Phylum Mollusca

What is a mollusk? PHYLUM MOLLUSCA Slugs, snails, and squids are all mollusk. There is a wide variety of animals in this phylum—from the slug that moves very slow to the squid who is jet-propelled. Most species live in the ocean, but some live in freshwater and moist land habitats. Some mollusks, such as oysters, live most of their lives attached to the ocean floor or a boat that has sunk. Others, like the octopus, swim freely in the ocean.

Phylum Mollusca Some mollusks have shells, and others do not. All mollusks have bilateral symmetry, a coelom, a digestive tract with 2 openings, a muscular foot, and a mantle. This is a membrane that surrounds the internal organs of the mollusk. In mollusks with a shell, the mantle secretes the shell.

A. How Mollusks Obtain Food Most mollusks have a structure called a radula that helps them obtain food. This structure is located within the mouth. It is a tongue-like organ with rows of teeth. It is used to drill, scrape, grate, or cut food.

A. How Mollusks Obtain Food Octopus and squid are predators that use their radulas to tear up the food they capture with their tentacles. Other mollusks are grazers and some are filter feeders.

B. Reproduction in Mollusks Mollusks reproduce sexually. In most aquatic species, eggs and sperm are released at the same time into the water, where external fertilization occurs. Many mollusks that live on land are hermaphrodites, with internal fertilization. Some marine mollusks have free-swimming larvae that propel themselves by cilia.

C. Nervous Control in Mollusks Mollusks have simple nervous systems that coordinate their movement and behavior. Some more advanced mollusks have a brain. Most mollusks have paired eyes that range from simple cups that detect light to the complex eyes of an octopus that have irises, pupils, and retinas similar to the eyes of humans.

D. Circulation in Mollusks Mollusks have a well-developed circulatory system that usually includes a 2 or 3 chambered heart. In most mollusks, the heart pumps blood through an open circulatory system. In this type system, the blood moves through vessels and into open spaces around the body organs. This adaptation exposes body organs directly to blood that contains nutrients and oxygen, and it also helps remove wastes from cell activites.

D. Circulation in Mollusks Some mollusks, such as octopi, move nutrients and wastes through a closed circulatory system. In this system, blood moves through the body enclosed entirely in a series of blood vessels. This system provides an efficient means of gas exchange within the body.

E. Respiration in Mollusks Most mollusks have respiratory structures called gills. These are specialized parts of the mantle that have a rich supply of blood for the transport of gases. Gills increase the surface area through which gases pass.

F. Excretion in Mollusks Mollusks are the oldest known animals to have structures called nephridia. These are organs that remove waste from an animal’s body. Mollusks have one or two nephridia that collect waste from the coelom, which is located around the heart only. Wastes are discharged into the mantle cavity, and expelled from the body by the pumping of the gills.

Diversity of Mollusks Most of the common and well-known species of mollusk are in 3 classes: Class Gastropoda Class Bivalvia Class Cephalopoda

A. Gastropoda This is the largest class of mollusks. They are also called the stomach-footed mollusks. This name comes from the way the animal’s large foot is positioned under the rest of its body.

Gastropoda Most members of this class have a shell but some do not. Some of the animals that do have a shell are abalones, conchs, periwinkles, whelks, limpets, cowries, and cones. These may be plant eaters, predators, or parasites.

Gastropoda A slug is one example of a gastropod that does not have a shell. It is protected by a thick layer of mucus.

Gastropoda Colorful sea slugs, called nudibranches, are protected in a different way. When sea slugs eat jellyfish, they take in the poisonous nematocysts into their own tissues. Any fish that tries to eat the sea slug will get stung by these nematocysts.

B. Bivalvia These are mollusks with 2 shells. Some examples are clams, oysters, and scallops. Most bivalves are marine. They have no distinct head or radula. Most use their large, muscular foot for burrowing in the mud or sand at the bottom of the ocean. A ligament, like a hinge, connects their two shells (valves) and strong muscles allow the valves to open and close over the body. One of the main differences between gastropods and bivalves is that bivalves are filter feeders.

C. Cephalopoda This class is the head-footed mollusks. It includes octopus, squid, cuttlefish, and the chambered nautilus. The only cephalopod with a shell is the chambered nautilus.

Cehphalopoda In this class, the foot has evolved into tentacles with suckers, hooks, or adhesive structures. They swim or walk over the ocean floor after their prey, capturing it with their tentacles. Once tentacles have captured prey, it is brought to the mouth and bitten with strong jaws. Then the food is torn and pulled into the mouth by the radula.

Cephalopoda

Cephalopoda Like bivalves, cephalopods have siphons (pumps) that expel water. They can expel water in any direction and move quickly. Squids can get up to speed of 20 m/s using this system of movement. Squids and octopi also can release a dark fluid to cloud the water. This “ink” helps to confuse their predators so they can make a quick escape.