Discovery Science: Earth – Invertebrates – Mollusks

Earth Science: Invertebrates – Mollusks

Mollusks (phylum Mollusca) are a diverse group of animals that include snails, mussels, clams, and cephalopods. Mollusks may live either in saltwater, freshwater, or on land.

All mollusks have a unique structure called the radula, a rasping organ similar to a tongue for feeding, though many species have only a reduced or embryonic radula.

A rigid outer shell of calcium carbonate is also common to many mollusks. The shell protects and supports the soft body tissue.


Snails (class Gastropoda) have a head with two tentacles and a muscular foot, flattened for crawling. The inner organs are located in the inner body cavity; in snails with shells, these organs are inside the spiral-shaped shell.

When threatened, the animal can fully retreat inside its shell. In slugs, the intestines are located in the foot. There are two types of snails, classified according to how they breathe: gilled snails with gills and pulmonate snails with lungs.


Just like snails, cephalopods (class Cephalopoda) also have a distinct head section. Attached tentacles are covered with many suckers. They often have highly developed eyes and beak-shaped jaws. Species living in light surroundings will often eject a dark tinted substance when threatened, so as to hide in the cloud of ink.

Others can adapt quickly to the surroundings by changing color so they disappear against the background. Common animals of this class are squid, cuttlefish, octopus, and the beautiful pearly nautilus.

Mussels and clams

Exclusively aquatic mussels and clams (class Bivalvia) have a two-part shell that can be opened and closed by strong muscles. Some species are fixed in one place, while others can crawl or swim by using their shells to create a propelling effect.

Mussels and clams are filter feeders, eating plankton and detritus from water that they suck in. Marine species usually reproduce through external fertilization.


the shipworm, which can grow to eight inches (20 cm), may look like a worm due to its elongated shape and softissue, but it is actually a clam with a reduced shell.

The tiny shells are razor sharp allowing these animals to bore into wood During the times when most ships were constructed with wood, the shipworm was especially unpopular for the damage it caused.


MANY SNAILS ARE HERMAPHRODITES, which means that they have both male and female germ cells. Before the actual act of mating, individuals of some species will shoot a tiny “Cupid’s arrow” into the foot of their partner.

The arrow introduces a slimy secretion, making the other snail more receptive to the sperm of the “shooter.” Successful shooters often have twice as many offspring as those who miss the target.