1 Module # 2 Component # 6 Phylum Mollusca Objectives: To gain an understanding of the morphology and biology of common molluscs. Expected Outcomes: To develop a good understanding of the internal and external morphology of terrestrial gastropods To become familiar with the taxonomic classification of the phylum describe their locomotion, nutrition and reproduction To understand their various forms Image: Ashley Kemp
2 Phylum Mollusca Molluscs form a large part of the animal kingdom, comprising over fifty thousand known and described species. The phylum includes animals as varied as clams, oysters, squid and octopus, slugs and snails, and bivalves. typical of molluscs: Generally, combinations of the following characteristics are The lower surface of the body has developed into a muscular, creeping foot. The body is unsegmented and bilaterally symmetrical. Respiration is by means of gills or lungs. The upper body has developed into a system of two folds known as the mantle. This mantle secretes the shell, and hides the respiratory organs. The body is generally well supplied with mucous secreting glands. The shell (if present) is dorsally positioned and has the shape and function of a shield. They have poorly developed heads. Most these animals are encountered in marine and other aquatic environments. Thus, the only molluscs that a field guide is likely to encounter are the terrestrial slugs and snails. Consequently, only the Gastropods, and of these, only the Order Pulmonary (freshwater and land snails and slugs) will be covered in this course material. However, for completeness other Classes include: C. Polyplacophora (Chitons) C. Cephalopoda (Squids, octopuses, nautiluses) C. Bivalvia (Clams, oysters)
3 Class: Gastropoda (Order: Pulmonata) There are approximately thirty-five thousand species of gastropods known world-wide, having had an unbroken fossil record from the Cambrian epoch (590 million years ago). For more on the fossils record, please see Module # 1, Component # 2 Evolution.
4 Characteristics of Gastropods (Pulmonata.) They have developed a well-defined head. Their shell has been modified from a shield into a retreat. The structure of the shell is always a univalve i.e. in one piece unlike those of the clams and oysters. They all undergo a peculiar physical change known as torsion.
5 The Shell The shell material is secreted by the mantle, and consists of three typical layers: The outside layer is known as the Periostracum and serves to waterproof and protect the shell. The middle layer is called the prismatic layer and consists of calcium carbonate. This serves to strengthen the shell and provide rigidity. The inner layer is called the nacreous layer and consists also of calcium carbonate, but this is deposited in very fine layers. During periods of drought or during winter, a layer called the epiphragm is secreted by the pedal glands. This prevents water loss. The shell is an asymmetrical spiral in its present form. It seems to have originated from the original planospiral shape (that of a hose-pipe coiled flat on the ground.) The spiral of the shell may be either dextral (right hand twist) or sinistral (left hand twist). The spiral forms around the Columella, a central pillar on which the whorls of the shell are deposited. This allows for a very compact design. Shells of this nature are carried obliquely to the long axis of the body. This shape has limited the efficiency of breathing through gills as in other molluscs. Consequently, many gastropods have developed a lung that opens to the atmosphere through the pneumostome. The tip of the shell is always the oldest, with successive layers of material being deposited around the opening in whorls. From the opening, the head and the foot protrude from the shell. Retractor muscles draw the exposed body parts back into the shell when the need arises.
6 Torsion A larval snail starts its life being bilaterally symmetrical i.e. each side of the body is equal in shape and dimensions. As the larva grows, one side of the body grows faster than the other and causes the body to undergo a flexing action. This action ultimately twists the entire visceral mass through 180 degrees. This results in an asymmetrical body, with the anus and viscera having been twisted around to be located near the head. Torsion is very rapid in some species and may only take a few moments to be achieved. The function of torsion is thought to be related to safety, in that the head can be withdrawn into the mantle cavity leaving only the foot exposed to predation.
7 Locomotion The foot, or the sole, is ciliated and has glands that produce mucous on which the animal "walks." This form of locomotion is extremely slow. This movement is accomplished by a complex system of waves that pulse backwards or forwards along the foot and propel the animal along as if in a liquid medium. Mucous left behind is also used by snails to home in on communal hibernating and breeding grounds. This is done by a pheromone trail left in the mucus.
8 Nutrition Most molluscs feed by means of a radula. This is a tongue - like a conveyer belt that has multiple chisel-shaped teeth on its surface. This is moved forward and upwards against the food source, and particles are rasped from the surface and swallowed into the gut. Most species are herbivorous, feeding on decaying and young plant matter. Others, however, are carnivorous, and many specialise in feeding on other species of snails. Image: Ashley Kemp
9 Reproduction Slugs and snails are monoecious i.e. they have both sexes within the same body. Mating takes place reciprocally, with each individual inserting its penis into the vagina of the other and transferring a spermatophore. Eggs are laid by each individual in a moist locality where they develop for up to a month. On hatching they are miniature replicas of the adult. Image: Ashley Kemp