The
Phylum Annelida contains about 9000 species belonging to the classes Hirudinea,
Oligochaeta and Polychaeta. Polychaetes are the largest class and are
mostly marine. They will be the only class discussed in this book. An
annelid wormÕs body is elongate, segmented and cylindrical. They have
a complete digestive tract, with the mouth and anus at opposite ends of
the body. They also have a circulatory system. Their coelem, or body cavity,
is well developed and forms a fluid-filled space between the body wall
and the digestive tract. It also acts as a kind of skeleton in which the
muscles can operate against to allow the worm to burrow, crawl and swim.
They also use their hydrostatic skeleton to extend and retract certain
structures that are used in feeding. Some organs are found in the coelem,
those dealing with reproduction or the regulation of water balance are
an example. Polychaete worms are very diverse. Some are burrowing, some
crawl or swim, while others build tubes. The latter have feather-like
cirri which are used to collect food particles; they also have a trap
door to close their tubes. Most polychaete worms that move about have
well-developed appendages called parapodia, a head with eyespots and sensory
appendages.
The
Phylum Annelida contains about 9000 species belonging to the classes Hirudinea,
Oligochaeta and Polychaeta. Polychaetes are the largest class and are
mostly marine. They will be the only class discussed in this book. An
annelid wormÕs body is elongate, segmented and cylindrical. They have
a complete digestive tract, with the mouth and anus at opposite ends of
the body. They also have a circulatory system. Their coelem, or body cavity,
is well developed and forms a fluid-filled space between the body wall
and the digestive tract. It also acts as a kind of skeleton in which the
muscles can operate against to allow the worm to burrow, crawl and swim.
They also use their hydrostatic skeleton to extend and retract certain
structures that are used in feeding. Some organs are found in the coelem,
those dealing with reproduction or the regulation of water balance are
an example. Polychaete worms are very diverse. Some are burrowing, some
crawl or swim, while others build tubes. The latter have feather-like
cirri which are used to collect food particles; they also have a trap
door to close their tubes. Most polychaete worms that move about have
well-developed appendages called parapodia, a head with eyespots and sensory
appendages.