The word Annelida was first coined by Lamarck. This phylum includes elongated worms like earthworms, nereids leeches etc. They show a segmented body. The annelids are distributed all over the world. Nearly 7,000 species are known today. The smallest annelid is Chetogasfer which measures 1 mm in length while the largest annelid is Megascolides australis which measures about 1l feet in length. Many of the annelids are marine but some are terrestrial.
- The body is elongated and bilaterally symmetrical. Cephaiization is seen. They show metameric segmentation.
- The body shows segments or metameres, which are arranged one after the other in a linear order.
- Cuticle is the outer covering of the body. It is secreted by the epidermis.
- The body wall is dermo-muscular. Muscles are circular and longitudinal.
- The locomotory organs are groups of chitinous setae or chaetae, embedded in setigerous sacs.
- The body is triploblastic with Ectoderms Mesoderm and Endoderm layers.
- Annelids are true coelomate animals. Coelome is a spacious cavity, except in leeches in which it is filled with botryoidal tissue.
- The alimentary canal is a straight tube starting with the mouth at the anterior end, and ending with the anus, at the posterior end of the body.
- Digestion is extracellular. Respiration is through the general body surface by diffusion.
- The blood-vascular system is well developed and closed type.
- The blood is red-colored fluid with hemoglobin dissolved in plasma.
- The excretory system consists of the nephridia, which are segmentally arranged.
- The nervous system consists of a nerve ring around the pharynx, and a doubly ventral nerve cord is seen extending the entire length of the body.
- Asexual reproduction is seen in many species.
- In hermaphrodite animals the development is direct.
E.g.: Oligochaeta, Hirudinea. In unisexual animals, the development is indirect with "trochophore" larva.
E.g.: Polychaeta, Archiannelida.
Classification of Annelida
The following classification is based on Parker and Haswell's system.
CLASS I: POLYCHAETA (Greek Poly many, chaete setae)
All are marine. Segments show well-developed lateral outgrowths, called parapodia, with many setae. A distinct head is present with eyes, tentacles, cirri, and palps. Sexes are separate Trochophore larva is seen. This is divided into 2 subclasses.
Sub-Class 1: Errantia
Free swimming forms are included. Segments are many and similar. The head shows a prostomium with sensory appendages. Parapodia are well-developed with acicula.
E.g.: Neanthes, Aphrodite, Eunice.
Sub-Class 2: Sedentaria
The body includes 2 or more regions, which are dissimilar. The prostomium is small. Parapodia are small, without acicula. Pharynx is non-protrusible. Jaws and teeth are also seen. They live in burrows or tubes.
E.g: Chaetpterus, Terebella.
CLASS II: ARCHIANNEUDA (Gr, Archi, first)
It is a minor group including a few genera. They are marine worms. They do not show seate or parapodia. Segmentation is internal. They are unisexual or hermaphrodite. Their larval form is a trochophore. Some zoologists consider Archiannelida as a class of Annelida.
E.g.: Polygardius, Dinophilus.
CLASS III: OLIGOCHAETA (U.C)
Fresh water and terrestrial worms are included. They do not show parapodia. The head is not distinct. Setae are in each segment. Clitellum is present. All are bisexual animals. Development is direct. This is divided into 4 orders.
Order 1: Plesiopora
Male gonophores are seen on the segment following the testis. Spermathecae are present in the region of the genital segments.
Example: Nais, Tubifex, Chaetogaster.
Order 2: Prosopora
Male gonophores are seen on the segment following the tests. Spermathecae are considerably in front of the genital segments.
Examples: Enchytraeus, Aspidodrilus
Order 3: Prosopora
Male gonophores in the same segment as the last pair of testis. Aquatic worms.
Examples: Bdellodrillus
Order 4: Opisthopora
Male gonophores are present at least one segment behind the last pair or of the testis. Aquatic or terrestrial animals are included.
Examples: Pheretima, Lumbricus, Megascolex.
CLASS IV: HIRUSDINEA (L HIRUDE, A LEACH)
Freshwater, marine, or terrestrial worms. They are parasitic or predators. The body is depressed, made with a fixed number of small segments (33) which are externally divided by secondary annuli. Setae are absent. Usually, the suckers are present. Coelome is much reduced, and the combat is filled with botryoidal tissue. Bisexual animals and fertilization are internal. It is divided into 4 orders.
Order 1: Acanthobdellida
The anterior sucker is absent, proboscis and jaws are absent. Setae are present in the first five anterior segments.
Example: Acanthobdella. (connecting link between Oligochaeta and Hirudinea.)
Order 2: Rhycobdellida
Jawless leeches with protrusible proposes are included in this order. Colorless blood is seen. They are aquatic worms.
Examples: Glossiphonia, Pontobdella.
Order 3: GnathobdelJida
- Proboscis is absent.
- Blood is red. Aquatic or terrestrial leeches.
- Examples: Hirudo, Hirudinaria.
Order 4: Pharyngebdellida
Jaws and teeth are absent, in freshwater or amphibious leeches.
Examples: Erpobdella, Dina.