Class Gastropoda
Organisms in this class include snails, limpets, and slugs. Class Gastropoda is the largest and most varied class. This class supplies organisms that act as intermediate hosts for important trematode parasites.
Torsion is where an organism in this class performs a 180 degree counterclockwise twist of the visceral mas, mantle, and mantle cavity. The reason organisms perform torsion is to position its gills, anus, and openings away from excretory and reproductive systems, which are behind the head and nerve cords. Organisms will perform torsion so they can remove their gills from being under the excretory system so that feces doesn't fall on the gills. Torsion also shapes the digestive tract into a U shape. Torsion is also helpful when a snail enters its shell. Without torsion foot enters the shell first, which leave the head exposed to predators. But, with torsion snails enter its shell head first, in this case the foot is protected by the operculum, which is a covering on the posterior dorsal part of the foot.
Older gastropods have a shell that is coiled in one place. Whereas, modern gastropods have an asymmetrical shell that is coiled into a more compact form. The modern style shell leaves less room on one side for the visceral mass.
Gastropods are able to move with their flat foot, which can be ciliated or have gland cells. Some snails used different methods in order to move. Small snails rely on their ciliated foot to move, while larger snail are able to move using waves muscular contractions.
A gastropod's radula is used to scrape algae or other small organism, these organisms can later be consumed. Some gastropods are herbivores and others can feed on scavengers or parasites. The flesh of other organism is easily digested using a modified digestive tract that becomes an extensible proboscis. ciliated digestive tartcs can trap food and push them to the protostyle, mucoid mass that extends to the stomach.
Modern gastropods only have one gill, compared to early gastropods that had two gills. In response modern gastropods have developed siphons, which serves as an inhalant tube. IN land snails gills can be lost or reduces, so they have developed a vascular mantle, which aids in gas exchange. Mantles contractions can circulate air and water through the cavity.
Gastropods have an open circulatory system.during circulation blood can leaves the vessels to bathe cells that are in sinuses, tissue spaces. The heart of gastropods is composed of one ventricle and one auricle. The blood in molluscs serves as a hydraulic skeleton. a hydraulic skeleton has fluid under pressure that may be within tissue spaces to extend or support the body.
the nervous system in gastropods is made up of mainly six ganlgia, which can be found in the head-foot and visceral mass. Torsion has caused nerves to be able to untwist and to concentrate nervous tissue into fewer ganglia.
Gastropods tend to have good sensory structures. Eyes can be at the base or tip of tentacles, the eyes may have pits of photo-receptor cells or they may have a lens and cornea. Gastropods have osphradia, chemo-receptors in the anterior walls of the mantle cavity, that serve to detect sediment and chemicals in the air. In predators, the osphradia can detect prey.
Gastropods can either be monoecious or dioecious. Marine gastropods (dioecious) posses gonads within their visceral mass and during fertilization gametes are released from ducts into the sea. In monoecious gastropods reproduction can be accomplished through internal cross fertilization or when one snail acts as a male and the other snail acts as the female. In external fertilization eggs can be released individual or in masses. In internal fertilization eggs are in gelatinous strings or masses. Terrestrial snails can deposit large yolky eggs in moist environments, such as a forest floor. Marine gastropods have spiral cleavage that can become a trochophore larvae, free swimming larva. The trochophore larva can become a veliger larva, a free swimming larva with a foot, eyes, tentacles, and a shell. Veliger larva is able to perform torsion.
Torsion is where an organism in this class performs a 180 degree counterclockwise twist of the visceral mas, mantle, and mantle cavity. The reason organisms perform torsion is to position its gills, anus, and openings away from excretory and reproductive systems, which are behind the head and nerve cords. Organisms will perform torsion so they can remove their gills from being under the excretory system so that feces doesn't fall on the gills. Torsion also shapes the digestive tract into a U shape. Torsion is also helpful when a snail enters its shell. Without torsion foot enters the shell first, which leave the head exposed to predators. But, with torsion snails enter its shell head first, in this case the foot is protected by the operculum, which is a covering on the posterior dorsal part of the foot.
Older gastropods have a shell that is coiled in one place. Whereas, modern gastropods have an asymmetrical shell that is coiled into a more compact form. The modern style shell leaves less room on one side for the visceral mass.
Gastropods are able to move with their flat foot, which can be ciliated or have gland cells. Some snails used different methods in order to move. Small snails rely on their ciliated foot to move, while larger snail are able to move using waves muscular contractions.
A gastropod's radula is used to scrape algae or other small organism, these organisms can later be consumed. Some gastropods are herbivores and others can feed on scavengers or parasites. The flesh of other organism is easily digested using a modified digestive tract that becomes an extensible proboscis. ciliated digestive tartcs can trap food and push them to the protostyle, mucoid mass that extends to the stomach.
Modern gastropods only have one gill, compared to early gastropods that had two gills. In response modern gastropods have developed siphons, which serves as an inhalant tube. IN land snails gills can be lost or reduces, so they have developed a vascular mantle, which aids in gas exchange. Mantles contractions can circulate air and water through the cavity.
Gastropods have an open circulatory system.during circulation blood can leaves the vessels to bathe cells that are in sinuses, tissue spaces. The heart of gastropods is composed of one ventricle and one auricle. The blood in molluscs serves as a hydraulic skeleton. a hydraulic skeleton has fluid under pressure that may be within tissue spaces to extend or support the body.
the nervous system in gastropods is made up of mainly six ganlgia, which can be found in the head-foot and visceral mass. Torsion has caused nerves to be able to untwist and to concentrate nervous tissue into fewer ganglia.
Gastropods tend to have good sensory structures. Eyes can be at the base or tip of tentacles, the eyes may have pits of photo-receptor cells or they may have a lens and cornea. Gastropods have osphradia, chemo-receptors in the anterior walls of the mantle cavity, that serve to detect sediment and chemicals in the air. In predators, the osphradia can detect prey.
Gastropods can either be monoecious or dioecious. Marine gastropods (dioecious) posses gonads within their visceral mass and during fertilization gametes are released from ducts into the sea. In monoecious gastropods reproduction can be accomplished through internal cross fertilization or when one snail acts as a male and the other snail acts as the female. In external fertilization eggs can be released individual or in masses. In internal fertilization eggs are in gelatinous strings or masses. Terrestrial snails can deposit large yolky eggs in moist environments, such as a forest floor. Marine gastropods have spiral cleavage that can become a trochophore larvae, free swimming larva. The trochophore larva can become a veliger larva, a free swimming larva with a foot, eyes, tentacles, and a shell. Veliger larva is able to perform torsion.