Phylum Gastropoda
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The document summarizes key aspects of gastropods (snails and slugs): - Gastropods undergo a process called torsion in development where the visceral mass and mantle cavity twist 180 degrees, positioning organs and openings optimally for protection and function. - Most gastropod shells coil asymmetrically into a compact form to accommodate internal organs. - Gastropods use their muscular foot for locomotion and have various feeding, digestive, gas exchange, sensory, excretory and reproductive systems adapted for their habitats. - With over 35,000 species, gastropods occupy a wide range of marine, freshwater and terrestrial habitats and fill important ecological roles.
Phylum Gastropoda
- 1. GASTROPODA
- 2. OUTLINES • Class Gastropoda • Torison • Shell Coiling • Locomotion • Feeding • Digestion • Other maintenance functions
- 3. CLASS GASTROPODA The class Gastropoda (gas-tropo-dah) (Gr. gaster, gut podos,foot) includes the snails, limpets, and slugs. With over 35,000 living species ,Gastropoda is the largest and most varied molluscan class. Its members occupy a wide variety of marine ,freshwater, and terrestrial habitats.
- 4. CONT.… • Most people give gastropods little thought unless they encounter Helix pomatia (escargot) in a French restaurant or are pestered by garden slugs and snails. • One important impact of gastropods on humans is that gastropods are intermediate hosts for some medically important trematode parasites of humans.
- 5. TORISON • One of the most significant modifications of the molluscan bodyform in the gastropods occurs early in gastropod development.Torsion is a 180°, counterclockwise twisting of the visceral mass,mantle, and mantle cavity. Torsion positions the gills, anus, and openings from the excretory and reproductive systems just behind the head and nerve cords, and twists the digestive tract into a U shape.
- 6. ADVANTAGE OF TORSION • The adaptive significance of torsion is speculative; however, three advantages are plausible. • First, without torsion, withdrawal into the shell would proceed with the foot entering first and the more vulnerable head entering last.
- 8. ADVANTAGE OF TORISON • With torsion, the head enters the shell first, exposing the head less to potential predators. In some snails, a proteinaceous covering, called anoperculum, on the dorsal, posterior margin of the foot enhances protection. When the gastropod draws the foot into the mantle cavity, the operculum closes the opening of the shell, thus preventing desiccation when the snail is in drying habitats.
- 9. CONTD • A second advantage of torsion concerns an anterior opening of the mantle cavity that allows clean water from infront of the snail to enter the mantle cavity, rather than water contaminated with silt stirred up by the snail’s crawling.
- 10. CONTD…. • The twist in the mantle’s sensory organs around to the head region is a third advantage of torsion because it makes the snail more sensitive to stimuli coming from the direction in which it moves. • Note in previous diagram that, after torsion, the anus and nephritis empty dorsal to the head and create potential fouling problems. However, a number of evolutionary adaptations seem to circumvent this problem. • Various modifications allow water and the wastes it carries to exit the mantle cavity through notches or openings in the mantle and shell posterior to the head.
- 11. CONT. • Some gastropods undergo detorsion, in which the embryo undergoes a full180° torsion and then untwists approximately 90°. The mantle cavity thus opens on the right side of the body, behind the head.
- 12. SHELL COILING • The earliest fossil gastropods had a shell that was coiled in one plane. • This arrangement is not common in later fossils, probably because growth resulted in an increasingly cumbersome shell. (Some modern snails, however, have secondarily returned to this shell form.)
- 13. CONT. • Most modern snail shells are asymmetrically coiled into a more compact form, with successive coils or whorls slightly larger than, and ventral to, the preceding whorl.
- 14. CONT. • This pattern leaves less room on one side of the visceral mass for certain organs, which means that organs that are now single were probably paired ancestrally. • This asymmetrical arrangement of internal organs is described further in the descriptions of particular body system.
- 16. LOCOMOTION • Nearly all gastropods have a flattened foot that is often ciliated, covered with gland cells, and used to creep across the substrate. • The smallest gastropods use cilia to propel them selves over a mucous trail. Larger gastropods use waves of muscular contraction that move over the foot. The foot of some gastropods is modified for clinging, as in abalones and limpets, or for swimming, as in sea butterflies and sea hares.
- 17. • Most gastropods feed by scraping algae or other small, attached organisms from their substrate. Others are herbivores that feed on larger plants, scavengers, parasites, or predators. FEEDING
- 18. DIGESTION • The anterior portion of the digestive tract may be modified into an extensible proboscis, which contains the radula. This structure is important for some predatory snails that must extract animal flesh from hard-to-reach areas.
- 19. CONT. • The digestive tract of gastropods, like that of most mollusk's, is ciliated. Food is trapped in mucous strings and incorporated into a mucous mass called the protostyle, which extends to the stomach and is rotated by cilia. • A digestive gland in the visceral mass releases enzymes and acid into the stomach, and food trapped on the protostyle is freed and digested. Wastes form fecal pellets in the intestine.
- 20. OTHER MAINTENANCE FUNCTIONS • Class gastropoda forms following maintenance functions. Gas exchange: • Gas exchange always involves the mantle cavity. Primitive gastropods had two gills; modern gastropods have lost one gill because of coiling. Some gastropods have a rolled extension of the mantle, called a siphon, that serves as an inhalant tube.
- 21. CONT. • Burrowing species extend the siphon to the surface of the substrate to bring in water. Gills are lost or reduced in land snails (pulmonates), but these snails have a richly vascular mantle for gas exchange between blood and air.
- 22. CONT. • Mantle contractions help circulate air and water through the mantle cavity. Gastropods, like most molluscs, have an open circulatory system. During part of its circuit around the body, blood leaves the vessels and directly bathes cells in tissue spaces called sinuses. • Molluscs typically have a heart consisting of a single, muscular ventricle and two auricles. Most gastropods have lost one member of the pair of auricles because of coiling (see figure )
- 23. BLOOD CONTROLLING SYSTEM • Molluscs typically have a heart consisting of a single, muscular ventricle and two auricles. Most gastropods have lost one member of the pair of auricles because of coiling.
- 24. CONT. .In addition to transporting nutrients, wastes, and gases, the blood of molluscs acts as a hydraulic skeleton. A hydraulic skeleton consists of blood confined to tissue spaces for support. A mollusc uses its hydraulic skeleton to extend body structures by contracting muscles distant from the extending structure. For example, snails have sensory tentacles on their heads, and if a tentacle is touched, retractor muscles can rapidly withdraw it.
- 25. CONT. However, no antagonistic muscles exist to extend the tentacle. The snail slowly extends the tentacle by contracting distant muscles to squeeze blood into the tentacle from adjacent blood sinuses.
- 26. NERVOUS SYSTEM The nervous system of primitive gastropods is characterized by six ganglia located in the head-foot and visceral mass. In primitive gastropods, torsion twists the nerves that link these ganglia. The evolution of the gastropod nervous system has resulted in the untwisting of nerves and the concentration of nervous tissues into fewer, larger ganglia, especially in the head.
- 27. SENSORY STRUCTURES • Gastropods have well-developed sensory structures. • Eyes may be at the base or at the end of tentacles. They may be simple pits of photoreceptor cells or consist of a lens and cornea. Statocysts are in the foot. Osphradia are chemoreceptors in the anterior wall of the mantle cavity that detect sediment and chemicals inhalant water or air. The osphradia of predatory gastropods help detect prey. Primitive gastropods possessed two nephridia.
- 28. EXCRETORY SYSTEM • Primitive gastropods possessed two nephridia. • In modern species, the right nephridium has disappeared, probably because of shell coiling. The nephridium consists of a sac with highly folded walls and connects to the reduced coelom, the pericardial cavity.
- 29. CONTD. • Excretory wastes are derived largely from fluids filtered and secreted into the coelom from the blood. The nephridium modifies this waste by selectively reabsorbing certain ions and organic molecules. The nephridium opens to the mantle cavity or, in land snails, on the right side of the body adjacent to the mantle cavity and anal opening.
- 30. CONT. Aquatic gastropod species excrete ammonia because they have access to water in which toxic ammonia is diluted. Terrestrial snails must convert ammonia to a less-toxic form—uric acid. Because uric acid is relatively insoluble in water and less toxic, it can be excreted in a semisolid form, which helps conserve water.
- 31. • Many marine snails are dioecious. Gonads lie in spirals of the visceral mass. Ducts discharge gametes into the sea for external fertilization. • Many other snails are monoecious, and internal, cross- fertilization is the rule. Copulation may result in mutual sperm transfer, or one snail may act as the male and the other as the female. REPRODUCTION
- 32. CONT. • A penis has evolved from a fold of the body wall, and portions of the female reproductive tract have become glandular and secrete mucus, a protective jelly, or a capsule around the fertilized egg. Some monoecious snails are protandric in that testes develop first, and after they degenerate, ovaries mature. Eggs are shed singly or in masses for external fertilization. Internally fertilized eggs are deposited in gelatinous strings or masses.
- 33. DEVELOPMENT • The large, yolky eggs of terrestrial snails are deposited in moist environments, such as forest-floor leaf litter, and a calcareous shell may encapsulate them. • In marine gastropods, spiral cleavage results in a free- swimming trochophore larva that develops into another free- swimming larva with foot, eyes,tentacles, and shell, called a veliger larva. Sometimes, the trochophore is suppressed, and the veliger is the primary larva. Torsion occurs during the veliger stage, followed by settling and metamorphosis to the adult.
- 34. GASTROPOD DIVERSITY • The largest group of gastropods is the subclass Prosobranchia. Its 20,000 species are mostly marine, but a few are freshwater or terrestrial. Most members of this subclass are herbivores or deposit feeders; however, some are carnivorous. Some carnivorous species inject venom into their fish, mollusc, or annelid prey with a radula modified into a hollow, harpoon like structure.
- 35. CONT. • Prosobranch gastropods include most of the familiar marine snails and the abalone. This subclass also includes the heteropods. These animals are voracious predators, with very small shells or no shells. • The foot is modified into an undulating “fin’’ that propels the animal through the water.
- 36. CONT. • Members of the subclass Opisthobranchia include sea hares,sea slugs, and their relatives . They are mostly marine and include fewer than two thousand species. The shell, mantle cavity, and gills are reduced or lost in these animals, but they are not defenseless. Many acquire undischarged nematocysts from Their cnidarian prey, which they use to ward off predators. The pteropods have a foot modified into thin lobes for swimming.
- 37. CONT. • The subclass Pulmonata contains about 17,000 predominantly freshwater or terrestrial species . These snails are mostly herbivores and have a long radula for scraping plant material. The mantle cavity of pulmonate gastropods is highly vascular and serves as a lung. Air or water moves in or out of the opening of the mantle cavity, the pneumostome.
- 38. CONT. In addition to typical freshwater or terrestrial snails, the pulmonates include terrestrial slugs.
- 40. REFERENCES • Book of zoology by Miller and Harley
- 41. Thank You☺