Phylum Annelida Notes:
The phylum Annelida (segmented worms) is diverse, containing the earthworms, leeches, and marine worms.
There are three major classes within the phylum Annelida:
- They are all triploblastic
- bilaterally symmetrical
- Eucoelomate
There are three major classes within the phylum Annelida:
- Class Polychaeta - mostly marine worms, such as Nereis (the clamworm)
Class Hirudinea - the leeches (predominantly freshwater), such as Hirudo
Class Oligochaeta - mostly freshwater and terrestrial worms, such as Lumbricus (the earthworms)
- 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. Annelids may be monoecious or dioecious. Larva may or may not be present; if present they are of the trochophore type. Some forms also reproduce asexually. They are protostomes, with spiral cleavage. Members of the Phylum Annelida can be found throughout the world, in marine, freshwater, and terrestrial environments. Ecologically, they range from passive filter feeders to voracious and active predators.
Phylum Annelid Classes:
The class Polychaeta (Poly = many, Chaeta = bristle) are the most diverse and most specious group of the Annelida containing over 5,500 species. They are mainly marine animals and are divided into the Errantia and the Sedentaria groups, depending on whether or not they live sedentary lives in holes or live more active lives. The Errantia have well developed heads and complex parapodia (paddles)that they can use for swimming. They are often dorsoventrally flattened. Most polychaetes are gonochoristic (meaning they are either male or female), however some are sequential hermaphrodites (meaning they are one sex first and then change to being the other sex). Reproduction of Annelids are often accompanied by the production of special reproductive segments which may, or may not, become independent of the parent worm before mating. These segments are destroyed during or immediately after they have released their sperms and ova.
The class Aelosomata contains about 25 species of small to minute worms with many chaetae. They live in the interstitial zone of both fresh and brackish water environments. They are hermaphrodites with each animal possessing one ovary and two testis. They are little known to science and their classification is disputed with some authors considering them to be part of the Oligochaeta.
The class Clitellata contains three distinct groups:
The class Polychaeta (Poly = many, Chaeta = bristle) are the most diverse and most specious group of the Annelida containing over 5,500 species. They are mainly marine animals and are divided into the Errantia and the Sedentaria groups, depending on whether or not they live sedentary lives in holes or live more active lives. The Errantia have well developed heads and complex parapodia (paddles)that they can use for swimming. They are often dorsoventrally flattened. Most polychaetes are gonochoristic (meaning they are either male or female), however some are sequential hermaphrodites (meaning they are one sex first and then change to being the other sex). Reproduction of Annelids are often accompanied by the production of special reproductive segments which may, or may not, become independent of the parent worm before mating. These segments are destroyed during or immediately after they have released their sperms and ova.
The class Aelosomata contains about 25 species of small to minute worms with many chaetae. They live in the interstitial zone of both fresh and brackish water environments. They are hermaphrodites with each animal possessing one ovary and two testis. They are little known to science and their classification is disputed with some authors considering them to be part of the Oligochaeta.
The class Clitellata contains three distinct groups:
- The subclass Oligochaeta (Oligo = few, Chaeta = bristle) are the second most numerous group of annelids with around 3,100 species. Oligochaeta live in marine, freshwater and terrestrial habitats. Generally they have a more rounded cross-section, a less distinct head and are less diverse in form than the Polychaetes. They are normally hermaphrodites, and possess a clitellum as adults, an organ which looks like a bandage of skin wrapped around the animal. This clitellum, from which the whole group takes its name has an important function in sexual reproduction, otherwise reproduction may asexual by fission. A few species are parasitic but most species are free living.
- The subclass Branchiobdella contains 147 species of small (about 1 cm long) aquatic whitish animals that are either commensals or parasites on Crayfish. They are mostly found in the northern hemisphere. Different species attach to their hosts at different places on the body, thus Branchiobdella parastica attaches to the under side of the abdomen while Branchiobdella astaci attaches to its hosts gills.
- The subclass Hirundinea contains the 500 or so species of animals commonly known as leeches. Leaches are well known for their blood sucking habits and their head to tail looping mode of locomotion. Except for the primitive Acanthobdella peledina leeches have no chaetae and 33 body segments. they have two suckers which in most cases are located one at the anterior (head) end of the body composed of segments 1-4 and the other at the posterior (tail) end composed of segments 25-33. Like the Oligochaeta from which they are believed to have evolved the Hirundinea occur in Fresh water, marine and terrestrial environments.