Animalia Life

The first gastropods appeared 600 million years ago. Among these mollusks are snails and slugs, related animals which have a single shell and an asymmetric body. There are thirty-five thousand species, varying hugely in structure and lifestyle. The smallest snails are barely visible, while the largest, sea slugs, weigh up to thirty pounds. They occur in marine, freshwater, and land habitats. The first gastropods were bilaterally symmetrical. Later, many evolved into asymmetric snails, with gills and anus above the head and coiled shells. In land snails, the gill-holding cavity became a lung. The snail head has eyes and tentacles, enabling good sight and smell. The mouth has a rasplike radula, used to harvest food. Digestive, nervous, circulatory, and reproductive systems are also well developed. Some snails have sexes and lay eggs in water, where they hatch, settle down, and mature. In many snails, fertilization is internal and hermaphroditic, and they can mate with any mature animal of the same species.

Physical Characteristics of Snails
Most snails have hard outer shells and slimy bodies. Slugs have no shells or internalized shells. On their heads, snails and slugs have two pairs of antennae. One pair holds the eyes. The other antennae sense the environment. The bodies of these gastropods grow from 0.1 inch to several feet in length, depending on species. The shell of a snail may be rounded, long and pointy, or flattened. Shells are homes and protection. When afraid, snails close them up via plates under their bodies. Snails and slugs eat algae, leaves, lichens, small insects, and small marine organisms. As they also eat decaying plant and animal matter, the gastropods are ecologically important. Snails are also important to food chains, serving as food sources for fish and birds. A snail or slug eats with a radula, a ribbonlike tongue having thousands of tiny teeth. Radulas are drawn along rocks, leaves, or plants to scrape off food. Carnivorous snails have radulas that bore holes through shells of other mollusks to eat their flesh. Snails move by wavelike muscle contraction along the bottom of their muscular feet. This motion is aided by cilia in aquatic snails or slugs and by a slime track on land.

Types of Snails
Abalone, marine snails of the family Haliotidae, live on underwater rocks near shores of warm ocean regions. Their flat, ovoid shells, often a foot long, are nice souvenirs. Their thick feet are tasty. The common garden snail, Helix aspersa, a land snail, inhabits Great Britain and continental Europe. It lives in moist, shady places, not restricted to gardens, is nocturnal, and can be four inches long. Conches, marine snails of the order Mesogastropoda, close their shells with their digging claws. The largest conches (queen conches) inhabit warm Atlantic regions. Their three-foot-long shells make beautiful ornaments. Humans eat conch feet. Great pond snails, order Bassommatophora, occur in ponds in Europe, Asia, and Africa. Their shells grow two inches long and one inch wide. Air-breathers, with lungs, they frequently come to pond surfaces for air. Limpets, marine snails of the order Archaeogastropoda, are found worldwide. Limpets have arched, nonspiral shells and are found clinging to rocks. They scrape out rock areas the size of their shells, returning to their safety nightly. The New England species is 1.5 inches long.

The Life Cycle of Snails
Most snails are hermaphrodites, having both male and female sex organs. However, they usually mate with another individual of the same species, passing sperm to its partner and getting sperm from the partner. Fertilized eggs hatch in two weeks to two months. They are usually laid on marine or land plants, depending on the species involved. The offspring pass through complex developmental cycles before hatching. Often, they hatch as miniature replicas of parents. In some cases hatchling snails have weak shells and very fragile bodies. Such snails become stronger and obtain hard shells, as they grow. Snails of most types are sexually mature at one to two years of age and live for up to ten years. Abundant snails and slugs are important to food chains. They are herbivores, carnivores, and omnivores. As they eat decaying flesh and plants, these gastropods have another ecological function, environmental cleanup. They are also important human foods. Escargot, in French cuisine, are cultivated land snails. Other edible snails are abalone, periwinkles, and queen conches. A few of these gastropods are harmful. For example, snails and slugs damage crops and gardens. Others are pests in oyster beds.

Snail Facts

Classification:
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Orders: Include Archaeogastropoda (limpets), Bassomatophora (land snails), Mesogastropoda (conches)
Geographical location: Europe, Asia, Africa, the Americas, and Australia
Habitat: Oceans; freshwater lakes, streams, ponds; moist land environments
Gestational period: Eggs hatch in two weeks to two months, yielding miniature versions of parents
Life span:Two to ten years, depending on species
Special anatomy: Antennae, radula, muscular foot for locomotion, univalve shells, eyes on stalks in head, hermaphroditic reproductive system

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