nilviäinen

A nilviäinen, commonly translated as mollusc, is a large and diverse phylum of invertebrate animals within the kingdom Animalia. They are characterized by a soft body, which may or may not be protected by a hard shell or a pair of shells. This soft body typically consists of a muscular foot used for locomotion, a visceral mass containing the internal organs, and a mantle, which is a fold of tissue that secretes the shell if one is present.

Molluscs exhibit an extraordinary range of forms and inhabit nearly every environment on Earth, from the deepest oceans to terrestrial habitats and freshwater systems. Their body plans are highly diverse, reflecting their evolutionary adaptations to various ecological niches. Common examples include snails, slugs, clams, oysters, mussels, squids, and octopuses.

Respiration in molluscs varies depending on their habitat. Aquatic species often possess gills for extracting oxygen from water, while terrestrial molluscs typically breathe using a primitive lung-like structure derived from the mantle cavity. Their feeding mechanisms are also varied, with some being herbivores, carnivores, detritivores, or filter feeders.

The phylum Mollusca is divided into several major classes, including Gastropoda (snails and slugs), Bivalvia (clams, oysters, mussels), Cephalopoda (squids, octopuses, cuttlefish), and Polyplacophora (chitons). While many molluscs are sessile or slow-moving, the cephalopods are renowned for their active predation and remarkable intelligence. Their ecological roles are significant, acting as food sources for other animals, influencing water quality, and sometimes being considered pests or agricultural nuisances.