THE INDIAN EARTH WORM

 class Oligochaeta  includes terrestrial earthworms and some other species that live in fresh water. As compared to many setae of polychaetes, they posses few locomotor setae borne directly by body segments which are devoid of parapodia. there are several genera of earthworms. Common  genus of Europe and North America is Lumbricus. In India, Drawida and Megascolex occur in South India and Eutyphaeus is found in the Gangetic Plain of North India.  Pheritima, which is commonly found in South East Asia, Japan, Sri Lanka and Australia is represented  by 13 species in the Indian soil.  An earth worm is usually studied as a type of Annelida because it is easily  available almost every where.

                                                     

            SYSTEMATIC POSITION OF PHERITIMA  POSTHUMA [ INDIAN EARTH WORM ]

                      Phylum :        Annelida 

                      Class     :       Oligochaeta 

                       order    :        Opisthopora 

                       Family  :        Megascolecidae

                       Genus    :       Pheritima 

                       Type       :       posthuma 

HABITS AND HABITAT :    Pheritima Posthuma is terrestrial earthworm living in burrows made in moist earth.  It prefers to live in the burrow during day and comes out at night and in damp cloudy weather.  It is thus nocturnal in habit.  During rainy season, after a heavy fall,  earthworms leave their burrows and are seen in large numbers  crawling on ground.  Earthworm makes its burrow partly by boring with its pointed anterior end and partly by sucking and swallowing the earth. It feeds on dead organic matter pressent in soil.   Food and soil are ingested together and the latter, along with undigested   food is finally egested in the form of worm castings.  Earthworm is are hermaphrodite, but they under go copulation for exchange of their spermatozoa.  Fertilization and development occur inside  a  cocoon.  Trochophore   larva  does not occur as  young warm hatches out of cocoon.  Earth worm  posses great power of regeneration. 

 EXTERNAL   MORPHOL :

 1] Shape and size : Earthworm is a bisymmetrical animal.  Its body is cylindrically elongated, pointed in front,  blunt behind and thickest a little behind the anterior end. It is well-adapted  for burrowing. A mature worm measures about 150 mm in length and 3  to  5 mm  in  width.

2] colour :   earthworm is of a glistening deep  brown or clay  colour.  Dorsal surface is darker than the ventral surface and  carries a dark coloured median line due to dorsal blood vessel which is seen through the integument. Brown colour of worm is due to the pigment porphyrin  present in body wall  and it protects the body against bright and strong light.

3] Segmentation : Soft and naked body of earthworm   is  divided into 100  to 120 similar segments 

, called metameres or somites. these are without parapodia.  Segments  are separated from each other by distinct ring-like grooves.  External segmentation  of body.

4] head : Earthworm lacks a distinct head and sense organs like eyes, cirri, and  tentacles.  First segments at the anterior end of the body is called buccal segment  or   peristomium bearing the terminal ,  crescentic  mouth.  It is prolonged anteriorly into a fleshy lobe,  the prostomium, which  overhang the mouth.

5] Clitellum : In mature worms, a conspicuous external feature is a girdle-like thick band of glandular tissue,  the clitellum, which completely and permanently surrounds the segments 14 to 16.  Due to its presence, the body is distinguished into peri-clitellar ,clitellar and post-clitellar regions.  Clitellum is a glandular organ which secretes mucus,  albumen and an egg case or cocoon for eggs.

6] Setae : About the middle of each segment there  is a ring of tiny curved bristles, called setae or chaetae,  formed of a horny nitrogenous organic substance, known as chitin. About 80 to 120  setae are present on each segment. But they are absent on peristomium , pygidium, and the clitellum.  Each seta is embedded in a small pit in body wall, called setigerous or setal sac.  It is formed by a single  formative cell present in the basal part of sac.  It has a faint yellow colour and is shaped like an elongated ''S" with a  swollen middle part, called  nodulus. About one third of its length projects above the  surface of skin in a contrcted segment.

7] Genital papillae : these are two pairs of conspicuous rounded elevations, one pair each in  the 17th and 19th segments, on ventral surface. Each papilla bears a shallow cup-like depression at its top which acts as sucker during copulation.