Phylum Annelida - Annelids/Segmented Worms скачать в хорошем качестве

Phylum Annelida - Annelids/Segmented Worms

NOTE - This is not a detailed zoology lesson, but a general overview of the phylum Annelida - annelids/segmented worms. Annelida is a large and diverse phylum of segmented worms that include earthworms, leeches, and a wide variety of freshwater and marine worms. Members of this phylum are found in marine, brackish, freshwater, and moist terrestrial habitats. While most annelids are free-living, some are symbiotic, and others are parasitic. The defining characteristic of annelids is their segmented body, which is divided into repeating rings or segments. This segmentation allows for greater control of movement, as different parts of the body can elongate or shorten independently to aid in locomotion. Traditionally, Annelida was broken up into 3 main groups or classes: Polychaeta, Oligochaeta, and Hirudinea. Polychaeta: Polychaetes are the largest and most diverse group of annelids. It consists primarily of marine species. Examples of polychaetes include clam worms, lugworms, fan worms, feather duster worms, Christmas tree worms, scale worms, bobbit worms, parchment worms, fire worms, ragworms, sandworms, blood worms, ice cream cone worms, spaghetti worms, peacock worms, and paddle worms. Most polychaetes are benthic, meaning they live on or within the seabed. They can be found under rocks, in coral crevices, in abandoned shells, or burrowed into mud or sand. Many polychaetes are sedentary and build their own protective tubes on submerged objects or within bottom sediments. Others adopt the tubes or homes of other animals. Some polychaetes, however, are pelagic, living freely in the open sea. Polychaetes are often characterized by having a pair of parapodia on each body segment. Parapodia are lateral, fleshy protrusions that assist in both movement and respiration. Each parapodium typically contains tufts of numerous setae, which are chitinous, bristle-like, movable structures. Setae help anchor the worm during locomotion to prevent backsliding. They also assist in swimming and help prevent the animal from being pulled out of its burrow, tube, or home. Polychaetes have a well-defined head equipped with eyes and tentacles. They have separate sexes, and reproduction often involves releasing eggs and sperm into the water, where external fertilization occurs. Oligochaeta: the best-known oligochaetes are earthworms. Most oligochaetes are terrestrial or freshwater organisms, though some are parasitic, and a few inhabit marine or brackish waters. Terrestrial oligochaetes are primarily burrowers and scavengers, feeding on decomposed or partially decomposed organic matter, which helps recycle nutrients and aerate the soil. Oligochaetes lack a well-defined head and parapodia (fleshy, paired appendages seen in polychaetes). They have only a few small setae (bristles) per segment, which assist in movement and anchoring. A distinguishing feature of oligochaetes is the clitellum, a saddle-like structure that appears as a thickened, band-like region around the body. The clitellum plays a critical role in reproduction. Oligochaetes are hermaphroditic, meaning each individual possesses both male and female reproductive organs and can produce both eggs and sperm. They reproduce through cross-fertilization, where two worms exchange sperm during mating. The clitellum secretes mucus to facilitate copulation and forms a protective cocoon that receives the eggs and sperm. Hirudinea: hirudinids, or leeches, are predominantly found in freshwater habitats, although some species are marine or have adapted to terrestrial environments in warm, moist areas. Leeches typically have a flattened body with a sucker at each end, which they use for attachment and movement. They lack appendages, parapodia, and setae, distinguishing them from other annelids. Most leeches are ectoparasites living on the outside of their host feeding on their blood. They attach using their suckers, bite through the skin with cutting plates or jaws, and extract blood and tissue fluids. However, many leeches are predators, preying on small invertebrates such as worms, slugs, or larvae. Leeches are hermaphroditic and possess a clitellum, although the clitellum is only visible during the reproductive period. The traditional 3-group division of Annelida into Oligochaeta, Polychaeta, and Hirudinea has been revised and is currently in flux. Oligochaetes and hirudinids, both possessing a clitellum, have been combined into a single group called Clitellata, whose members are referred to as clitellates. Polychaeta is no longer recognized because it is paraphyletic. Many polychaete species have been reassigned to new or redefined clades, such as Errantia (free-moving species) and Sedentaria (sedentary, tube-dwelling species), though this classification remains under debate. Additionally, studies have incorporated several former phyla into the phylum Annelida. #annelida #segmentedworms #zoology #biology #beckbio