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General Characteristics of Annelids

Annelids, or ringed worms, are represented by more than 17,000 modern species, including earthworms and leeches. There are many different subcategories of annelids, but each, as can be deduced from the name, has a segmented body which creates a ringed effect along its body. They can and do live in almost every environment on earth in some variety or another, but they prefer moist and damp conditions.

External Characteristics
- All annelids have a long, cylindrical and symmetrical body. The segments on the body are usually separated by septa, dividing cavities which do the same job as the middle part between a human's nostrils. They also possess pogonohora and echiura, small, hairlike appendages along the side of their body which are used for sense. Some species also have sipuncula, a pair of leglike limbs which are used for movement. All categories of annelids have a mouth at the front and an anus at the end of the tail.
Internal Characteristics
- The internal structure of annelids is much more uniform across the species. They have a brain and an esophagus located behind the mouth. Further down are five pairs of hearts. They have one long digestive tract running almost the entire length of the body, but not a real digestive system. They have no respiratory organs --- they instead "breathe" through the skin --- and no circulatory system. There is a very basic nervous system controlled by a central nerve chord.
Species Characteristics
- Several subspecies of annelids have their own external features. Polychaetes, which make up the majority of annelid species, have multiple hairs per segment and always have the sipuncula to help with movement. The majority of these live in salt water. Oligochaetes have just a few hairs per segment and mainly live on land and tend to burrow underground. Clitellates also tend to live on land, but they have a saddle, or clitellum, somewhere down their body. Hirudinea are the bloodsucking variety of annelids and include leeches.
Reproductive Characteristics
- These animals reproduce both asexually and sexually. Polychaetes, which are mostly saltwater animals, are asexual, meaning there is neither a male nor female of the species. They reproduce by splitting or budding off a new organism. Other polychaetes reproduce asexually at one time of the year and sexually during another. Other varieties of annelids may lay eggs which are fertilized afterward and produce miniature adults rather than a larval form. Others engage in a very basic form of intercourse. Some annelids may reproduce only once in a lifetime, while others will reproduce several times a year. Most annelids are classified as hermaphrodites, as they are capable of changing gender roles.

Animals & Science