CLASS HIRUDINEA (hirdin, leech’)
The class Hirudinea contains five hundred species of leeches. Most leeches are fresh water. Some are marine. Some are completely terrestrial. Leeches prey on small invertebrates or they feed on the body fluids of vertebrates.
EXTERNAL STRUCTURE AND LOCOMOTION
Leeches lack parapodia and head appendages. Setae are absent in most of leeches. Setae occur in anterior segments in few leeches. Leeches are dorsoventrally anteriorly. They have 34 segments. The segment are divided by annuls. Therefore, it is difficult to distinguish the segment externally. Anterior to and posterior segments are modified into suckers. The leeches have modified patterns of body-wall muscles and coelom. It influences locomotion of leech.
The musculature of leeches is more complex than annelids. A layer of oblique muscles is present between the circular and longitudinal muscles layers. These muscles are responsible for the flattening of leech. The coelom of leech has lost its metameric partitioning. Septa are lost. Connective tissue has filled the coelom. It produces many interconnecting sinuses.
These modifications have changed the patterns of locomotion in leeches. Therefore, leeches do not use independent coelomic compartments. The leech has a single hydrostatic cavity. It uses a looping type of locomotion. Leeches also swim by undulations of the body.
FEEDING AND THE DIGESTIVE SYSTEM
Many leeches feed on body fluids. They may eat the bodies of other invertebrates. Some feed on the blood of vertebrates, including human blood. Leeches are parasites. But the association between a leech and its host is very brief. Therefore, leeches are described as predator. Leeches are also not specie specific.
The mouth of a leech opens in the middle of the anterior sucker. In some leeches, the anterior digestive tract is modified into a protrusible proboscis. It is lined inside and outside by a cuticle. In other leeches. the mouth is armed with three chitinous jaws. A leech attaches to its prey by anterior sucker during feeding. They extend their proboscis into the prey. Or they use its jaws to cut through host tissues. Salivary glands secrete an anticoagulant called hirudin. Hirudin prevents blood from clotting.
There is muscular pharynx behind the mouth. It pumps body fluids of the prey into the leech. The pharynx opens into esophagus and oesophagus open into a large stomach. Stomach has lateral cecae. Most leeches ingest large quantity of blood or other body fluids. They store it in their stomachs and lateral cecac. It increases their body mass 2 to 10 times. The leech can tolerate periods of fasting after storage of this fluid. This period may last for months. The digestive tract ends in a short intestine and anus.
GAS EXCHANGE AND CIRCULATION
Leeches exchange gases though the body wall. Some leeches retain the basic annelid circulatory pattern. But most leeches have coelomic sinuses in place vessels. Coelomic fluid has taken over the function of blood. Respiratory pigments are absent except in to orders..
NERVOUS AND SENSORY FUNCTIONS
The leech nervous system is similar to other annelids. Ventral nerve cords are not fused. The suprapharyngeal and subpharyngeal ganglia and the pharyngeal connectives all fuse to forms a nerve ring. This nerve ring surrounds the pharynx. Ganglia of posterior end of the animal are also fused.
1. Different types of epidermal sense organs are scattered over the body. Most leeches have photoreceptor cells in pigment cups. Pigments cups are present along the dorsal surface of the anterior segments. Leeches are negatively phototactic. But the behavior of some leeches changes during searching of food. Thus they become positively phototactic. It helps in search of prey.
2. Hirudo medicinalis has a well-developed temperature sense. It helps it to detect the higher body temperature of mammalian prey. Other leeches are attracted by the prey tissues.
3. All leeches have sensory cells with terminal bristles. They are present in a row along the middle annulus of each segment. These sensory cells are called sensory papillae. Their function is unknown. But they are taxonomically important.
Leeches have 10 to 17 pairs of metanephrindia. One nephridium is present per segment in the middle segments of the body. Their metanephridia are highly modified. It possesses a capsule. This capsule is involved in the production of coelomic fluid. Chloragogen tissue spread through the body cavity of most leeches.
REPRODUCTION AND DEVELOPMENT
All leeches reproduce sexually. They are monoecious. Asexual reproduction or regeneration is absent in them. They have a single pair of ovaries. They have four to many testes. Leeches have a clitellum. Clitellum is composed of three body segments. The clitellum is present only in the spring. Spring is breeding season of leeches.
A penis is used to transfer sperm between individuals. A few leeches transfer sperm by expelling spermatophore from one leech into the integument of another. It is a form of hypodermic impregnation. Special tissues are connected to the ovaries by short ducts. Cocoons are deposited in the soil. or they attached to underwater objects. There are no larval stages. The offspring become mature in next spring.