According to their parasitic mode of life leeches show a number of adaptations in their morphology, habit and habitat:
1. Habit and habitat:
Leeches swim actively in search of hosts and inhabit in such a pond or lake where cattle, men and other vertebrates visit frequently.
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2. Slime glands:
These are present in body wall. Their secretion keeps the body moist and slippery so that the host can’t remove it easily while it sucks blood.
These are the organs that help in locomotion as well as in making firm adhesion to the host’s body while feeding.
4. Jaws and pharynx:
Triradiate jaws are equipped with denticles, able to make incision in the skin of the host. A muscular suctorial pharynx works as a suction pump to suck more and more blood, oozing out from the wound.
Hirudin is an anticoagulant, secreted by salivary gland when it sucks the blood to prevent it from coagulation. It faciliates the feeding.
6. Spacious crop:
The availability of the host to the leeches is not regular. Therefore its digestive tract is modified in order to store a great quantity of blood in a single meal whenever it gets chance to feed.
To accomodate the large amount of blood the crop is spacious, thin walled, elastic and capable of great dilation.
7. Slow digestion:
Blood from crop is poured into stomach drop by drop because digestion is very slow. So that after a full meal a leech can stay for one year or more without feeding.
For this reason also, leeches lack elaborate digestive juices and enzymes for digestion.
8. Sense organs:
These are well developed and provide the animal with greater opportunities of life.
This doubles the rate of reproduction as after copulation both individuals lay eggs.
It takes place in cocoon with full safety and efficiency. It is very quick and takes only a fortnight.