Segmented worms make up the Phylum Annelida. The phylum includes earthworms and their relatives, leeches, and a large number of mostly marine worms known as polychaetes. Various species of polychaete are known as lugworms, clam worms, bristleworms, fire worms, and sea mice. Annelids can be told by their segmented bodies. Polychaetes (meaning "many bristles") have, predictably, many bristles on the body, while earthworms and leeches have fewer bristles. There are about 9000 species of annelid known today.All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Segmentation is also called metamerism. Segments each contain elements of such body systems as circulatory, nervous, and excretory tracts. Metamerism increases the efficiency of body movement by allowing the effect of muscle contraction to be extremely localized, and it makes possible the development of greater complexity in general body organization.Besides being segmented, the body wall of annelids is characterized by being made up of both circular and longitudinal muscle fibers surrounded by a moist, acellular cuticle that is secreted by an epidermal epithelium. All annelids except leeches also have chitonous hair-like structures, called setae, projecting from their cuticle. Sometimes the setae are located on paddle-like appendages called parapodia. Annelids are schizocoelous and with a large and well-developed true coelom.The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. Each segment typically contains a pair of nephridia. The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some forms have fairly well developed eyes, including lenses.
| Development: Protostomous |
| Body cavity (between endo- and ectoderm): Coelom |
| Digestive System: Complete |
| Circulatory System: Closed (completely contained within vessels) |
| Excretory System: Proto- or metanephridia connected to coelomoducts |
| Nervous System: Brain, ventral nerve cord, ganglion in each segment |
| Reproductive system: Separate sexes in most Polychaetes, Hermaphroditism in Oligochaetes (earthworms), sequential hermaphroditism (first male, then female) in Hirundinids (leeches) |
| Claim to Fame: Strength, flexibility, and modularity through segmentation |
Diagram of a worm
Sea Worm
Precursor to Human Systems: Closed Circulatory System
