The collars that give choanocytes their name are made up of finger-like cytoplasmic membrane extensions called microvilli (like the ones that line your small intestine) surrounding tail-like flagella. Some can be four- or six- pointed others are hooked. The spicules can have any number of points the fossils of the glass sponge Hydnoceras from the Devonian (390-360 mya) look for all the world like they're wrapped in plaid thanks to their six-pointed spicules which appear four-pointed in the two dimensions apparent on its exterior. The exterior of sponges is coated with epidermal cells in the wall of the sponge are wandering amoeboid cells that perform some immune-system like duties and secrete crystalline spicules (usually calcium carbonate or silicic acid) or proteinaceous fibers that support the sponge special cells called porocytes can encircle the inflowing wall pores and distinctive choanocytes, or collared cells, line the interior spaces of the sponge. Click here for a nice diagram that helps illustrate the following text. Like algae, the thinness of the sponge body plan and the fact that most of its cells are exposed to circulating water (provider of oxygen and food, remover of wastes) makes organs like hearts, kidneys, and digestive tracts unnecessary.Ī typical barrel or tube sponge is in essence a perforated sac. Click image for link.Ī sponge is, in essence, a multicellular organism with no organs or tissues, but with specialized cells, which distinguishes it from small multicellular protists. Some sort of vertebrate hides inside a gorgeous blue barrel sponge off Bonaire in the Caribbean.
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