General Characteristics
- Includes the snails, limpet, and slugs.
- Largest andmost varied molluscan class with 35, 000 different species
- They occupy marine, freshwater, and terrestrial
- Can be used as an intermediate host for some trematoda parsites
Torsion
This occurs early in development and is the 180 degree counterclockwise twisting of the visceral mass, mantle, and mantle cavity. Torsion positions the gills, anus, and opening from the excretory and respiratory structures behind the head causing the digestive tract to form a U shape. There are three evolutionary advantages from torsion, the first allows the head to enter the shell before the foot, which allows for the ehad to brotected from predation. The presence of an operculum furthers protection from predation.
Shell Coiling
Fossils: one plane coils, Modern: asymmetrically coiled= more compact, successive coils slightly larger than preceding whorl. Patterns leaves less room on one side of the visceral mass for certain organs leading to singles rather than pairs.
Locomotion
Use ciliated flattened foot covered with gland cells. Cilia propel themselves over a mucous trail (small). Waves of muscular contraction move over the foot (larger). Foot can be modified for clinging or swimming.
Feeding and Digestion
Feed by scraping with radula. Can be predatory or parasitic or herbivorous. Anterior portion of the digestive tract may be modified into an extensible proboscis containing the radula. This structure is important for predation. Ciliated digestive tract like most molluscs. Food is trapped in muouc strings and incorporated into a mucoid mass (protostyle) extending to the stomach and rotated by cilia. Food trapped on the protostyle is freed and digested via enzymes and acid in the stomach.
Maintenance and Other Functions
Have one gill as apposed to two in precursors. Siphon is a rolled extension of the mantle is an inhalant tube. Gills are loss/reduced in land snails but have a muscular mantle for gas exchange between blood and air. Mantle contractions circulate air and water through the mantle cavity. Open circulatory system allows bllod to flow from the vessels to cells in tissue spaces (sinuses). Heart consists of single, muscular ventricle and two auricles. Most gastropods have lost one auricle because of coiling. Blood of molluscs acts as a hydraulic skeleton which is pressurized to tissue spaces to extend body structures and support. Blood is moved through contraction.
Nervous system consisted of six ganglia located in the head-foot and vsceral mass and torsion twists the nerves that link these ganglia-> modern ganglia have untwisted nerves and concentrated nervous tissues into fewer but larger ganglia especially in the head.
For sensory they have eyes at the base or at the end of tentacles that could either be a pit of photoreceptors or lens/cornea. Statocysts (sense of directino via gravity) are in the foot. Osphradia are chemoreceptors in the anterior wall of the mantle cavity that detect sediment and chemicals in inhalant water/air. Osphradia help detect prey.
Right and left nephridia->only a left nephridia because of shell coiling. Nephridium consists of sace with highly folded walls and connects to the reduced coelom (pericardial cavity). Excretory wastes come from filtered fluids and secretions from blood into the coelom which is modified by selectively reabsorbing certain molecules. Nephridium opens to the mantle cavity or in land snail (right side of body adjacent to the mantle cavity/anal opening). Aquatic gastropods excrete ammonia. Terrestrial snails convert ammonia to uric acid. Uric acid is relatively insoluble in water and can be excreted in a semisolid form to conserve water.
Nervous system consisted of six ganglia located in the head-foot and vsceral mass and torsion twists the nerves that link these ganglia-> modern ganglia have untwisted nerves and concentrated nervous tissues into fewer but larger ganglia especially in the head.
For sensory they have eyes at the base or at the end of tentacles that could either be a pit of photoreceptors or lens/cornea. Statocysts (sense of directino via gravity) are in the foot. Osphradia are chemoreceptors in the anterior wall of the mantle cavity that detect sediment and chemicals in inhalant water/air. Osphradia help detect prey.
Right and left nephridia->only a left nephridia because of shell coiling. Nephridium consists of sace with highly folded walls and connects to the reduced coelom (pericardial cavity). Excretory wastes come from filtered fluids and secretions from blood into the coelom which is modified by selectively reabsorbing certain molecules. Nephridium opens to the mantle cavity or in land snail (right side of body adjacent to the mantle cavity/anal opening). Aquatic gastropods excrete ammonia. Terrestrial snails convert ammonia to uric acid. Uric acid is relatively insoluble in water and can be excreted in a semisolid form to conserve water.
Reproduction and Development
Many marines are dioecious with gonads lying in spirals of visceral mass. Ducts discharge gametes into the sea for external fertilization. Many other nsial are monoecious and internally fertilize via cross fertilization. Copulation results in mutual sperm transfer. Penis is evolved from a fold of the body wall and portions of the female reproductive tract have become glandular and secrete mucus (protective jelly) around the fertilized egg. Some monoecious snails are protandric (testes develop before ovaries) and afterward degenerate having ovaries mature. Eggs are shed singly or in masses for external fertilization. Internal fertilization are eggs deposited in gelatinous strings or masses. Terrestrial snails deposit eggs in moist environments and encapsulate them in a calcareous shell. Marine gastropod larva develops into trochophore larva with develops into a veliger larva (free swimming larva with foot, eyes, tentacles, and shell). Sometimes trocho is suppresed nad veli is the primary larva. Torsion occurs during the veilger stage followed by settling and metamorphosis to adult.
Gastropod Diversity
Prosobranchia is the largest subclass. Some carnivorous species inject venom with a radula modified into a hollow, harpoonlike sturcture. Heterpods have a fin instead of foot used for propulsion. Opisthobranchia are marine with reduced shells, mantle cavity and gills. They have undischarged nematocysts from their cnidarian prey. Foot is modified into thin lobes for swimming. Pulmonata are mostly herbivores and have a long radula for scraping plants. Highly vascular mantle cavity. Air/water moves in/out of the opening of the mantle cavity (pneumostome).