Annelida refers to a phylum of invertebrate worms commonly known as the segmented worms or annelids. With approximately 17,000 described species, annelids are found in various habitats such as marine, freshwater, and terrestrial ecosystems. The name "annelida" is derived from the Latin word "annellus," which means little ring, referring to the segmented body composition of these organisms.
Annelids are characterized by their elongated, cylindrical body with numerous repetitive segments. Each segment contains a set of organs, including a pair of bristles called setae used for locomotion and burrowing. The body is divided into three distinct regions: the head, the segmented trunk, and the terminal region called the pygidium.
These organisms exhibit a broad range of feeding habits, including filter feeding, predation, scavenging, and parasitism. They have a well-developed digestive system that may vary depending on the species, with some possessing specialized structures like jaws or retractable proboscises.
The nervous system of annelids consists of a ventral nerve cord and a ganglionic brain, allowing for coordinated movement and sensory perception. They also possess a closed circulatory system, with blood pumped by a contractile dorsal blood vessel.
Reproduction commonly occurs through sexual reproduction, although some annelids can also reproduce asexually. Fertilization is usually external, with most species exhibiting separate sexes. Development can range from direct development to complex indirect development, involving a larval stage.
Annelids play crucial roles in various ecosystems, contributing to nutrient recycling through decomposition and serving as prey for other organisms. Additionally, some species of annelids serve as valuable indicators of environmental pollution due to their high sensitivity to changes in water quality.
