In this article we will discuss about the dissection of leech. Also learn about:- 1. The Alimentary System 2. Dissection of Nervous System.
Quite a few methods, all equally good, are used for killing leeches. Wash the specimens with clear water and place them in a container partly filled with water. Add slowly a small amount of 90% alcohol to the water.
Continue adding alcohol at intervals till the movements of the leeches stop. Take the specimens out of the container, wash with water and gently squeeze with fingers to press out blood from the crop as far as possible. The leech can also be killed with chloroform.
Stretch the animal (Fig. 3.1) as fully as possible and fix it on the dissecting tray keeping the dorsal surface upwards, with the help of fine pins. The pins should pass through the lateral sides of the anterior and posterior suckers. While fixing the anterior end, care should be taken not to damage the brain.
Cut the skin slightly lateral to the mid-dorsal line and proceed both anteriorly and posteriorly up to the ends. Separate the skin from the underlying tissues with a fine arrowhead needle. Special care should be taken in removing the skin as it is connected to the crop with strong strands of tissues. Pin down the separated flaps of the skin.
The Alimentary System:
The alimentary canal is fully exposed with the separation of the skin from the underlying tissues. It is a straight tube (Fig. 3.2) running from the mouth to the anus, the anterior two- thirds being modified into a crop.
A cup-shaped depression, ventral to the anterior sucker.
A triradiate opening at the base of the preoral chamber guarded by the velum.
A short chamber following the mouth. It opens in the pharynx. Three jaws, two ventrolaterals and one dorsomedian are present.
An oval muscular chamber between the buccal cavity and the oesophagus.
A short narrow tube ending in the crop.
Occupies about two-thirds of the length of the alimentary canal. It has ten pairs of lateral caeca. The anterior pair are smallest and the posterior pair largest.
A small, thin walled chamber in the 19th segment, broad anteriorly and narrowing posteriorly, ends in the intestine.
A narrow tube ending in rectum.
The last part of the gut. It is wider than the intestine and narrows down posteriorly to end in the anus.
A narrow opening at the junction of the posterior sucker and dorsal body surface, on the mid-dorsal line.
Dissection of Nervous System:
It is situated at the anterior end of the body, being oriented vertically. The pharynx runs through it.
a. Supraoesophageal or cerebral ganglia:
Two in number, fused to form the dorsal part of the nerve ring or so-called brain.
b. Peripharyngeal connectives:
Two in number, constitute the lateral sides of the ring round the pharynx.
c. Sub-pharyngeal ganglia:
Two ganglia fused to form the ventral part of the ring.
Ventral nerve cord:
Situated ventrally; the two cords joined by ganglia run backward from the sub-pharyngeal ganglia to the posterior end of the body.
a. The nerve cords bear a ganglion in each segment. The terminal ganglionic mass is ovoid.
b. Two pairs of nerves arise from each ganglion.