External Morphology of Leech

 

The leech is an ectoparasite. The leech, Hirudinaria, is a segmented worm found in  fresh­water tanks, ponds, and shallow weedy lakes.   Leech can easily suck blood from farmers as well as from the cattle. Such type of feeding habit is called a sanguivorous feeding habit.

The blood-sucking habit of the leech had been used  by the older physicians for the purpose of treating old wounds. Hence it is also called as medicinal leech..

 

2. External Feature of Leech:

 The animal has a soft elongated body with a sucker at each end and when fully expanded it measures about 2.5 cms in length. It is flattened dorso-ventrally but the dorsal surface is a bit more convex than the ventral. When contracted, the animal appears to be elliptical and does not exceed three or four inches in length. The skin is moist and slimy due to constant secretion of mucus, and bears beautiful colour patterns.

The dorsal surface is dark-green  or olive green in colour and the ventral surface is pale yellow in colour.. The entire body of a leech is divided externally into a large number of rings or annuli separated by transverse grooves.

The body of leech consists of 33 segments.

 

 Metamerism in leech

          When segmentation in bilateral animal such as annelids involves a longitudinal division of body into a linear series of similar section of parts is termed as metamerism. Or  metameric segmentation.

Body of annelids is metamerically segmented. Internal and external metameism is visible in most of the annelids.  The body of leech is divisible into many regions as follows.

1.      Cephalic region                     I-V

2.      Pre-clitellar region               VI-VIII

3.      Clitellar region                    IX-XI

4.      Middle region                      XII-XXII

5.      Caudal region                      XXII-XVII

6.      Posterior sucker                   XXVIII-XXXIII

 Each segment is divided by ring also called as annuli. The segment is either uni-annulate, bi-annulate, tri-annulate or Quinqua-annulate. Segment with five annuli is called quinqua -annulate segments or complete segment. The distribution of annuli in different regions of body is as follows.

 

Sr. No.	Body region	Segment No.	Distribution of annuli	Complete or incomplete
1	Cephalic region	I-V	I and II         Uni-annulate	Incomplete
			III -                 Biannulate	Incomplete
			IV and V       Tri-annulate	Incomplete
2	Pre-clitelar Region	VI-VIII	VI-       Tri-annulate	Incomplete
			VII and VIII Quinqua-annulate	complete segment
3	Clitelar region	IX I XI	Quinqua-annulate	complete segment
4	Middle region	XII-XXII	Quinqua-annulate	complete segment
5	Caudal region	XXIII- XVII	XXIII           Tri-annulate	incomplete segment
			XXIV and XXV     Bi-annulate	incomplete segment
6	Posterior sucker	XXVIII – XXXIII	Uni- annulate.	incomplete segment

 

 A typical segment or metamere from the middle of the series includes five rings, of which the first bears the segmental receptor organs arranged in a ring round the body. The receptors are the sense organs of the leech and they appear as small black spots on the surface of the skin. Each metamere bears four pairs of segmental receptors on its dorsal surface and three pairs on the ventral surface.  The first annuls marks the boundry of the segment.

To both the end of body there is presence of sucker.. the anterior sucker is oval and cup like.  The anterior sucker consists of first 5 segments. On dorsal surface of anterior sucker, there are presence of five pairs of prominent eyes.

The posterior sucker is composed of the last seven segments. It is a muscular disc, circular in Outline, and is larger in size than the anterior sucker. It is used as an organ for adhesion and locomotion.

(1) The mouth is a triradiate opening situated ventrally in the centre of the cup-shaped anterior sucker.

(2) The anus is a small opening situated in the middle line on the dorsal surface of the 26th segment, in front of the posterior sucker.

(3) The urinary openings are the seventeen pairs of minute nephridio­pores on the ventral surface of the body. A pair of nephridiopores occur in each segment, beginning from the sixth and extending up to the twenty-second.

(4) Leech is hermaphrodite. The repro­ductive openings or gonopores are situated in the mid-ventral line, and are separated from one another by five rings,

(a) The male gonopore lies in the mid-ventral line of the tenth segment and a filamentous penis may occasionally be seen protruding through this opening,

(b) The female gonopore is situated in the mid- ventral line of the eleventh segment.

A saddle-shaped clitellum is formed in summer during the breeding season of the leech. At this time the skin of segments 9th, 10th and 11th swells up and is changed in colour and tex­ture. The clitellum produces a protective capsule or cocoon for the developing embryo.

 

3. Body Wall and Body Cavity of Leech:

 The body wall of leech consists of a thin cuticle, the epidermis, the dermis, the musculature and the botryoidal tissue. It is best studied in a cross-section passing through the middle of the animal