Mollusks are shelled invertebrates. These are mostly marine-dwelling organisms like cuttlefish, squid, octopi, clams, mussels, oysters, but also includes the terrestrial slugs and snails. Some of these organisms have an easily recognizable outer shell, others have an inner shell, and others have the organ that CAN make a shell, but doesn’t. I’ve included a picture here of an extinct organismA living individual made up of cells. called an ammonite. They are like a chambered nautilus, but they are coiled, not spiraled. I just want to remind you that we cover living animals, but there are many extinct versions.
Mollusks have three anatomical components: Mantle, visceral mass, foot. The visceral mass contains what we’d call the guts. Note that the digestive tract isn’t just a tube, it’s got pockets, twists and turns, and increased surface area. Marine mollusks have gills for respirationThe process of gas exchange, including ventilation, external and internal respiration., like fish, and they have a dedicated blood supply, trapped in blood vessels, like the annelids.
These are bilateral organisms with a nervous systemThe organ system that controls body functions using electrical and chemical signals. that’s pretty well organized. There’s a big cluster of nervous tissue which we can actually call a brain. Note the axons (like extension cords) that control each individual tentacle of this squid.
Most bivalves, or mollusks with two shells are filter feeders, filtering suspended particles from the waterThe universal solvent essential for life.. They have an incurrent siphon that takes in water, and an excurrent siphon that spits out the filtered water.
Other mollusks, like gastropods, or land snails and slugs, are herbivores that like to graze on beds of algae. In factA statement based on direct observation that is repeatedly confirmed., snails are beneficial for fish tanks as the snails like to scrap the algae off the glass. They do this with a specialized feeding structure called a radula. This is essentially a tongue, but it has lots of blunt spikes to increase surface area. Cephalopods, like squid and octopi, are carnivorous animals, with a beak at the center of their tentacles. Yes, a break…for crushing.
All molluscs have an organ called a mantle that is able to secrete a shell. This cuttlefish here has an internal shell. Squid have the same, but their shell is like cellophane. It’s called a pen, and yes, you can remove the ink sac and write with the pen and ink. Some of the bivalves, or molluscs with two shells, have a muscular foot. They use this foot to dig themselves into the sand, pointing their siphons upwards for filtering. The cephalopods, octopi, squid, and cuttlefish use their siphons for propulsion, spewing the water out to push them forward.
List of terms
- organism
- respiration
- nervous system
- water
- fact