Feeding and Digestion

All mollusks are heterotrophs, some are carnivorous while some are herbivorous.

Cephalopods, octopuses and squids, are active predators. They capture their prey with their tentacles and holds a firm grasp with their suckers. They have a two part mouth, a radula, to rasp on their food, and a beak to cut their prey into pieces before being swallowed. With their radula and chitinous beak to kill and eat their prey. Their diet mainly consist of fish, crustaceans, and other mollusks.

The Bivalvia mollusks are filter feeders, extracting the nutritious matter from the sea where they live. They draw water from their gills through the beating of the cilia. Suspended food are trapped in the mucus of the gill. Food they eat are phytoplankton, zooplankton, algae and other nutrients and particles in water. The trapped food and then transported to the mouth, where they are eaten, digested and expelled as feces. They filter about 5L of water per hour. Bivalves that are buried under the ground extend a siphon to the surface for feeding.

Most gastropods are herbivores and scavengers. They diet consists of fungi, dead animal material, leaves, stems, bulbs and algae. A few are carnivorous, and usually the larger gastropods. They prey on other snails and other smaller organisms. They use their radula for feeding, a tongue like structure that is covered by rows of rasping teeth for scraping or cutting food before it enters the esophagus.

Cephalopods' muscular stomach is located roughly in the midpoint of the visceral mass. After digestion in the stomach, the bolus moves to a long, white organ, caecum for digestion. Lastly the food goes to the liver, for nutrient absorption and solid waste is excreted through the rectum. Octopuses are said to have external digestion. They will use their beak to break through the crab's armour and excrete a digestion liquid onto the crab's internal organs. The liquid contains a nerve poison that paralyses the prey and proteases, enzymes, that digest its organs, so the crab's interior can be sucked by the octopus.

When food particles enter the mouth of the bivalves, they travel down the esophagus to the stomach. In the stomach mechanical and chemical digestion is taken place to break down into smaller particles with the churning motion created by the muscles and enzymes to help in breaking down food. With ciliary tracts, the bolus in the stomach is carried to the digestive diverticulum for intracellular digestion. There are digestive cells that collect the nutrients and store into food vacuoles within the cell. The intestine transfer the wastes into the rectum where it is later discharged from the anus.

For gastropods, after the food have been swallowed, the food is moved to the digestive tract through the currents of the ciliary movements. In carnivores, this doesn't happen, but instead it depends on the muscular action of the esophagus. Enzymes are secreted by various salivary glands and digestive glands and led into the buccal cavity or the stomach, sometimes to both area. Digestion then occurs in the stomach and food is broken down. If they are inactive, digested food can be stored in the apical digestive gland, or what is known as the "liver".