Nervous System

· The nervous system of earthworm is not well developed and is divided into the Central nervous system (CNS), Peripheral nervous system (PNS) and Sympathetic nervous system (SNS).

a. Central nervous system (CNS)
· Central nervous system passes through the centre of the body and consists of a brain ring that is present around the pharynx in segments 3 and 4. 
· It is made up of:
(a) two fused cerebral (Supra-pharyngeal) ganglia lying mid-dorsally and 
(b) two fused sub-pharyngeal ganglia lying mid-ventrally.
· The cerebral and sub-pharyngeal ganglia are connected by a ring-like circum-pharyngeal connective.

Nervous system of Earthworm

(a) Ventral nerve cord: It is paired, solid and show segmental ganglion from the 5th to the last segment. Each segmental ganglion is made up of two fused ganglia as evidenced by two segmental nerves that emanate from it. The nerve cord is made up of neurons and fibres and is covered by the visceral peritoneum and epineurium. Bipolar and multipolar cells are present in it.

b. Peripheral nervous system
· It consists of 8-10 nerves from the cerebral ganglia which innervate the body wall, prostomium, buccal chamber and pharynx.
· Two nerves from peripharyngeal connective and three from sub-pharyngeal ganglia supply nerves to visceral organs of the segments 1-4.
· Two pairs arise from each segmental ganglia of the visceral nerve cord and supply to segmental organs.

c. Autonomic nervous system (ANS)
· ANS consists of nerve plexuses on the gut wall and internal organs. These are connected by fine nerves to the circum-pharyngeal connective of the brain.
· Earthworm has sensory (afferent) and motor (efferent) fibres. These operate in the following manner:
· Stimuli- Epidermal sense organs- Afferent fibres- Efferent fibres- Muscle contraction

Receptors or Sense Organs

The organs concerned with the reception of the stimuli are receptors or sense organs. In earthworms, there are three types of receptors which are described in the following table.
Epidermal receptorsBuccal receptorsPhotoreceptors = ‘little eye’ (light)
Receptors covered by elevated cuticleIt lies in the buccal liningOccur in the deeper layer of dorsal skin in the region of prostomium and peristomium
Sensory cells have sensitive hair at the outer end, a nucleus at the centre and sensory nerve fibre innervations-Consists of an optic cell with a nucleus, lens (transparent) and cytoplasm
Basal cells and epidermal cells are present.-Retinal lens made up of neurofibrils present in the cytoplasm
a. Touch (tangoreceptor), sensitive to vibration
b. Chemoreceptors
a. Olfactory (smell)
b. Gustatory (taste)
Detect the presence of light, negatively phototrophic

Reproductive System

· Earthworms are monoecious or hermaphrodite i.e. both male and female reproductive organs are present in the same individual.
· Self-fertilization does not occur as the male and female organs do not mature at the same time.
· The testes mature earlier than the ovaries (protandry).

a. Male reproductive system

Reproductive system of Earthworm

The male reproductive system of earthworms consists of
· Two pairs of Testes (10th - 11th segments)
· Two Testis sacs (10th - 11th segments)
· Two pairs of seminal vesicles (11th - 12th segments) [IOM 2068]
· Two pairs of spermiducal or seminal funnels or spermatic funnels (10th - 11th segments)
· Two pairs of vasa differentia (12th - 18th segments)
· One pair of Prostate glands (16th or 17th to 20th or 21st segments)
· Two pairs of Accessory glands (17th and 19th segments)
· Testis sac, which is thin-walled, large bilobed present in segments 11 and 12. The former is somewhat larger. It is mesodermal in origin.
· Testes are two pairs present in segments 10 and 11 and enveloped by the testes sac.
· Seminal vesicles are in pairs, each in segments 11 and 12. It receives the spermatozoa from the testes sac which are stored here after undergoing further development and maturation.
· Vas deferens is the posterior part of the testes sac and bears a large ciliated funnel-like structure called spermidual funnel. This leads to a ciliated narrow sperm duct or vas deferens.
· Prostate glands: A pair of them extends from the segments 17 - 20 and is large and lobulated. They produce a prostatic secretion that serves as a medium for the transfer of sperms.
· Accessory glands
· These are two pairs of glandular masses situated internally in the 17th and 19th segments. Each of them opens outside on each genital papilla. Slimy secretion of them facilitates copulation.

· Spermatogenesis and Sperm Maturation
· Spermatogonia are round, mononucleated and are formed in the digitate processes of testes.
· They are released in the testes sacs, reach seminal vesicles and form flagellated sperms.
· Once formed, the sperms return to the testes sacs and then finally enter the vas deferens via. spermidual duct.

b. Female reproductive system
The female reproductive system consists of:
· A pair of Ovaries (13th segment)
· A pair of Oviducal funnels (13th segment)
· A pair of Oviducts (13th - 14th segments)
· Four pairs of Spermathecae or Receptacula seminales (6th - 9th segments)
· Ovaries: These are digitate, paired, present in segment 13 on either side of the nerve cord, containing ova in various stages of the development. The oldest and the matured ova are at the tip.
· Oviducts: The oviduct funnel lies behind the ovary and conducts ova in the ciliated oviduct. The left and right oviducts unite in segment 14 and open on the ventral surface by the one female genital pore [MOE 2061 Kartik].
· Spermathecae: These are four pairs in segments 6-9. They open intersegmental via. spermathecal pore present on 5/6, 6/7, 7/8 and 8/9 segments. Each spermatheca has a pear-shaped ampulla, a narrow neck and a caecum.
· Spermathecae are meant for receiving sperms for another worm during copulation and temporary storage of sperms. [IOM 2001]

Copulation and Cocoon formation
Cocoon formation in Earthworm

· The two worms apply to each other during the rainy season [MOE 2052] by their ventral surfaces with heads pointing in opposite directions.
· Earthworms lack penis and vagina. The male genital pore of each lies against a pair of spermathecal pores of the other. The genital papillae are inserted into the spermathecal pores and the sperms along with prostatic spermatic secretions are transferred to the spermathecae.
· The sperms and eggs are passed into the cocoon which is secreted by the clitellar glands.
· Fertilization is therefore external (occurring inside the cocoon; IOM 2069) and cross.
· The cocoon is yellowish in colour, 2.4 mm in diameter and is deposited in a moist place.
· Only one embryo develops per cocoon.
· Cocoons are formed by glandular epithelium. [MOE 2003, 2008; Ind. Emb. 2007]
· Cleavage is holoblastic, spiral and determinate.
· Development is direct.
· There is no larval stage.
· The young hatch out of the cocoon in 2-8 weeks.
· It does not have a clitellum at this time.
· Some of the reproductive events are suggested to be controlled by the hormones secreted by the neurosecretory cells present in the cerebral ganglia.
Example: development of clitellum, accessory sexual organs and maturation of sperms and eggs, the release of gametes and gamete formation. 

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