mention four charactericts of annelida
Answers
Explanation:
There are six major kinds of sensory structures found in annelids. These include palps, antennae, eyes, statocysts, nuchal organs and lateral organs (Fig. 5). Palps and antennae are located on the head of many polychaetes.
Answer:
Characteristics of Annelida: Plesiomorphies and Other Features
Greg W. Rouse and Fredrik Pleijel
1. Symmetry and Size
Annelids are all bilaterally symmetrical animals. They range in size from much less than 1 mm in length to more than 3 m. The smallest annelids described to date would appear to be Neotenotrocha (Dorvilleidae, Eunicida) reaching only a few hundred microns in length (Eibye-Jacobsen and Kristensen, 1994). Also within Eunicida may be found the largest of annelids. A preserved specimen of Eunice (Eunicidae) at the Museum of Natural History in Washington D. C. is close to 3 m in length and around 5 cm in diameter in its preserved and contracted condition. Some earthworms, such as the Giant Gippsland Earthworm (Megascolides australis), can reach 1 m in length. The largest leech in the world, Haementeria ghilianii, is found in South America and reaches more than 40 cm in length.
2. Coelom
Nearly all annelids have a fluid-filled cavity between the outer body wall and the gut, and this is referred to as a coelom (Figure 1). The coelom is often used as a storage area for gametes and acts as a hydrostatic skeleton for locomotion. The coelom (formed via schizocoely) is generally organized as a series of compartments divided by intersegmental septa. This compartmentalization means that if the worm is damaged the coelomic contents will only be lost from a few segments, and locomotion can be maintained. In other annelids there may only be a few septa dividing the coelom.
3. Body wall
Annelids have a body covered by an external cuticle that is never shed or molted. Epidermal microvilli secrete a network of fibers that are in part collagenous and also contain scleroprotein. Chaetae are also cuticular structures, but contain large amounts of chitin. Beneath the epidermis and its basal lamina lies a layer of circular muscle. The circular muscle layer forms a nearly continuous sheath around the body, except in polychaetes with well-developed parapodia. Beneath the circular muscle layer lie thick longitudinal muscles. In many annelids the longitudinal muscles are present as four distinct bands
4. Parapodia
Parapodia are unjointed segmental extensions of the body wall found in many polychaetes, though many do lack these features. They are also absent in Clitellata and Echiura. Parapodia are equipped with musculature derived mainly from the circular muscle layer and usually carry chaetae. Parapodia vary in structure but basically can be considered to consist of two elements; a dorsal notopodium and a ventral neuropodium .