Aplacophorans: Aplacophora

Aplacophorans (ah-plak-oh-FOR-ans) are mollusks and are related to clams, mussels, octopuses, and squids. Their worm-shaped bodies range from long and slender to almost ball-shaped and measure between 0.039 and 3.9 inches (1 to 100 millimeters) or more in length. A thick sheet of skin, or mantle, covers their bodies. The mantle makes up the shells, or valves, in most mollusks. However, aplacophorans are the only mollusks that do not have any valves. Instead, their mantles produce sharp, needlelike projections or small, scaly plates that are imbedded in the mantle over the back. These projections and scales make most aplacophorans look shiny. Like the valves of other mollusks, these projections and plates are hard and made up of a mineral called calcium carbonate. The surface of the mantles themselves are smooth, rough, bumpy, or spiny.

The head is poorly developed, and there are no eyes or tentacles. At the back of the mouth of most species is a rough structure called the radula (RAY-jeh-leh). The radula is covered with lots of rows of small teeth. The edges of the teeth are sawlike and made up of even smaller teeth called denticles (DEHN-te-kelz). The radula is used to scrape bits of food off rocks and other hard surfaces. In most mollusks, the radula is ribbonlike in shape but not in aplacophorans. Their radulas are part of their gut. There is a bundle of nerves, or ganglion, inside the head that is attached to a nerve chord that runs along their underside. The body cavity is small. The circulatory system is open, and the blood is not always contained inside blood vessels. There are no kidneylike organs, and there is either one or a pair of reproductive organs.

They do not have a well-developed muscular foot like other mollusks. They move slowly, either with the help of tiny bristles on their bodies called cilia (SIH-lee-uh) or on a track of mucus produced by a groove underneath their bodies. The mucus smoothes the way for the aplacophoran as it glides over the track. At the end of the body is a cavity that has the openings to the reproductive organs and the anus. The anus is the opening at the end of the digestive system where solid waste leaves the body. Breathing organs may or may not be present.