Annelida

.

Annelid, common name for about 9000 species of wormlike invertebrate animals with well-developed segmentation. The three major classes of the annelid phylum are the bristle worms (about 5300 species), which are mainly marine and often luminescent; the oligochaetes (about 3100 species), which are mostly freshwater or inhabit the soil, such as the earthworm; and the leeches (about 300 species), which are mainly freshwater but may also be marine or terrestrial.

Although annelids are diverse in form, the bristle worms, which generally are considered the ancestral stock, can exemplify their structure. The elongate body is rounded in cross section, with a mouth (sometimes jawed) at one end and an anus at the other, and it exhibits bilateral symmetry. The body is made up of several similar units, or segments, separated externally by furrows and internally by septa (partitions). The segments generally bear parapodia (lobes) with setae (bristles) that are used in locomotion. The coelom (fluid-filled body cavity) provides a kind of structural support, so that the body is flexible. The gut is a straight tube and the nervous system fairly simple, with sense organs poorly developed. The head may bear eyes, feelers, or tentacles; a few bristle worms have highly developed eyes. Bristle worms grow by adding segments at the posterior end.

Oligochaetes have fewer setae and no parapodia. The body is streamlined and well adapted to burrowing, and the head does not have eyes or feelers. Leeches are flattened and can swim or crawl by looping movements; they usually lack setae.

Bristle worms generally have separate sexes and reproduce by releasing sperm and eggs into the water. By contrast, the basic pattern in earthworms and leeches is hermaphroditism, with cross-fertilization occurring internally. These animals produce cocoons, and some leeches care for the young.

Scientific classification: Annelids make up the phylum Annelida. Bristle worms make up the class Polychaeta, oligochaetes the class Oligochaeta, and leeches the class Hirudinea.

Types of Invertebrates

.

Annelida -> Segmented worms with a muscular body wall used for burrowing. External hairs called setae aid in traction during burrowing. An internal coelom is divided into compartments by walls known as septum. The digestive system stretches from the mouth to the anus, differentiated into regions, each with a different function. Reproduction is sexual. – Examples: Lugworms, earthworms, leeches

Pogonophora -> Deep-sea worms that live in chitinous tubes attached to the ocean floor. Their long, slender body has a beard of tentacles at the head end. There is no mouth or digestive system and these animals absorb all nourishment through the body surface. They reproduce sexually. – Examples: Bead worms

Vestimentifera -> Giant deep-sea worms that live in chitinous tubes attached to the ocean floor. They derive nutrition using a specialized organ called a trophosome to digest sulfide-oxidizing bacteria. – Examples: Llamellibrachs

Sipuncula -> Marine worms with a saclike body and a long proboscis. These worms withdraw their narrow head into the fatter posterior portion. The head end bears tentacles used in burrowing and gathering food particles. Lacking a cardiovascular system, they use internal fluid to transfer oxygen and food to body tissues. They reproduce sexually and some have a larval form. -> Peanut worms

Echiura -> Plump marine worms that take shelter in sand burrows or rock crevices. They use a mucous net or a scoop-shaped proboscis to capture food particles. Reproduction is sexual. – Examples: Spoon worms

Priapulida -> Cucumber-shaped, marine worms with spiny heads. During movement the barrel-shaped proboscis withdraws into the trunk of the worm. Reproduction is sexual. – Examples: Priapulus

Tardigrada -> Microscopic animals with four pairs of stubby legs that live in marine and freshwater sediments and on the surface of mosses and lichens. They use sharp stylets protruding from their mouths to suck food from plant cells. These animals have a remarkable ability to withstand extreme dryness and low temperatures. Reproduction is sexual. – Examples: Water bears

Pentastomida or Linguatulida -> Parasitic worms that live in the lungs of snakes, crocodiles, and some mammals and birds, feeding on blood and tissue. The head bears four leglike claws and a snoutlike mouth. The body is covered by a cuticle that is molted during larval development. They reproduce sexually. – Examples: Tongue worms

Phoronida -> Cylindrical, marine worms that live in a chitinous tube embedded in sand or attached to rocks, shells, or other objects in shallow water. Protruding out of the tube is the animal's lophophore, a structure of ciliated tentacles arranged in a horseshoe shape, that is used for feeding. They have a U-shaped digestive tract. Adults are sedentary and larvae are free-swimming. Reproduction is asexual in at least one species, but most species reproduce sexually. – Examples: Horseshoe worms

< next page >