Mollusks: Phylum Mollusca

• Mollusks are invertebrates with soft bodies belonging to the Phylum Mollusca.
• Phylum Mollusca is the second-largest phylum after Phylum Arthropoda.
• Most mollusks live in marine environments, but some inhabit freshwater and land.
• Phylum Mollusca contains over 80,000 species that are herbivores, parasites, or predators.

Mollusk Characteristics

• Mollusks usually protect their soft bodies with a calcium carbonate shell that can be external or reduced internal shells.
• They have a fleshy cloak called a mantle that wraps around the body and secretes the shell on the outer surface.
• Aquatic mollusks have gills in the mantle cavity for gaseous exchange, while land-dwelling mollusks have a lung-like space.
• Mollusks have a large, flat muscular foot for locomotion.
• Viscera, or body organs, are located above the foot in a concentrated mass.
• Some mollusks have a radula, which is a belt of teeth resembling a rasp tool, in their mouth.
• The body space containing blood is referred to as a hemocoel, and mollusks also have a coelom (body cavity).
• Some mollusks have an open circulatory system with hemolymph, while others like squids have a closed circulatory system.
• Mollusks possess digestive, circulatory, respiratory, excretory, reproductive, and nervous systems.
• The larval stage, called trochophore, is similar in all mollusks, developing into a veliger larva in many species.

Types of Mollusks

• Mollusks are classified into Gastropoda, Bivalvia, Cephalopoda, and Polyplacophora (chitons).
• The three major groups of mollusks are gastropods, cephalopods, and bivalves.

Gastropods

• Gastropods belong to the class Gastropoda, the largest class of mollusks, with over 70,000 species.
• Gastropods are primarily found in marine areas, freshwater, or on land and have a well-developed head with tentacles.
• Gastropods use their muscular foot for crawling.
• Most gastropods have a spirally coiled external shell, while others have a flattened shell or no shell.
• Land-dwelling gastropods with a lung-like structure in their mantles are called pulmonates.
• Snails and slugs are examples of gastropods.

Bivalves

• Bivalves belong to the class Bivalvia, with approximately 10,000 species.
• They mainly inhabit fresh and marine waters with a pH above 5.
• Bivalves have two hinged shells (valves) connected by a flexible ligament.
• An adductor muscle keeps the valves tightly closed for protection.
• Bivalves are filter feeders, using gills to collect food and they have no head.
• Examples of bivalves include clams, scallops, mussels, and oysters.

Cephalopods

• Cephalopods belong to the class Cephalopoda and include approximately 650 species.
• Cephalopods are primarily predators or scavengers.
• They have a funnel in their mantle that produces jet propulsion movement.
• The mouth is surrounded by 8 to 90 arms or tentacles with suckers for catching prey.
• Cephalopods have well-developed eyes that can form images.
• They can change body color and produce a thick black liquid from their ink sac to avoid predation.
• Examples of cephalopods include the squid, octopus, nautilus, and cuttlefish.

Mollusk Examples

• Snails

  • Belong to the Class Gastropoda.
  • They have an external shell, such as a conch shell.
  • Land snails are pulmonate.

• Octopus

  • Belong to Class Cephalopoda.
  • Have eight arms with suckers surrounding their mouth.
  • They also have complex eyes.

• Oysters

  • Belong to Class Bivalvia.
  • They have two hinged shells.
  • They are mainly filter feeders.