Invert Book Chapter (Cnidaria)

Phylum Cnidaria Classification:

Introduction provided by: Jessica Comeau

Cnidarians are organisms found exclusively in aquatic habitats. Of the over 10,000 aquatic Cnidarian species discovered, most inhabit marine environments. All species are diploblastic organisms that have radial symmetry which can exhibit itself in two major body forms, polyp or medusae. Although the structure of various organisms differ, all individuals maintain a lack of cephalization of the nervous system, and no specialized respiratory, circulatory, or excretory system. 

The polyp body form consists of a gastrovascular cavity, gastrodermis, epidermis, and mesoglea. The shape of a polyp is usually elongated and features thin body walls. A hold-fast is present on the aboral end, the end opposite of the oral opening, to attach itself to a substrate. The hold-fast anchors the organism for life making it a sessile creature. Tentacles covered with cnidocytes, stinging cells,surround the organism’s oral opening and point upward for filter-feeding. Cnidocytes are the most important characteristic that distinguishes organisms as belonging to the Cnidarian phylum. Polyps may live individually or in colonies made up of large numbers of individuals joined together that share a gastrovascular cavity. Each polyp in a colony is genetically identical, but may differ in function based gene expression 

The medusae body form closely resembles that of the polyp form containing a gastrovascular cavity, gastrodermis, epidermis, and mesoglea. Unlike polyps, medusae have a thicker mesoglea that resembles a bell shape. Instead of attaching itself to a substrate, medusae float in the aquatic habitat and swim with their tentacles dangling downward, or drift with the current. Dangling tentacles surround the oral opening which is now facing towards the seafloor. These carnivores use their tentacles to move protista, small fish, and crustaceans into the oral opening to the gastrovascular cavity.

The gastrovascular cavity function for digestion and circulation. The multifaceted function of one system causes it to be less efficient than having specialized organs. The gastrovascular cavity has only one body opening where food and waste must pass. This means that the organism may not feed and digest food at the same time. Movement can also causes a decrease in efficiency. The movement distorts the gastrovascular cavity interrupting circulatory and digestive processes. An additional function of the opening of the gastrovascular opening is to release of gametes and embryos in sexual reproduction.

Asexual and sexual reproduction both present themselves in the Cnidarian phylum. Sexual reproduction involved complex life cycles of both the polyp and medusa forms. Asexual reproduction may occur by strobilation or budding. Cnidarians are divided into two subphylums, the Medusozoa and Anthozoa. These  sub classes are then classified into five classes; Scyphozoa, Cubozoa, and Hydrozoa, which are Medusozoan, and  Hexacorallia and Octocorallia which are Anthozoans.


General Content:

Coral Bleaching

How Can You Sleep if You Haven’t Got a Brain? Jellyfish May Hold the Answer

Jellyfish dominating the oceans

The Portuguese man o’ war


Lab Work:

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Scyohozoa – under 100x total magnification
Photo credit: Marisa Benjamin


Aurelia strobila – under 400x total magnification
Photo credit: Simone McEwan


Aurelia ephyra – under 400x total magnification
Photo credit: Simone McEwan

Anthozoa (Hard Corals) – no magnification
Photo credit: Devon Audibert



Term definitions provided by: Devon Audibert

Medusa– Dome shaped, free-footing Cnidarian such as a jellyfish.
Mouth- Gets the food ingested past the cilia.
Tentacle- Brings food into gastrovascular cavity.
Gastrovascular Cavity- Location of digestion and circulation.
Cnidocyte– Explosive cell, used for protection to defend off potential predators.
Cnidae- Capsules containing condensed tubules along with venom.
Operculum– A “cover” or “door” that exposes the organism to the outside world.
Cnidocil- Hairlike structure that protrudes from the cnidoblast. Releases discharge from the nematocyst.
Discharged- When the organism has its cnidocil out and ready to attack.
Undischarged- When cnidocil is closed off, not attacking.
Manubrium- Structure that connects the mouth to the gastric cavity. Surrounded by oral arms.
Radial Canals- Forms a portion of the water vascular system, found existing on the tentacles.
Oral Arms- Hold stunning cells for jellyfish and also release sperm. Located around the mouth of the organism.
Gastric Pouch- Pouches that open around the stomach. Location of digestion.
Gonad– Location where gametes exist, sex glands.
Gastric Filament- Filament that is lines with nemocysts, kill organisms before entering the organisms stomach.
Zooanthellae- Single celled dinoflaggelates that are able to live inside other cnidarians including corals and jellyfish.
Pinnate– Resembling a feather in structure.
Planula Larvae- A free swimming, ciliated type of cnidarian larvae.
Scyphistoma– Stage in the life cycle of jellyfish where it reproduces asexually.
Strobilation– Asexual reproduction by division into segments of the body in tapeworms and jellyfish.
Ephyrae– Free swimming jellyfish larvae formed by fission.
Velum– A membrane typically covering a cavity, membrane projecting inward on a jellyfish.
Stolon– A rootlike extension of the body, produces organisms from buds on the tips of the structures.
Hydranth- Terminal part of the hydroid polyp where the mouth and stomach are located.
Perisarc– Outer case protecting hydrozoans.
Hydratheca– Part of the perisarc covering the hydranth.
Thecate– Having or being incased in a theca
Zooids- An organism arriving from another from budding or division.
Gastrozooid- A zooid with a mouthing full digestive system.
Gonozooid- A sexual zooid or medusa bud of a hydroid.
Dactylozooid- A mouthless zooid with tentacles found in some hydrozoans that perform tactile functions.
Siphonoglyph– Groove leading from the mouth down to the gullet, found in some anthozoans
Pharynx- Canal found immediately behind the mouth.
Mesenteries- Folds that are attached to the stomach and other major organs.

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