Mollusca Flashcards
What are the major classes of marine Mollusca?
Gastropods, cephalopods, and bivalves
What are the defining traits of mollusks?
The dorsal part of the mantle secretes calcareous spicules, plates or shells.
Radula (ring of teeth in esophagus, absent in bivalves)
There are two pairs of main nerve cords.
Ventral body wall muscles develop into a locomotory/attachment foot.
Describe the mantle of a mollusc
The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat, to sea, fresh water or air.
The cavity was at the rear in the earliest molluscs, but its position now varies from group to group. The anus, a pair of osphradia (chemical sensors) in the incoming “lane”, the hindmost pair of gills and the exit openings of the nephridia (“kidneys”) and gonads (reproductive organs) are in the mantle cavity.
The whole soft body of bivalves lies within an enlarged mantle cavity.
Describe the shell of molluscs.
The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate), except the outermost layer, which in almost all cases is all conchiolin (see periostracum).
Molluscs never use phosphate to construct their hard parts, with the questionable exception of Cobcrephora. While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.
Describe the foot of molluscs.
The underside consists of a muscular foot, which has adapted to different purposes in different classes.[34]:4
The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.
In bivalves, the foot is adapted for burrowing into the sediment; in cephalopods it is used for jet propulsion, and the tentacles and arms are derived from the foot.
Describe the circulatory system of Molluscs
Molluscs’ circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton. The blood contains the respiratory pigment hemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.
The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine. A pair of nephridia (“little kidneys”) to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.
Describe mollusc respiration
Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills’ cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.
Descrive mollusc digestion
Members of the mollusc family use intracellular digestion to function. Most molluscs have muscular mouths with radulae, “tongues”, bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks, and is associated with the odontophore, a cartilaginous supporting organ. The radula is unique to the molluscs and has no equivalent in any other animal.
Descrive mollusc nervous system
Two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot.
Describe mollusc feeding ecology
Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as a ‘rake’ to comb up filaments from the sea floor. Others feed on macroscopic ‘plants’ such as kelp, rasping the plant surface with its radula.
