kontaktiseläimet

Kontaktiseläimet, also known as echinoderms, are a phylum of marine invertebrates. The name echinoderm comes from the Greek words echinos meaning "spiny" and derma meaning "skin", referring to the characteristic spiny skin found in many species. This diverse group includes starfish, sea urchins, sea cucumbers, brittle stars, and sea lilies.

A defining feature of echinoderms is their radial symmetry, usually pentaradial (five-part) symmetry, which develops during their larval stage. Adult echinoderms possess this symmetry externally, though their internal organs often exhibit bilateral symmetry. Another unique characteristic is the presence of a water vascular system, a network of fluid-filled canals that operate tube feet. These tube feet are used for locomotion, feeding, respiration, and sensory perception.

Echinoderms inhabit all the world's oceans, from shallow coastal waters to the deepest trenches. They play important roles in marine ecosystems, acting as predators, herbivores, scavengers, and filter feeders, influencing the distribution and abundance of other marine organisms. Reproduction in echinoderms is typically sexual, with external fertilization being common, though some species also reproduce asexually through fragmentation. Their life cycle usually involves a free-swimming larval stage that eventually settles and metamorphoses into the adult form.