Earthworm: Morphoogy

The morphology or external features are explained under following headings,

i)                   Shape & size:- Body is elongated, cylindrical with bilateral symmetry. The anterior end is pointed while posterior end is blunt. The widest part of body is present behind anterior end and measures about 150 mm in length and (3-5) mm in width.

       

ii)                Colour:-  It is glistening dark brown coloured due to pigment porphyrin derived from chlorophyll. The Porphyrin also protects the body from bright light. The dorsal surface is darker in colour and contains a median dark line throughout length which is mid-dorsal blood vessel visible through integument.

iii)              Segmentation:- The body of earthworm is metamerically segmented and is differentiated into about 100 to 120 segments called as metameres or somites by transverse grooves. External segmentation corresponds with internal segmentation. All segments are alike except first and last segment, at same level of maturity and interdependent with each other.

iv)              Head:-  A distinct head with sense organs is absent. However; first segment of body bears mouth and is called as buccal segment or peristomium. A fleshy projection is present in first segment called as prostomium.

v)                 Clitellum:-  A thick band of glandular tissue encircles 14^th , 15^th  and 16^th  segments of body called as clitellum. It produces mucus, albumen, and cocoon. Due to its presence body remains differentiated into 3-regions:   

      * pre-clitellar region: from 1^st segment up to 13^th segment in front of clitellum.

      * clitellar region:from: from14^th up to 16^th segment where clitellum lies and

      * post-clitellar region: from 17^th segment up to last segment bend cliellum.

vi)              Setae:-  About 80 to 120 setae are present in each segment except in 1^st , last and clitellar segments. All setae of a segment are present at the middle of segment forming ring of setae. This type of arrangement is called as perichaetine arrangement. It is elongated s- shaped with swollen middle part nodulus. About 1/3^rd  part remain outside body surface called as neck and about 2/3^rd part remain embedded in body wall called as base. It is made up of nitrogenous organic substance chitin. They remain embedded in setigerous or setal sac. Their movement is controlled by a pair of protractor and single retractor muscles. The setae of dorsal surface help to move inside burrows while of ventral surface help in crawling on ground.

       

Fig.: Showing arrangement of setae in body wall

vii)   Apertures of body:-

a)  Mouth:-  Anterior opening of alimentary canal present in first segment and remains overhung by prostomium.

b) Anus:-  Posterior opening of alimentary canal in last segment.

c)  Male genital pores:- A pair of openings of common spermatic and prostatic duct each on ventro-lateral position of 18^th segment.

d) Female genital pore:- A single median opening of common oviduct on ventral surface of 14^th segment.

e) spermathecal pores:- Four pairs of openings of spermatheca each pair in ventro-lateral position of inter-segmental groove between 5/6, 6/7, 7/8 and 8/9 segments.

f)   Nephridiopores:- A number openings of integumentary nephridia scattered in each segment except in anterior 2 or 3 segments.

g)  Dorsal pores:- Openings of coelomic chambers present mid-dorsally in each inter segmental groove behind 12^th segment except in last inter segmental groove.

Viii) Genital papillae:- Two pairs of genital papillae each pair in ventro-lateral position of 17^th and 19^th  segments. Each papilla is small elevated area with cup shaped depression and contains a number of openings of accessory glands.They act as sucker during copulation.