What Are the Differences Between Sponges and Cnidarians?

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Cnidarians have groups of similar cells that work together as tissues, while sponges have no tissues, only disconnected regions of specialized cells. Each group has a type of cell unique to their group: Sponges have collar cells, and cnidarians have nematocysts. No sponges are capable of movement as adults, while some cnidarians move as adults.

Sponges use their collar cells for filter feeding, pushing water through pores in their surface and capturing small food particles. Cnidarians' nematocysts are also used for gathering food but have a very different structure. Nematocysts are found in a cnidarian's tentacles, and each is a cell with a barbed thread coiled inside. On contact with another animal, the nematocysts fire off these barbed threads, which contain deadly poisons. While some cnidarians are also filter feeders, many species are able to catch much larger prey.

Sponges vary in complexity, but all have superficially similar body plans, with stiff body walls full of pores that lead to central chambers. Water is pumped into these chambers and escapes through one or more larger holes. Cnidarians have some more superficial variation, although all possess tentacles. Cnidarians' bodies take two forms: medusae and polyps. Medusae are the rounded, free-floating forms such as jellyfish and have tentacles dangling beneath. Polyps are the stalk-like bodies like sea anemones, which have tentacles protruding above.