Phylum Mollusca: Characteristics and Diversity

General Characteristics

• Mollusks are a diverse group that includes snails, squid, octopi, clams, and oysters.
• Mollusks have a visceral mass containing organs, a foot (large muscle), and a mantle (outer covering that may secrete a shell).
• Examples are chiton, snail, bivalve, and octopus

Phylum Mollusca

• Mollusca consists of over 112,000 species of soft-bodied animals that display Bilateral Symmetry
• They exhibit reduced segmentation, and their main body parts are visceral mass, foot, and mantle
• Mollusks have internal organs (heart, digestive, reproductive & excretory) and many respire through gills.
• Nitrogenous wastes are excreted through nephridia
• They have an open circulatory system with a dorsal heart and vessels, and a hemocoel (primary body cavity aiding fluid return to heart).
• Mollusks have a complete digestive system with a mouth and radula (used for grazing/drilling) and also a nervous system consisting of a dorsal brain, nerve ring, and ventral nerve cords.
• Reproduction is sexual, with trochophore larvae developing into veliger larvae for swimming
• They are true coelomates with a reduced main body cavity, called the hemocoel and are protostomes, with the mouth developing first in the embryo

Main Classes

• Amphineura (Polyplacophora), Bivalvia, Gastropoda, and Cephalopoda
• Bivalves and cephalopods are the focus

Bivalves

• Bivalves have a two-part shell with examples of clams, oysters, mussels, scallops
• The mantle secretes the shell for protection, and muscles hold the shell closed
• The foot varies between species and clams use a large foot for digging, while scallops move by jet propulsion and can swim. Oysters attach to rocks for a permanent home
• Bivalves mainly live in marine intertidal zones and have a large visceral hump, a muscular foot, a reduced head, and a thin mantle
• Their mantle cavity contains gills and two siphons (incurrent and excurrent).

Bivalve feeding

• Filter feeding occurs as water with food enters through the incurrent siphon, food is filtered, and excess water exits through the excurrent siphon.
• The oldest part of the shell near the hinge is called the umbo.
• Bivalves have no radula (teeth-like structure).
• Clams are an example of bivalve anatomy
• Gills perform respiration and filter food
• The digestive path of bivalves involves food moving from labial palps to the mouth, esophagus, stomach, intestine, and anus.
• The intestine is long and coiled, ending at the anus for waste expulsion.
• The heart is in the visceral mass, pumps blood into the body cavity, and supplies oxygen.
• Bivalves have an open circulatory system with a dorsal heart and vessels.
• Geoducks burrow in sand/rocks or attach via byssal threads and reproduction involves separate sexes with external fertilization

Cephalopods

• Cephalopod means "head foot" and examples are octopuses, squid, cuttlefish, and nautilus
• All cephalopods live in the ocean and are advanced invertebrates with a highly developed nervous system.
• Cephalopods have learning and muscular control and can be trained and have highly advanced and different eyes than humans.
• Movement is by jet propulsion: take in water slowly and expel it quickly to move forward
• Cephalopods have a chitinous beak for feeding on animals with shells or exoskeletons and also a radula (toothed tongue) for grazing/drilling.
• An ink sac is used to cloud water and escape predators, and chromatophores are special pigment cells for changing color and texture.
• Octopuses use chromatophores for camouflage, while cuttlefish use them for attracting mates and warning off competition
• Respiration occurs through gills in the mantle cavity and have a closed circulatory system, with circulatory fluid contained within vessels.
• Jet propulsion is to escape predators by expelling water through a siphon.
• During feeding, cephalopods catch prey with tentacles and inject poison with their radula (toothed tongue).
• Reproduction is sexual with internal fertilization, followed by egg laying.

Explore the diverse phylum Mollusca, including snails, squid, and clams. Learn about their unique features such as the visceral mass, foot, and mantle. Discover their complex systems, including respiratory, circulatory, and digestive.