BIO 5 GENERAL BIOLOGY

PHYLUM MOLLUSCA

 

 

1.         The Phylum Mollusca is a large and diverse group of animals that includes clams, scallops, oysters, snails, slugs, squid, octopus, and chamber nautilus. The Phylum shares features with other animal phyla. The mollusks are bilaterally symmetrical, have an organ system level of body organization, have a complete digestive system, and a coelom (small in size). Additionally these animals have a circulatory system and a respiratory system. But, the defining characteristics of the group are special features found only in this animal phylum. These features are 1) the shell, 2) the mantle, 3) the foot, and 4) the radula.

2.         The mollusks are mostly aquatic but are found both in fresh water and in the ocean. A few mollusks (snails and slugs) are terrestrial.

3.         Since the phylum is so diverse, most of this discussion will focus on the clam, an aquatic mollusk. Clams, like sponges, are filter feeders. The soft body of the clam is protected from predators by a two-parted shell that is hinged. The shell is produced by a tissue called the mantle that is adjacent to the shell. The clam has a pair of very large gills, and the surface of the gills is covered with cilia. The cilia beat in a coordinated fashion, and the movement of the cilia cause water to move into and through the clam. Food-containing water enters through the incurrent siphon and passes over the gills. The gills produce a sticky, glue-like material called mucus. Food (small organisms and food particles) becomes trapped in the mucus. The cilia are also responsible for transporting the trapped food to flap-like structures called labial palps. The labial palps gather the food and place it into the clam's mouth. After water passes over the gills, it exits the clam through the excurrent siphon (located next to the incurrent siphon.) Filter feeding mollusk process quite a large amount of water. An average oyster filters three liters of water an hour.

4.         Clams use their muscular food for digging into the sand or soil forming the bottom of the body of water in which the clam lives. The movement of clams is quite limited, and these are sedentary animals.

5.         The nervous system of clams consists of three pairs of ganglia connected by nerve cords. Nerves connect the ganglia and nerve cords to sensory cells. Different cells are specialized for various functions. Some sense touch. These are located at the edge of the mantle. Light sensors are located on the siphons, and osphradia (chemical sensors) are located on the incurrent siphon.

6.         Clams (and all mollusks) have a complete digestive system. It consists of a mouth where food is ingested, a short connecting tube called the esophogus, a stomach which temporarily holds food, and an intestine where food digestion and absorption takes place. A digestive gland surrounds the stomach and releases digestive enzymes into the food within the stomach. Digestion is extracellular. This means that digestive enzymes break food down into food particles and food molecules within the hollow intestine. Food molecules diffuse or are actively transported into the cells lining the intestine.

7.         Food is distributed to the cells of the body by blood. Blood is pumped by the heart into blood vessels. The blood leaves the vessels and flows into cavities called sinuses. Sinuses surround body organs and extend into all parts of the clam. As the blood moves past the intestine, it picks up food molecules and carries these molecules throughout the clam. The blood eventually enters the pericardial cavity (coelom) which surrounds the heart. It then enters the heart though pores, and is then pumped into blood vessels. A circulatory system such as that of the clam in which blood is not always contained within the heart or blood vessels is called an open circulatory system.

8.         Digestive waste leaves the digestive system through the anus. This is located near the excurrent siphon, and the digestive waste is carried out of the clam by water moving outward. The blood circulating through the clam picks up nitrogen waste produced by cells. The nitrogen waste (ammonia) is carried into the pericardial cavity. There the kidney removes nitrogen waste from the blood and releases it into water moving out of the clam through the excurrent siphon.

9.         The release of carbon dioxide and acquisition of oxygen is called gas exchange. Gas exchange is the function of the respiratory system. Blood carries carbon dioxide to the gills. There it is released into the water passing over the gills. There also the gills pick up oxygen from the water moving through the clam. The blood then distributes oxygen to all cells of the body.

10.       The mollusks are grouped into eight classes. The great diversity of habitat, form and function shown by the mollusks is considered to be the result of adaptive radiation. This is a process in which an unspecialized ancestor evolves into very different groups of organism, each adapted to its own environment and own "lifestyle". The ancestral mollusk is believed to have been a marine herbivore (animal that eats plants) that had a shell for protection from predators and for support, a mantle that produced the shell, a foot for movement, and radula for chewing algae. In the Class Pelecypoda animals such as the clam have evolved into filter feeders. These animals have a shell for protection from predators and for support, a mantle that produces the shell, a foot for digging, and lack a radula. In the Class Gastropoda animals such as the snail are terrestrial herbivores. These animals have a shell for protection from predators and for support, a mantle to produce the shell and to serve as a lung for breathing air, a foot for movement, and a radula for chewing plants. In the Class Cephalopoda, animals such as the octopus are marine carnivores. These animals lack a shell or have only a reduced shell because they are fast-moving predators. They have a mantle that has become modified into a siphon. The siphon allows the octopus to move by "jet propulsion". The foot in these animals has become modified into tentacles with sucker for grasping prey. They have a radula that has become modified into a beak for tearing flesh. Each of these animals have the special features found in mollusks, but in each group the ancestral features have become modified in ways that allow the animals to exploit specific food within their specific environment.

11.       People uses members of the Phylum Mollusca for food. Examples of this are clams, oysters, mussels, scallops, abalone, snails (escargot), squid, and octopus. People have defined some mollusk produces as valuables. For examples, pearls are produced by mollusk and some Native Americans used mollusk shells as currency. Some mollusks are pests. Snails and slugs eat flowers and crop plants. Some mollusks are intermediate hosts of human parasites. For example, immature stages of liver flukes and blood flukes live in snails.