Zoology

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Cnidocyte

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is an explosive cell containing one giant secretory organelle or cnida (plural cnidae) that defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used for prey capture and defense from predators.

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Medusa

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a free-swimming sexual form of a coelenterate such as a jellyfish, typically having an umbrella-shaped body with stinging tentacles around the edge. In some species, medusae are a phase in the life cycle that alternates with a polypoid phase. Compare with polyp.

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Polyp

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is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are roughly cylindrical in shape and elongated at the axis of the vase-shaped body.

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Nematocyst

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a specialized cell in the tentacles of a jellyfish or other coelenterate, containing a barbed or venomous coiled thread that can be projected in self-defense or to capture prey.

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Mesoglea

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Mesoglea refers to the tissue found in jellyfish that functions as a hydro-static skeleton. Mesohyl generally refers to tissue found in sponges

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Radial symmetry

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a basic body plan in which the organism can be divided into similar halves by passing a plane at any angle along a central axis, characteristic of sessile and bottom-dwelling animals, as the sea anemone and starfish

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Nerve net

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A network of nerve cells connected with each other by synapses or fusion. The nervous system of certain invertebrates (e.g. coelenterates and echinoderms) consists exclusively of a nerve net in the body wall.

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Gastro vascular cavity

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is the primary organ of digestion and circulation in two major animal phyla: the Cnidaria and Platyhelminthes. The cavity may be extensively branched into a system of canals.

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Epidermis

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The epidermis of a more complex invertebrate is just one layer deep, and may be protected by a non-cellular cuticle. The epidermis of a higher vertebrate has many layers, and the outer layers are reinforced with keratin and then die.

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Tissue

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is a cellular organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

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Gastrodermis

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the inner layer of cells that serves as a lining membrane of the gastrovascular cavity of Cnidarians. The term is also used for the analogous inner epithelial layer of Ctenophores.