There are three parts in the nervous system, namely, Central, Peripheral and Sympathetic nervous system.
The central nervous system is enclosed in the ventral haemocoelomic channel and consists of a nerve ring on the dorsal side of which is a cerebral or supra-pharyngeal ganglion. It is connected by a pair of lateral circumpharyngeal connectives with the ventral sub-pharyngeal ganglion. Ventral nerve cord arises from the sub-pharyngeal ganglion and runs backward from 6th to 26th segment and make one ganglion in each segment. The ventral nerve cord ends in a large terminal ganglionic mass, situated within the posterior sucker.
Peripheral nervous system consists of paired nerves arising from segmental ganglia of the central nervous system. A pair of optic nerves arises from cerebral ganglion and supplies to the 1st pair of eyes. Four pairs of optic nerves arise from subpharyngeal ganglion and supply to the other 4 pairs of eyes.
Sympathetic nervous system consists of a nerve plexus spread beneath epidermis, within muscles and on the gut wall.
There are specially modified epidermal cells to serve as receptor organs. These cells remain scattered and sunk within the body wall. The different receptors found in leech are:
There are five pairs of eyes, one pair each on the first five segments. Each eye is cylindrical and cone-shaped, whose broad outer surface is formed by transparent epidermal layer covered with cuticle. The cone is filled with gelatinous material and photoreceptor sensory cells arranged in longitudinal rows. Each cell is large and bears a crescentic hyaline lens in its cytoplasm. Each cell is connected to the optic nerve by a nerve fibre. These eyes can distinguish light from darkness and cannot form a proper image.
They are papillae arranged in a row on each annulus of the segment. Each annulus bears eighteen receptors on the dorsal and eighteen on the ventral side. These receptors are made of sensory cells and supporting cells and are tactile in function.
Segmental receptors are present as papillae on the first annulus of each segment of the body. Four pairs of these papillae occur on the dorsal surface and three pairs on the ventral surface. The sensory cells found in these receptors are tactile as well as photoreceptor in nature.
In addition to these receptors there is large number of free nerve endings scattered between the epidermal cells of the skin which are tactile as well as chemoreceptors in function.
Like earthworm leech is hermaphrodite and male and female reproductive organs occur in the same animal. Cross fertilization occurs by copulation in which mutual exchange of spermatophores takes place.
Male Reproductive Organs
Testes are located inside testis sacs which are eleven pairs located in segments 12-22nd, one pair in each segment. Spermatogonia are budded off from testes and float in the coelomic fluid within each testis sac and mature into spermatozoa.
Each testis sac is connected to a common longitudinal vas deferens running on each side of gut, by small vasa efferentia, through which the mature spermatozoa pass into vas deferens. In the 10th segment each vas deferens forms a compact mass of tubules, the epididymis. In the two epididymes spermatozoa undergo maturation.
A short narrow ejaculatory duct connects epididymis with atrium, which is a pyriform sac situated in the ninth and tenth segments. The atrium is made of an anterior prostate chamber and a posterior penis sac, the former is covered with several layers of unicellular prostateglands that secrete seminal fluid. The penis sac contains a coiled evertible penis which is used to transfer sperms into the vagina of another leech.
In prostate chamber the spermatozoa are glued by the secretion of prostate glands into spermatophores, which pass through the penis into the vagina of other leech during copulation.
Female Reproductive Organs
The female reproductive organs consist of a single pair of ovaries, situated in the eleventh segment, which are coiled nucleated cords from which ova are budded off. The ovaries remain floating in the fluid of ovisacs. The oviducts of two sides unite to form a common oviduct in the middle of eleventh segment, where there is a mass of unicellular albumen glands opening into it. The common oviduct opens into a pear-shaped muscular vagina, which opens to the exterior through a mid-ventral female genital aperture in the eleventh segment. Ova are fertilized in the vagina and zygotes are release into the cocoon.
Copulation takes place in March-April. During copulation two leeches come together pointing in opposite directions so that the male aperture of one leech lies opposite the female aperture of the other. The penis of each leech is inserted into the vagina of the other and spermatophores are exchanged. Copulation may occur on land or in water and lasts for about an hour after which the two leeches separate. Fertilization is internal inside the vagina. The fertilized eggs are deposited into the cocoon in which further development of the embryo occurs.
The cocoons are secreted by the clitellum, which is formed around segments 9-11 during breeding season. The clitellar glands secrete albumen into the cocoon which is used as nourishment by the developing embryo. The cocoon is then passed over the head of leech as the leech withdraws its anterior end backwards by rhythmic movements of body. The prostomial glands secrete two polar plugs to close the two ends of cocoon. The cocoons are laid in moist soil where eggs develop into tiny leeches without undergoing through a larval stage.
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