Gastropoda is generally called snails. It is the largest class of mollusks, including 75, living species and 15, fossil species. Gastropoda is also widely distributed. In the ocean, from the species floating in the ocean to the seabed with different depths and properties, they are distributed in all kinds of fresh water areas. In particular, Gastropoda spirochetes are the species that really conquer the land environment and can live on the ground. Gastropoda is the most prosperous class of mollusks. Gastropoda has an obvious head and a spirally curled shell outside. The head, feet, visceral sac and mantle can all be retracted into the shell. In the process of development, the body is twisted, causing the nerves to twist into an "8" shape and the internal organs to lose symmetry. Some species undergo torsion in development and then reverse torsion, and the nerves are no longer "8"-shaped, but the organs lost in torsion no longer occur, and the internal organs of the body still lose symmetry. Including Prosobranchia, Opisthobr-anchia and pulmonata. Basic introduction to Chinese scientific name: Gastropoda Latin scientific name: Gastropoda Cuvier, 1797? Boundaries: Animalia: Mollusca Subclass: Gastropoda's development history, shape characteristics, growth habits, growth and reproduction, distribution range, habitat, species classification, Prebranchiae Subclass, Postbranchiae Subclass, and Pulmonary Snail Subclass. The development history of some gastropods in the early Cambrian strata is also symmetrical, for example, the shell of a fossil gastropod Strepsodiscus is symmetrical and circled in a plane. Later, after studying the embryonic development of Gastropoda, people also found that the trochophore of Gastropoda is also symmetrical, but when the larvae arrive at the face plate, the body suddenly twists, followed by an asymmetric growth process, and finally the adult becomes an asymmetric system. Therefore, the studies of comparative morphology, paleozoology and genetics have proved that the early system of gastropods was bilateral symmetry, while the asymmetry of most species was formed in the process of evolution. So how did gastropods form an asymmetric system during evolution? It is speculated that gastropods also evolved from ancestors like protomollusks. Firstly, due to the increase of their volume and the fact that their heads and feet often shrank under their shells, the visceral sac developed fully and constantly bulged on the back of their bodies. As a result, the mantle hanging from the top of visceral sac and the shells secreted by the mantle increased in height with the uplift of visceral sac, making the back and shells of their bodies oblong and conical. This shape is not conducive to the movement and survival of animals in the water, so it gradually appeared that the top of the visceral sac began to make a plane coiling, and the shell formed later surrounded the first shell. At this time, the shell and visceral sac were still symmetrical, just as the fossil species found in the early strata circled in a plane. However, this kind of plane hovering makes the diameter of the shell very large, but the space in the shell is very small and the shell is not firm, so the shell that appears in the later stage is no longer plane hovering, but spiral hovering from top to bottom along a central axis. The diameter of this spiral shell is reduced, and the internal volume of the shell remains unchanged, but the firmness of the shell is increased, and the shell becomes a short cone. At the same time, the axis of the shell is no longer perpendicular to the long axis of the body, but inclined to the long axis of the body, so that the center of gravity of the increased visceral sac moves to the near front end to facilitate movement. The spiral spiral of the shell may explain why gastropods only have a pair of gills, a pair of kidneys and a pair of contractile muscles. Appearance characteristics Gastropoda has an obvious head and a spirally curled shell outside. The head, feet, visceral sac and mantle can all be retracted into the shell. The heads are well developed, with one or two pairs of antennae and a pair of eyes. The eye is born at the base, middle or top of the antenna. The teeth and tongues in the mouth are developed and used for feeding and drilling. The shell is mostly spiral. The head is developed with eyes and tentacles. Foot developed, leaflike, located on the ventral side, so it is called Gastropoda. Foot has foot glands, which are unicellular mucinous glands. The growth habits of Gastropoda feed on algae, fungi, lichens and bryophytes. Sensory organs are developed, the number of teeth and tongues is reduced, and sea cucumbers and crabs can be eaten or the nutrient solution in shells can be sucked. There are also a few kinds of parasitic life. Growth and reproduction of hermaphroditism or allogamy, oviparous. Gastropoda fossils are widely distributed in Asia, North America, Europe and Oceania. The habitats of Gastropoda range from gardens, forests, deserts to mountainous areas, from ditches, rivers to lakes, from estuaries, mudflats, rocky intertidal zones and sand bottoms to deep sea, and some are parasitic. Species Classification of Primitive Gastropoda Archaeogastropoda of Prebranchiata Mesogastropoda Neogastropoda (Narrowglossa) Postbranchiata Cephalaspidea Anaspidea Shell Pteropoda Thecosomata Naked Pteropoda (Pteropoda) g Ymnosomata Sacoglossa Shellless Order Acochlidiacea Dormanta Notaspidea Nudibranchia Basommahora Stellate Order Stylommahora.