CLASS GASTROPODA (Or. gaster, gut + podos, foot)
The class Gastropoda includes the snails. limpets, and slugs. It has 35.000 living species. It is the largest class of molluscs. Its members occupy a wide variety of marine. freshwater; and terrestrial habitats. Gastropods are intermediate hosts for some medically important trematode parasites of humans.
Torsion is a 1800, counterclockwise twisting of the visceral mass, mantle, and mantle cavity. It occurs during early development. Torsion brings of gills, anus, and openings from the excretory and reproductive.systems just behind the head and nerve. It also twists the digestive tract into a U shape.
Advantages of torsion
There are three advantages of torsion.
1. First advantage: There is problem in withdrawal ( un..,..L51) of body into shell without torsion. The foot enters first and the head enters in the last. Thus head can be attacked by predator. After torsion. the head enters the shell first. Therefore, it does not expose the head to predators. In somc. snails a proteinoceous covering operculum is present on the dorsal, posterior margin of the foot. It enhances protection. The gastropod draws the foot into the mantle cavity and the operculum closes the opening of the shell. Thus it prevents the desiccation (dehydration) when the snail is in drying habitats.
2. Second advantage: Water enters through posterior opening without torsion. The snail’s crawling contaminates water with silt. After torsion, opening comes at anterior end. Therefore, the anterior opening of the mantle cavity allows clean water to enter into the mantle cavity front the front of the snail.
3. Third advantage: Mantle’s sensory organs are twisted due to torsion. These sense organs come around the head. It makes the snail more sensitive to stimuli coming ioni the direction in which it moves.
Disadvantages of torsion
Th anus and nephridia open dorsal to the head after torsion. It creates folding problems. However number of evolutionary adaptations solves this problem.
1. Various modifications produce notches or openings in the mantle and shell posterior o the head. Therefore, the mantle cavity removes the water and wastes through these opening.
2. Some gastropods undergo detorsion. In this case the embryo undergoes a full 180° torsion and then untwists approximately 90°. The mantle cavity thus opens on the tight side of the body behind the head.
The shell in the earliest fossil gastropods coiled in one plane. The growth has developed an increasingly huge shell. Therefore, this arrangement is not common in later fossils. Mo t modern snail shells are asymmetrically coiled into a more compact form. The successive coils or whorls are slightly larger then the preceding whorl. This pattern leaves less space on one side of the visceral mass for certain organs. It means that the organs that are now single were paired in ancestor.
All gastropods have a flattened foot. It is often ciliated and covered with gland cells. Foot is used to creep across the substrate. The smallest gastropods use cilia to propel themselves over a mucous path. Larger gastropods use waves of muscular contraction that move over the foot. The foot of some gastropods is modified for clinging and swimming.
FE DING AND DIGESTION
Mot gastropods scrap algae or other small, attached organisms from their substrate. Some are herbivores. They feed on larger plants. Others are scavengers, par sites or predators.
The anterior portion of the digestive tract is modified into an extensible proboscis. This proboscis contains radula. This structure is important for some predatory snails. Thee snails extract animal flesh from hard-to-reach areas: The digestive tract of gastropods is ciliated. Mucous trap the food. It forms a mucoid mass called protostyle. It is rotated by cilia and extends to the stomach. Digestive gland in the visceral mass releases enzymes and acid into the stomach. These enzymes free the food from protostyle and digested. Wastes form fecal pellets in the intestine.
OTHER MAINTENANCE FUNCTIONS
Mantle cavity is involved in the gas exchange. Primitive gastropods had two gills. Modern gastropods have lost one gill because of coiling. Some gastropods have a rolled extension of the mantle called siphon. Isacts as an inhalant tube. Burrowing species extend the siphon in to the substrate to bring water. Gills are lost or reduced in land snails (pulmonates). These snails have a richly vascular mantle for gas exchange between blood and air. Mantle contractions circulate air and water through the mantle cavity.
Gastropods have an open circulatory system. Blood leaves the vessels and directly bathes cells in tissue spaces called sinuses. Molluscs have a heart. It consists of a shgle. muscular ventricle and two auricles. Most gastropods have lost one auricles due to coiling. Blood transports nutrients, wastes, and gases.
The blood of molluscs acts as a hydraulic skeleton. In hydraulic skeleton blood in tissue spaces provides support. A mollusc contracts muscles and uses its hydraulic skeleton to extend body. For example, snails have sensory tentacles on their heads. If a tentacle is touched, retractor muscles can rapidly withdraw it. ‘However, no antagonistic muscles exist to extend the tentacle. The snail contract muscles and squeeze blood into the tentacle. Thus these tentacles are slowly extended.
The nervous system of primitive gastropod consists of six ganglia. These ganglia are located in the head-foot and visceral mass. In primitive gastropods. torsion twists the nerves that link these ganglia. Evolution in gastropod nervous system took place. It untwisted the nerves. Therefore, concentration of nervous tissues takes place in larger ganglia, especially in the head.
Sense organ: Gastropods have well-developed sense organs. These are:
1.Eyes: Eyes are present at the end of tentacles. Eyes may be simple pits of photoreceptor cells. Or it has a lens and cornea.
2. Statocysts are present in the foot.
3. Osphradia are chemoreceptors. They are present in the anterior wall of the mantle cavity. It detects chemicals in inhalant water or air. The osphradia of predatory gastropods detect prey.
Primitive gastropods possessed two nephridia. The right nephridium has disappeared in modern species due to shell coiling. The nephridium consists of a sac. The sac has highly folded walls. It is connected to pericardial cavity. Excretory wastes are formed from fluids filtered and secreted into the coelom from the blood. The nephridium selectively reabsorbs certain ions and organic molecules and modifies this waste. T he nephridium opens in to the mantle cavity. In land snails, it opens on the right side of the body adjacent to the mantle cavity and anal opening.
(a)Aquatic gastropods: Toxic ammonia is diluted in excess water in aquatic gastropod. Therefore, they excrete ammonia. (b) Terrestrial gastropods: Terrestrial snails convert ammonia to a less-toxic uric acid. Uric acid is insoluble in water and less toxic. Thus it can be excreted in a semisolid form. It conserves water.
PRODUCTION AND DEVELOPMENT
Many marine snails are dioecious. Gonads are present in spirals of the visceral mass. Ducts discharge gametes into the sea. External fertilization takes place in them. Many other snails are monoecious. But cross fertilization take place. Fertilization is internal. There is mutual sperm transfer during copulation. Or one snail may act as the male and other as the female. A penis is formed by the folding of the body wall. The portion of the female reproductive tract is glandular. It secretes mucus or a capsule around the fertilized egg. Some monoecious snails are protandric. In this case, testes develop first, a after they degenerate. Then ovaries become mature.
Eggs are shed singly or in masses for external fertilization. Fertilized eggs are deposited in gelatinous masses. The large, yolky eggs of terrestrial snails are deposited in moist environments like forest-floor leaf litter. A calcareous shell encapsulates
th in. Two larvae are produced in gastropods:
(a) Trochophore larva: In marine gastropods spiral cleavage take place. It produces a free-swimming trochophore larva.
(b) Veliger larva: Trochophore larva then develops into veliger larva. Veliger larva is free-swimming larva with foot, eyes, tentacles and shell. Sometimes, the trochophore is not produced. Therefore, the veliger is the primary larva. Torsion occurs during the veliger stage. It then settle and metamorphosed in to the adult.
Subclass Prosobranchia is the largest group of gastropods. It has 20,000 species. They are mostly marine. A few are freshwater or terrestrial. Most members of this subclass are herbivores or deposit feeders. Some are carnivorous. Some carnivorous species inject venom into their fish, mollusc or annelid prey with a radula. Their radula is modified into a hollow, harpoon like structure. Gastropoda have following subclasses:
1. Prosobranchia: They are most familiar marine snails and abalone. This subclass also includes the heteropods. These animals are voracious predators. They have very small shells or no shells. The foot is modified into an undulating fin. It propels the animal through the water.
2. Opisthobranehia: This subclass includes sea hares, sea slugs. and their relatives. They are mostly marine. This class has two thousand species. The shell, mantle cavitn and gills are reduced or lost in these animals. Many of these animals acquire undischarged nematocysts from their cnidarian prey. They use these namatocyts to save themselves from the predators. The preropods have a foot modified into thin lobes. It is used for swimming.
3. Pulmonata: It contains about 17, 000 species. Most of these species are freshwater or terrestrial. The snails are mostly herbivores. They have a long cedilla for scraping plant material. The mantle cavity of pulmonate astropods is highly vascular. It acts as a lung. Air or water moves in or out of the opening of the mantle cavity. The pulmonates include terrestrial slues.