Segmented worms
Segmented worms, classified under the phylum Annelida, are characterized by their segmented body structure, which shows a remarkable organization and complexity. Their bodies consist of a series of rings or segments, each featuring similar fundamental structures, except for the digestive system. Annelids exhibit metamerism, where body segments are repeated, allowing for specialized functions and enhanced adaptability. The segments contain muscles that facilitate movement and a coelom that acts as a hydrostatic skeleton and accommodates the circulatory and excretory systems.
Annelids are mostly hermaphroditic, with various reproduction methods depending on the species. Polychaetes, for example, often have external fertilization in marine environments, with unique breeding behaviors such as the palolo worm's synchronized reproductive events. Oligochaetes and leeches also reproduce similarly, involving the exchange of sperm and the development of eggs in protective cocoons. With their sophisticated systems and regenerative capabilities, segmented worms represent a significant group within the invertebrate animal kingdom, showcasing the diversity and adaptability of life forms.
Segmented worms
Segmented Worm Facts
- Classification:
- Geographical location: Worldwide
- Habitat:Polychaeta: mostly marine, found near the shore or on the bottom of shallow areas, with a few species living in brackish water or freshwater; Oligochaeta: freshwater and terrestrial forms found burrowing in soil or leaf mold; Hirudinea: freshwater and terrestrial environments often found attached to the body of a host
- Gestational period: Varies among species, but most species lay eggs within a few days after fertilization; eggs usually hatch within a few days to a few weeks after being deposited
- Life span: Varies among species; can be as short as a year in some polychaetes and up to several years in some earthworms
- Special anatomy: Elongated, metameric (segmented) bilateral invertebrates with appendages (parapodia) and chitinous setae in many species but lacking in Hirudinea; possess a true coelom (body cavity lined with epithelial tissue) divided by septa with a closed circulatory system, complete digestive system and an excretory system (nephridia) in each segment; leeches have specially adapted mouthparts for attaching to the body of the host
The Annelida are segmented worms in which the body wall, coelom (body cavity), epidermis, circular muscle, longitudinal muscle, and peritoneum are arranged into a longitudinal series of rings or segments. Over 15,000 species in the Annelida phylum belong to this group. True segmented animals exhibit metamerism, a repetition of a structure or organ from segment to segment. Each segment has the same fundamental structures as the others. Except for the digestive system, the major organ systems of the annelids are metameric in structure. Young annelids generally have few segments, but as they grow, new segments are formed by the division of the terminal segment. Annelids represent the most highly organized animals capable of complete regeneration.
General Characteristics of the Annelids
The mouth lies between the first and second segments and forms one segment called the prostomium. In leeches, the mouth contains suckers for attaching to the body of a host. The brain originates in the prostomium and develops a pair of cricopharyngeal nerve rings that reach around the pharynx to form the ventral nerve cord, which appears as a chain of ganglia, one pair in each segment. In the Polychaetes, a pair of swimming or crawling parapodia are located on most of the segments. Both the Polychaetes and Oligochaetes contain external setae to assist in locomotion.
The annelid body is covered with a thin cuticle. Each segment has a ring of circular and longitudinal muscles that contract to either elongate or shorten the segment. A spacious coelom, divided by septa, lies between the body wall and an internal digestive tract. The coelom is filled with fluid and serves as a hydrostatic skeleton in all annelids except the leeches. The coelom also contains the circulatory and excretory systems. A system of large vessels (hearts) pump blood through a ventral vessel into capillary beds that invade all of the tissues. The blood is returned to the hearts via the dorsal vessel. Each segment, except the first and last, contains a pair of nephridia, which collect wastes and deliver them to the outside.
Reproduction in the Annelids
In many species of polychaete worms, fertilization is external and occurs in the open seawater. The palolo worm (Palola viridis) provides a good example of polychaete reproduction. Throughout most of the year, the worms exist as sexually immature animals called atokes, but during the breeding season, the posterior segments develop gonads, and the coelom becomes filled with gametes. On the night of breeding, individuals back out of their holes, and the posterior portion, called an epitoke, breaks free. The epitokes swim to the surface for a few minutes and burst, shedding eggs or sperm and leaving a rapidly disintegrating body. Reproduction in the palolo worms is tied to an annual cycle designating the month, a lunar rhythm designating the day, and a diurnal cycle designating the hour of reproduction. Over 90 percent of the population breeds within a single two-hour period of the entire year.
Oligochaete worms are hermaphroditic, with a pair of testes in the tenth and eleventh segments and a pair of ovaries in the thirteenth segment. During copulation, two worms exchange sperm. Once the eggs are fertilized, the clitellum, a swollen glandular region of the epidermis, secretes a membrane that slips forward along the body so that the eggs are laid directly into it as it passes. Finally, the cocoon slips off the head, and the eggs develop into tiny worms, which later emerge from the cocoon. The reproductive system of leeches is very similar to that of the Oligochaetes.
Many segmented worms have the unique ability to regenerate, and some species can regenerate their body from only a tiny remaining piece of tissue. Many species regenerate using epimorphosis, which involves the creation of a regeneration bud that eventually restores the missing part of the body. However, this ability varies widely between species. Some can regenerate only their tail, and some have lost the ability to regenerate any piece of their body.
Principal Terms
Hermaphroditic: A condition where both male and female reproductive organs are located in the same animal
Nephridia: Excretory tubules specialized for excretion and osmoregulation, with an external opening and with or without an internal opening
Parapodia: Paired lateral projections on each side of most segments, variously modified for locomotion, respiration, or feeding
Septa: A thin, muscular tissue used to separate segments into a series of ringlike cavities
Setae: A needlelike, chitinous structure of the integument used for locomotion
Bibliography
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