Earthworm

EXCRETORY SYSTEM

Nephridia are the excretory organs of earthworm, which are of three types, namely, septal, integumentary and pharyngeal.

Septal Nephridia

 A typical septal nephridium consists of Nephrostome or ciliated funnel that drains coelom. Body of nephridium consists of a short straight lobe and a long twisted lobe with a narrow apical part. Lobes are made of syncytial mass, inside which run coiled ciliated tubules that end up in a narrow terminal excretory duct that opens into the segmental excretory canal and eventually into the intestine.

Integumentary Nephridia

These nephridia lie scattered in the body wall of each segment, except the first two. There are 200-250 nephridia in each segment but on clitellum their number increases to 2000-2500 per segment which is called the forest of nephridia. These nephridia lack nephrostomes and their terminal ducts open on body surface.

Pharyngeal Nephridia

These nephridia are located on either side of pharynx and oesophagus in segments 4^–6. Each nephridium consists of hundreds of coiled branched tubules that lack nephrostomes. The nephridial ducts of 4^th and 5^th segments open into pharynx, while those of 6^th segment open into buccal cavity. Ammonia and urea are the excretory products.

 NERVOUS SYSTEM

The nervous system of earthworm is well developed and consists of a pair of cerebral ganglia forming the brain situated in the third segment above the pharynx. Circum- pharyngeal connectives encircle the pharynx and meet with a pair of subpharyngeal ganglia below the pharynx. The ventral nerve cord runs from the subpharyngeal ganglia to the last segment of the body in the middle on the ventral side. In each segment there is one fused paired ganglion called segmental ganglion, from which arise 3 pairs of peripheral nerves. It includes the nerve plexuses situated in the wall of the alimentary canal and some other internal segment.

SENSE ORGANS

The sense organs or receptors of earthworm are as follows:

• Epidermal receptors: These receptors are scattered all over the epidermis making the skin sensitive to touch. The receptors are groups of long sensory tactile cells surrounded by supporting cells.
• Photoreceptors: Photoreceptors are solitary cells embedded in the epidermis which contain a crystalline structure in the cytoplasm and a nerve connection at the base. They are distributed on the anterior side of body.
• Buccal receptors: They are also bunches of sensory cells found in the buccal cavity that perceive chemical stimuli and help in smelling and tasting food.

REPRODUCTIVE SYSTEM

The earthworms are monoecious or hermaphrodite but protandrous, i.e. their testes mature earlier than ovaries to avoid self fertilization.

Male reproductive organs

There are two pairs of whitish testes in segments 10 and 11 which are enclosed in thin-walled coelomic spaces called testis sacs. Spermiductal funnels are attached to each testis sac that continues with the vas deferens that eventually lead to prostate gland of its side on 18^th segment. One seminal vesicle is also attached to each testis sac in which maturation of sperms takes place. They lie in segment 11 and 12.

Prostate glands that extend from 16^th to 21^st segment are made of glandular and non-glandular parts. Ducts of prostate glands open to the exterior independently by a pair of male genital pores on the ventral side of the 18^th segment.

On segments 17 and 19 is a pair of accessory glands which open to the exterior by a number of ducts on two pairs of genital papillae. Their secretion helps in holding the two worms together during copulation.

Female reproductive organs

A pair of small white ovaries lies in 13^th segment on either side of ventral nerve cord. Each ovary is whitish made of finger like lobules in which ova are arranged in various stages of development. An oviductal funnel lies immediately behind each ovary in 13^th segment and it leads behind into a short oviduct. The two oviducts run backwards and open to the exterior by the female genital aperture.

Spermathecae

 There are 4 pairs of small pear shaped bilobed spermathecae which receive sperms from another worm during copulation and store them in their diverticula in Pheretima and in ampullae in other earthworms.

Copulation

July to October is the breeding season of earthworms. During copulation two earthworms come to lie close together with their anterior ends pointing in opposite directions. In this position the male genital apertures of each worm lie opposite to the spermathecal openings of the other and genital papillae help the worm to hold each other in this position. Mutual exchange of sperms takes place between the two copulating worms. Copulation lasts for about an hour after which the worms separate.

Cocoon formation

 The cocoon secreting glands located on 14-16th segments secrete a girdle like cocoon around the clitellum, inside which ova are released. Albumen glands release albumen into the cocoon. Then the worm starts withdrawing itself from the girdle. As the cocoon passes over the spermathecae sperms are released through spermathecal pores and ova are fertilized to form zygotes. As the cocoon is released from the anterior end of body, the elasticity of its walls closes its two ends and the cocoon is deposited in a moist and protected place.

A cocoon contains many fertilized eggs but only one embryo completes development while other ova serve as nurse cells. The development is direct without free larval stage. After development of 2-3 weeks the young worm crawls out of the cocoon.