The Molluscan World: Life in a Shell (or not) Note: These links do not work. Use the links within the outline to access the images in the popup windows. This text is the same as the scrolling text in the popup windows. I. What is a mollusc? (Page 1) Mollusca: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mollusca.html Mollusca is another Phylum in the Protostomia line. It is most closely related to Phylum Annelida which contains the segmented worms, such as earthworms. While it may seem that molluscs, such as a snail or a clam, have little in common with segmented worms, animals in both Phyla have larvae that are almost identical. Recent DNA evidence also indicates a close relationship between the two groups. Body Plan: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/body_plan.html The major external features of molluscs are illustrated in this generalized body plan. The mantle is a tough sheath that covers the dorsal side of the body. The shell lies above the mantle. There is a thick, muscular structure called the foot on the ventral side of the body, and a specialized feeding structure, the radula, within the mouth. The mollusc shown here represents the hypothetical ancestor from which modern molluscs are derived. We shall see that these four features are modified to various extents in different molluscan groups. Mantle: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/mantle.html Molluscs have a tough dorsal covering called the mantle. It may be seen projecting from beneath the shell of the chiton shown here. It is thought that the mantle first appeared in a flatworm-like ancestor to provide a protective shield over the animal’s soft body. The mantle began to secrete spicules of calcium carbonate which increased its strength, and these eventually fused to form plates and finally the solid shell of modern molluscs. Foot: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/foot.html This photograph is the ventral surface of the chiton seen is the previous image. The thick, muscular foot occupies most of the ventral surface. Undulations of the foot coupled with mucus secretion provides a slow, creeping locomotion similar to that of planarians. Indeed, some of the tiny molluscs also bear flatworm-like cilia on the foot to facilitate locomotion. This is one indication that molluscs evolved from a flatworm ancestor. Radula: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/radula.html The radula is a fascinating feeding device found only in molluscs. In its generalized form, as shown here, the radula is a belt-like structure bearing teeth which is located inside the mouth. It can be extended out through the mouth when feeding. Extension and retraction of the radula are controlled by muscles within the head. Scrape: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/scrape.html The radula of primitive molluscs was originally designed to scrape food, mainly vegetation, from surfaces such as rock. This function has been retained in several groups of modern molluscs and is illustrated by this diagram which shows teeth scraping a rock as the belt-like radula moves back and forth. The photograph on the left is the radula of a slug. On the right we see the more complex radula of a whelk. Shell: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/shell.html Perhaps the most important innovation achieved by molluscs is the development of a protective shell. The shell is secreted by the mantle and consists of at least two hard layers containing crystals or sheets of calcium carbonate. These layers are similar to bone (which also contains calcium salts), but have an arrangement of molecules that make shell harder. On the other hand, there is less protein in shell causing it to be more brittle than bone. The inner shell layer is harder than the middle layer and has a smooth, glossy sheen. It is called the nacreous layer and is the source of “mother o’ pearl”. The outer shell covering is a tough, fibrous layer rich in protein. Outer Surface: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/outer_surface.html Here we see a highly magnified view of a shell surface. Many shells are distinguished by a characteristic pattern of lines and ridges on the outer surface. Increase in Size: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/increase_in_size.html The molluscan shell fits loosely around the body as compared to other animals with external skeletons. It offers some support for the body, but has primarily a protective function. Since the shell is not shed as the animal grows, it must grow as well. The rings in this shell indicate seasons of shell growth, somewhat like the rings in a tree trunk which reveal the tree’s age. Advantages: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/advantages.html The advantages of a shell are obvious. Molluscs like the snail have a moveable home into which they can retract when danger threatens. The sedentary clam can tightly close its pair of shells and remain safe within. The shell protects the soft-bodied molluscs from predators, mechanical damage and also from desiccation, when they are out of the water. In some specialized molluscs, the shell serves additional purposes such as burrowing, boring or containing gas for buoyancy. Evolutionary Trend: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/evolutionary_trend.html Although the shell is an effective protective device, it is heavy and limits locomotion to speeds as slow as, well, a snail. Thus, it should not be surprising that some molluscs have lost their shell, trading protection for speed and mobility. In the images shown here, we see two shelled molluscs in the bottom panels and two of their shell-less relatives (sea slugs) at the top. The squids and octopi are prime examples. They belong to a molluscan group in which almost all members have lost the protective shell. Advanced: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/advanced.html Unlike the cnidarians and flatworms, molluscs have a digestive tract with both a mouth and an anal opening. This allows one-way movement of food through the tract and leads to development of specialized digestive regions and the attachment of enzyme secreting organs. Digestion is thus more efficient in molluscs and is entirely extracellular. In parallel with formation of a complete digestive tract, a separate circulatory system appears in the molluscan line. A heart is present and housed within a small cavity. Both the digestive organs and the cavity are embedded within a mass of mesodermal tissue, called the visceral mass, that comprises most of the mollusc’s body. Thus the cavity is surrounded by mesodermal tissue and meets the definition of “coelom”—a body cavity lined by mesoderm. In molluscs, the coelom is very small, but in animals of most other Phyla, such the segmented worms, it occupies most of the body’s interior. Molluscs are also advanced in acquiring oxygen; they have developed a gill for this purpose. More details on molluscan respiration will follow shortly. II. What are the kinds of molluscs? (Page 2-5) Primitive Features: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/primitive_features.html The body plan of chitons has several primitive features, especially regarding internal anatomy. In other ways though, it is specialized for a life adhering to rocks and shells. The dorsal surface of the chiton’s body is covered by 8 overlapping plates. This differs from the solid shells of other molluscs and is an adaptation that permits flexibility while conforming to the shape of an uneven surface. The mantle extends out from beneath the shell plates and bears scales, bristles or calcareous spicules in some species. The broad ventral foot is surrounded by gills, which are protected by the over-hanging mantle. Chitons do not have a well defined head, but the mouth opens ventrally at the anterior end. Low Tide: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/low_tide.html Chitons are usually inactive when exposed at low tide, but begin to feed when covered by water. They tend to avoid light and are often found in crevices or under ledges during the day. It is not unusual to see one chiton clinging to the shell of another. Long Radula: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/long_radula.html The radula of chitons is unusually long compared to that of other molluscs and bears teeth that are mineralized to increase hardness and durability. These adaptations are necessary for a life spent scraping hard, abrasive surfaces. Interestingly, some of the teeth are capped by magnetite (an iron-containing mineral) and are actually attracted to a magnet. Clamp: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/clamp.html Chitons can cling tenaciously to rocky surfaces. For chitons living in the intertidal zone, this important capability prevents them from being carried away in the surging tide. When necessary, the chiton uses the mantle as well as the foot for attachment. It presses the outer edge of the mantle to the rock, then raises the inner portion to create a vacuum that acts like a suction cup. With this combined action of foot and mantle it is almost impossible to pry a chiton from its rock without injuring the animal. Many Species: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/molluscs/molluscs_popups/many_species.html Chitons vary little in appearance. Most are within the 3-12 cm size range and are colored in drab shades of red, brown, yellow or green.