Phylum Annelida

Monday, August 4, 2014

The most familiar annelids are earthworms. The name of the phylum Annelida is derived from the Latin word meaning "little ring", because the ring-shaped segments that make up these animals are very apparent. They are all soft-bodied worms and are found in the oceans, fresh water, and soil. More than 9,000 species of annelids have been described. All have well-developed organs, most of which are segmented. They have nerve ganglia, excretory organs, gonads, smooth muscle, and blood vessels repeated in their segments. Only the digestive systems are unsegmented. Most annelids have appendages on the segments, which bear stiff bristles called "setea". They move their appendages by means of muscles, but their appendages are not jointed. Earthworms have no appendages, but they do have setea (bristles). Furthermore, these setea have muscles and earthworms use the setea in locomotion.

How do they move?

Annelids do not have a rigid skeleton. Instead their coeloms (cavities) are filled with fluid that serve as a hydraulic skeleton, transmitting pressure from one part of the worm's body to another. When any muscle in the body wall of an annelid contracts, it presses the body wall against the fluid in the coeloms. This in turn causes the stretching of any relaxed muscles. By this means, annelids can crawl and burrow far faster and more effciently than ribbon worms, which have no hydraulic skeletons.