Jan 182013
 


In biology, earthworm is also known as Pheretima. It is one of the organisms which can be found in most of the parts of the world. It lives in soil which is rich in decaying organic matters. Some examples of such places are lawns, gardens, fields, pastures, banks of rivers, lakes, ponds etc. It is nocturnal in nature so it becomes active at night only. These are some characteristic features of earthworm. Here, in this post,we are going to

discuss about the external morphology of earthworms which include the following factors:

  1. Shape and Size: The body of earthworm is long, cylindrical, narrow, bilaterally symmetrical and nearly circular in cross section. The anterior end is tapering while the posterior end is more or less blunt. A mature earthworm measures about 150 mm in length and 3 to 5 mm in thickness. The thickest part of the body is little behind the anterior end. The dorsal surface is marked by the presence of a dark median line of dorsal blood vessel which runs along the length of the body just below the skin. Likewise the ventral surface is marked by the presence of genital openings and papillae in the anterior part of the body. The body is of shiny dark-brown colour. The dorsal surface is darker than the ventral surface. The brown colour of worm is due to the present of pigment called porphyrin in the body wall.
  2. Segmentation: The body of earthworm consists of about 100-120 segments. The segmentation is true in which the external segmentation corresponds with the internal segmentation of the body. Hence, the segmentation in the earthworm’s body is known as metameric segmentation or metamerism. The whole body surface is divided by a distinct series of circular grooves. All the segments are alike except the first and the last.
  3. Peristomium: Earthworm has no distinct head, eyes and tentacles. The first segment of the body is called peristomium or the buccal segment, which bears a terminal, crescentric mouth. IT is prolonged anteriorly to a fleshy lobe known as the prostomium which overhangs the mouth of the earthworm.
  4. Clitellum: Clitellum is a girdle-like thick band of glandular tissue which is found towards the anterior end of the body. It is also known as cingulum. The clitellum is formed in mature worms and extends over 14th-16th segments. The glands of clitellum secrete mucus, albumen and egg case or cocoon for the eggs. Due to the presence of clitellum, the body is earthworm can be divided into pre-clitellar, clitellar and post-clitellar regions.
  5. Setae: Setae are chitinoid structures of faint yellow colour which are shaped like an elongated “S”. They have a swollen middle part which are called the nodulus. Except the first, the last and the clitellum segment, each segment contains a ring of about 80 to 120 chitinous setae which are embedded in the epidermal pit of each segment. Such an arrangement is known as perichaetine arrangement. About one-third of setae length projects above the surface of skin and the remaining gets embedded in a setal sac. The setae help in locomotion by holding the earth since they are directed backwards. Their movement is controlled by two types of muscles which are protractor and retractor.