscles contractions cause the bell-shaped body to close like an umbrella and push water out of the bell moves medusa forward

•Most cnidarians reproduce both sexually and asexually

• Polyps can reproduce by budding

one type begins with a swelling on the side of an existing polyp that breaks off and becomes a new polyp

another is when polyps produce tiny medusas that separate and become new

individuals

• In most cnidarians, sexual reproduction takes place with external fertilization, which the process in which the eggs are fertilized outside the female’s body

Class Hydrozoa

• polyps grow in branching colonies that sometimes extend more than 1 meter

within colony, polyps are specialized to perform different functions

(ex.: some digest food held by tentacles while others make eggs and sperm)

• most common freshwater hydrozoans are hydras

lack medusa stage, instead live as solitary polyps

reproduce sexually and asexually

many get nutrient from small prey and some from symbiotic photosynthetic protists that live in their tissues

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anemonmone..

Class Anthozoa “flower animal”• only have polyp stage in their lives• many species are colonial• use nematocysts to capture prey for nutrition• new polyps produced by budding• as colony grows, they secrete underlying skeleton of calcium carbonate, or limestone

Worldwide distribution of corals determined by few factors:1) Temperature 2) water depth 3) light intensitylight- corals rely on mutualistic relationships with algae that

capture solar energy, recycle nutrients, & help corals lay down limestone

Harmed from human activities, logging, farming, mining, etc. Coral bleaching caused by high temperatures can kill algae that usually live in the tissues of corals, leaving behind onlytransparent cells atop ghostly white skeletons

cnidocytes, or stinging cellsnematocyst, a poison-filled, stinging structure that contains a tightly coiled up dart. gastrovascular cavity digestive chamber with a single opening in which cnidarians, etc., digest foodgastroderm is the inner lining of the g.c, where digestion takes placeepidermis is the outer layer of cellsmesoglea is the layer that lies between the epidermis and gastroderm extracellular, digestion takes places outside of cellsIntracellular digestion takes place inside of cellsnerve net which is a loosely organized network of nerve cells that together allow cnidarians to detect stimulistatocysts, which are groups of sensory cells that help determine direction of gravity ocelli, which are eyespots made of cells that detect lighthydrostatic skeleton which is a layer of circular and longitudinal muscles that, with the water in the gastrovascular cavity, enable it to move

jet propulsion this is when muscles contractions cause the bell-shaped body to close like an umbrella and push water out of the bell

external fertilization, which the process in which the eggs are fertilized outside the female’s body

• Cnidarians are…• Soft-bodied, carnivorous, and have stinging tentacles• Simplest organism with body symmetry and specialized tissues• Named after their cnidocytes, or stinging cells, on their

tentacles – Within cnidocytes, are nematocysts, poison-filled stinging structure with

a dart

• Include: jellyfishes, hydras, sea anemones, and corals• Body Plan• Radial symmetry• Two stages: Polyp and Medusa• Both have body wall surrounding a gastrovascular cavity which

is lined with the gastroderm, where digestion takes place• Epidermis is the outer layer of cells• Mesoglea is the layer between the epidermis and gastroderm

• Feeding, Respiration, Circulation, and Excretion• Feeding:

– paralyze the prey, pull it through the mouth, into the gastrovascular cavity, where extracelluar digestion, or outside of cells, occurs

– food is then absorbed by gastroderm, where digestion is completed intracellularly, or within the cells

– Food not digested, leaves through the mouth

• Other body processes occur by diffusion• Response and Movement• Cnidarians have a nerve net, or a loosely organized network of

nerve cells that allow the cnidarians to detect stimuli• They also have statocysts- groups of sensory cells that help

determine direction of gravity• Also, they have ocelli- eyespots with cells that detect light• Movement:

– Sea anemones- hydrostatic skeleton, which is a layer of muscles that, with water, enables it to move

– Medusas- jet propulsion, works by forcing the bell shaped body to close like an umbrella, and push water out of the bell

• Reproduction• Asexually

– Polyps reproduce by budding, either by a piece of the polyp swelling and breaking off into a new polyp, or by producing and releasing medusas

• Sexual– Male release sperm, female release eggs, and external

fertilization occurs, which is the process when the eggs are fertilized outside the female’s body

• Hydras and Their Relatives• Class Hydrozoa• Polyps grow in branching colonies and each is

specialized for a certain job• Hydras are the most common freshwater hydrozoans

– lack medusa stage– reproduce sexually and asexually– get nutrients from small prey and some from symbiotic

photosynthetic protists living in their tissues

• Sea Anemones and Corals• Class Anthozoa- “flower animal”• Only polyp stage• Many are colonial• Use nematocysts to capture prey• New polyps produced by budding• As it’s growing, it secretes a skeleton of calcium

carbonate, or limestone• Where corals grow is affected by temperature, water

depth, and light intensity• Corals rely on mutualistic relationships with algae that

capture solar energy, recycle nutrients, and help lay down limestone

• Corals are harmed by human activities, logging, farming, mining, etc.

• Coral bleaching caused by high temperatures kill algae in corals