Class Gastropoda ("Belly-Foot")
Why is Torsion necessary?
-(1) Without torsion, withdrawal into the shell would proceed with the foot entering first and the head entering last.
- With Torsion the head enters shell first, exposing head to less predators
- In some snails a covering called an operculum, on the dorsal, posterior margin of the foot provides protection. (refer to "Molluscan vocabulary").
-(2) Anterior opening of the mantle cavity allows clean water to enter from the front of the snail to mantle cavity, rather then risking contamination of silt stirred up by the snail's crawling.
-(3) The twist on the mantle's sensory organs around the head is a significant advantage because it makes the snail more sensitive to stimuli coming from the direction in which it moves.
-(1) Without torsion, withdrawal into the shell would proceed with the foot entering first and the head entering last.
- With Torsion the head enters shell first, exposing head to less predators
- In some snails a covering called an operculum, on the dorsal, posterior margin of the foot provides protection. (refer to "Molluscan vocabulary").
-(2) Anterior opening of the mantle cavity allows clean water to enter from the front of the snail to mantle cavity, rather then risking contamination of silt stirred up by the snail's crawling.
-(3) The twist on the mantle's sensory organs around the head is a significant advantage because it makes the snail more sensitive to stimuli coming from the direction in which it moves.
* An adaption allows water and wastes to exit through the mantle cavity through openings in the mantle and shell posterior to the head.
* Some gastropods undergo detorsion, where the embryo undergoes a 180 torsion and then untwists 90 degrees. Mantle cavity opens on the right side of the body behind the head.
* Some gastropods undergo detorsion, where the embryo undergoes a 180 torsion and then untwists 90 degrees. Mantle cavity opens on the right side of the body behind the head.
Shell Coiling
* Earlier gastropods had a coiled shell. This arrangement is not commonly found in modern fossils, because growth resulted in an cumbersome shell.
* Modern snail shells are asymmetrically coiled into a compact form with coils that get larger as it swirls around. This pattern leaves less room on one side of the visceral mass for certain organs, which is evidence that organs that are now single, use to be paired.
* Modern snail shells are asymmetrically coiled into a compact form with coils that get larger as it swirls around. This pattern leaves less room on one side of the visceral mass for certain organs, which is evidence that organs that are now single, use to be paired.
Locomotion
* Almost all gastropods have a flattened foot that is used to creep along surfaces with its cilia, and gland cells.
*The smallest of the snails use cilia to help propel themselves over a mucous trail, while larger snails use waves of muscular contraction that move over the foot.
-foot of some is modified for clinging or swimming
*The smallest of the snails use cilia to help propel themselves over a mucous trail, while larger snails use waves of muscular contraction that move over the foot.
-foot of some is modified for clinging or swimming
Feeding and Digestion
* Most feed my scraping small organisms like algae with their radula.
* Others are herbivores that feed on larger plants, scavengers, parasites, or predators.
* A modification includes the anterior part of the digestive tract forming an extensible proboscis that contains the radula (important for predatory snails that extract hard-to-reach animal flesh).
* Digestive tract is ciliated.
* Food gets stuck in mucous strings that are in a mucoid mass called the called the protostyle. It extends to the stomach and is rotated by cilia. (refer to "Molluscan vocabulary"). A digestive gland in the visceral mass releases enzymes and acid into the stomach and the trapped food in broken down. Wastes form fecal pellets in the intestine.
* Others are herbivores that feed on larger plants, scavengers, parasites, or predators.
* A modification includes the anterior part of the digestive tract forming an extensible proboscis that contains the radula (important for predatory snails that extract hard-to-reach animal flesh).
* Digestive tract is ciliated.
* Food gets stuck in mucous strings that are in a mucoid mass called the called the protostyle. It extends to the stomach and is rotated by cilia. (refer to "Molluscan vocabulary"). A digestive gland in the visceral mass releases enzymes and acid into the stomach and the trapped food in broken down. Wastes form fecal pellets in the intestine.
Other Maintenance Functions
*Primitive Gastropods had two gills, Modern Gastropods lost one due to coiling of the shell.
* Some have a rolled extension of the mantle, called a siphon, used as inhalant tube. (refer to "Molluscan vocabulary").
* Burrowing species extend the siphon to the surface to pull in water. Gills are lost or reduced to land snails, these snails has a great vascular mantle for gas exchange between blood and air.
- mantle contractions help circulate air and water through the mantle cavity.
* Gastropods have an open circulatory system. Blood leaves the vessels and directly fills cells in tissue spaces called sinuses.
*Typically mollusks have two heart auricles and a ventricle, but gastropods lost one auricle because of coiling.
* The blood of mollusks acts as a hydraulic skeleton. (refer to "Molluscan vocabulary").
* Mollusks contract muscles to force fluid, blood, forward.
*The nervous system of primitive snails had six ganglia and torsion twisted the nerves that link the ganglia. The evolution of of the gastropod nervous system has resulted in the untwisting of nerves and the concentration of nervous tissues into fewer but larger ganglia.
* Gastropods have well-developed sensory structures.
-eyes at base or end of tentacles
- pits of photoreceptor cells or may have lens and cornea.
* Statocysts are in foot
*Primitive snails had twp nephridia. In modern snails the right nephridia is absent, because of coiling.
* Excretory wastes are derived largely from fluid filtered and secreted into the coelom from the blood.
* Nephridium modifies waste by selectively reabsorbing certain ions and organic molecules.
- Nephridium opens to mantle cavity or right side of body next tp mantle cavity (land snails).
*Aquatic snails excrete ammonia.
*Land snails have to convert ammonia to uric acid (less toxic form).
* Some have a rolled extension of the mantle, called a siphon, used as inhalant tube. (refer to "Molluscan vocabulary").
* Burrowing species extend the siphon to the surface to pull in water. Gills are lost or reduced to land snails, these snails has a great vascular mantle for gas exchange between blood and air.
- mantle contractions help circulate air and water through the mantle cavity.
* Gastropods have an open circulatory system. Blood leaves the vessels and directly fills cells in tissue spaces called sinuses.
*Typically mollusks have two heart auricles and a ventricle, but gastropods lost one auricle because of coiling.
* The blood of mollusks acts as a hydraulic skeleton. (refer to "Molluscan vocabulary").
* Mollusks contract muscles to force fluid, blood, forward.
*The nervous system of primitive snails had six ganglia and torsion twisted the nerves that link the ganglia. The evolution of of the gastropod nervous system has resulted in the untwisting of nerves and the concentration of nervous tissues into fewer but larger ganglia.
* Gastropods have well-developed sensory structures.
-eyes at base or end of tentacles
- pits of photoreceptor cells or may have lens and cornea.
* Statocysts are in foot
*Primitive snails had twp nephridia. In modern snails the right nephridia is absent, because of coiling.
* Excretory wastes are derived largely from fluid filtered and secreted into the coelom from the blood.
* Nephridium modifies waste by selectively reabsorbing certain ions and organic molecules.
- Nephridium opens to mantle cavity or right side of body next tp mantle cavity (land snails).
*Aquatic snails excrete ammonia.
*Land snails have to convert ammonia to uric acid (less toxic form).
Reproduction and Development
*Many marine snails are dioecious.
* Gonads are located in spirals of visceral mass. Ducts discharge gametes into the sea for external fertilization.
* Many other snails are monoecious, interal cross-fertilization.
*Copulation results in mutual sperm transfer. or snails reverse sex roles.
* Penis has evolved from fold of body wall. Also portions of female reproductive tract have become glandular and secrete a protective mucous around eggs.
* Eggs are shed singly or in big groupings for external fertilization
* Internally fertilized eggs are deposited in gelatinous strings or masses
* Trochophore larva- develops into another free-swimming larva with feet, eyes, tentacles, and shell called veliger larva
* Gonads are located in spirals of visceral mass. Ducts discharge gametes into the sea for external fertilization.
* Many other snails are monoecious, interal cross-fertilization.
*Copulation results in mutual sperm transfer. or snails reverse sex roles.
* Penis has evolved from fold of body wall. Also portions of female reproductive tract have become glandular and secrete a protective mucous around eggs.
* Eggs are shed singly or in big groupings for external fertilization
* Internally fertilized eggs are deposited in gelatinous strings or masses
* Trochophore larva- develops into another free-swimming larva with feet, eyes, tentacles, and shell called veliger larva
Gastropod Diversity
* Largest group of gastropods in subclass Prosobranchia
* 20,000 species are mostly marine
* Mostly herbivores or deposit feeders (some carnivores)
* includes heterpods: voracious predators
* 20,000 species are mostly marine
* Mostly herbivores or deposit feeders (some carnivores)
* includes heterpods: voracious predators