Annelida Digestive System
There are three major classes of Annelida:
Class Polychaeta- mostly marine worms or clam worms
Class Hirudinea- the leeches (mostly freshwater)
Class Oligochaeta- moslty freshwater and terrestrial worms like earthworms
The digestive system of annelids consists of an unsegmented gut that runs through the middle of the body from the mouth, located on the underside of the head, to the anus, which is on the pygidium. The gut is separated from the body wall by the body cavity, called the coelom. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue, called septa, which are perforated by the gut and by blood vessels. Except in the leeches, the coelom is fluid filled and functions as a skeleton, providing the animal with rigidity and the resistance necessary for muscular movement. If the worm is punctured, it loses its ability to move properly, since functioning of the body muscles is dependent on the maintenance of the fluid volume in the coelom. In primitive annelids each compartment of the coelom is connected to the outside by ducts for the release of sex cells, and by paired excretory organs, or nephridia. These openings are closed except when functioning, thus preventing the loss of coelomic fluid. In more advanced species both excretory and reproductive functions are sometimes served by a single type of duct, and ducts may be absent in certain segments.
The phylum Annelida (segmented worms) is diverse, containing the earthworms, leeches, and marine worms. They are all triploblastic (containing three different segments) and bilaterally symmetrical. In addition, annelids have a body wall with both longitudinal and circular muscle layers,a complete digestive tract, a nervous system, a closed circulatory system, and an excretory system.
Class Polychaeta- mostly marine worms or clam worms
Class Hirudinea- the leeches (mostly freshwater)
Class Oligochaeta- moslty freshwater and terrestrial worms like earthworms
The digestive system of annelids consists of an unsegmented gut that runs through the middle of the body from the mouth, located on the underside of the head, to the anus, which is on the pygidium. The gut is separated from the body wall by the body cavity, called the coelom. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue, called septa, which are perforated by the gut and by blood vessels. Except in the leeches, the coelom is fluid filled and functions as a skeleton, providing the animal with rigidity and the resistance necessary for muscular movement. If the worm is punctured, it loses its ability to move properly, since functioning of the body muscles is dependent on the maintenance of the fluid volume in the coelom. In primitive annelids each compartment of the coelom is connected to the outside by ducts for the release of sex cells, and by paired excretory organs, or nephridia. These openings are closed except when functioning, thus preventing the loss of coelomic fluid. In more advanced species both excretory and reproductive functions are sometimes served by a single type of duct, and ducts may be absent in certain segments.
The phylum Annelida (segmented worms) is diverse, containing the earthworms, leeches, and marine worms. They are all triploblastic (containing three different segments) and bilaterally symmetrical. In addition, annelids have a body wall with both longitudinal and circular muscle layers,a complete digestive tract, a nervous system, a closed circulatory system, and an excretory system.