Introduction to Mollusks: What is a Mollusk?
by EOL staff
Mollusks are very efficient in the use of their body parts. They never settle for one function when an organ could serve two or six purposes at once.
A good example of this is the mantle, a membranous projection of a mollusk body wall. The mantle encloses and protects the animal’s internal organs, leaving room for an open internal space called the mantle cavity. The cavity is positioned differently in different mollusk groups and is filled with air or water—whatever is in the outside environment of the animal. It can serve as a space to exchange carbon dioxide for oxygen from that air or water (respiration, in either case), a chamber through which to pump water and filter out food particles, a sampling area for sensory organs to test the air or water, a threshold through which to dump waste products, or a safe place to keep eggs while they mature. Some groups use it for all five.
The mantle also secretes the shell, in those mollusks that have one. In many gastropods and cephalopods, the mantle is brightly colored and important for communication. In giant clams, the outer mantle tissue is colonized by symbiotic algae that provide their host with food energy in exchange for shelter. Since in bivalves the mantle is the tissue closest to the outside world, it’s the best place to put sensory organs, like eyes or sensory tentacles, or both. In many bivalves and snails, and in cephalopods, part of the mantle is modified into a siphon, which can be used to pump water through the mantle cavity for respiration, feeding and/or jet propulsion.
Freshwater and marine mollusks have gills (called ctenidia) for respiration, located in the mantle cavity. In most bivalves, these are enlarged and serve to trap food particles as well…
(read more: Encyclopedia of Life)
images: Common Octopus by Elaine Soulanille; Helix pomatia by Azchael | Wiki; Aplachophorans by Chris Allen; Sea Angel by Roy Hopcroft | Arctic Ocean Diversity; Limpet by BioImages; Glaucus atlanticus by Wang, TC, WJ, Chen & Robert; Chiton by Don Loarie; Giant Clam by Ria Tan