32 Reading: Mollusks
This laboratory exercise covers the following animals. You should learn this classification scheme and be able to classify the animals into these categories.
- Phylum: Mollusca (Mollusks)
- Class: Polyplacophora (Chitons)
- Class: Gastropoda (snails)
- Class: Bivalvia (Clams)
- Class: Cephalopoda (Nautilus, Squid, Octopus)
All mollusks have a visceral mass, a mantle, and a foot. The visceral mass contains the digestive, excretory, and reproductive organs. The mantle is a covering. It may secrete a shell. The foot is muscular and is used for locomotion, attachment, and/or food capture.
The mantle and foot can be seen in figure 1. The visceral mass is underneath the gill.
There may be a radula, a structure that resembles a tongue but contains hard plates and is often used for scraping food. The coelom is reduced and limited to the region near the heart.
Most mollusks have an open circulatory system but cephalopods (squids, octopus) have a closed circulatory system. The blood pigment of mollusks is hemocyanin, not hemoglobin. The heart of a clam can be seen in the photograph below. Bivalves have three pairs of ganglia but do not have a brain.
Most mollusks have separate sexes, but most snails (gastropods) are hermaphrodites. Some marine mollusks have a ciliated larval form called a trochophore.
Chitons (Class: Polyplacophora)
Chitons have a dorsal shell composed of 8 plates. A ventral foot is used for locomotion and for attachment to rocks. It pulls itself close to rocks for protection. Observe the chiton on display.
Snails—Class Gastropoda
Gastropods have an elongated, flattened foot and usually a head and shell although nudibranchs (sea slugs) and terrestrial slugs lack a shell.
Most are marine but there are also numerous freshwater and terrestrial species. Herbivorous gastropods use a radula to scrape food from surfaces. Carnivores may use a radula to bore a hole through surfaces such as bivalve (clam) shells. Some gastropods such as the slug (below) do not have a shell.
The larvae undergo torsion during development. It is a twisting that positions the visceral mass so that the anus is above the head. It is due to one side of the visceral mass growing faster than the other. The advantage (or function) of torsion is uncertain, but it may be to balance the animal or it may be to allow the head to be withdrawn into the shell first when predators approach.
Observe the snails and slugs on display.
Gas Exchange (Respiration)
Some gastropod species are terrestrial and have lungs for gas exchange, others are aquatic and use gills.
The space near the entrance to the shell that is bound by the mantle is the mantle cavity. Aquatic gastropods have gills located in the mantle cavity. The mantle of terrestrial gastropods functions as a lung.
Bivalves—Clams and relatives (Class: Bivalvia)
Bivalves have two shells (valves) held closed by powerful muscles. The shell is produced by the underlying mantle; it grows along the outer margins. They