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Sunday, September 12, 2010

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Earthworm [Part 2]

5) Locomotion:
Locomotion in earthworm is caused by setae and the musculature of the body wall. When as earthworm starts to crawl, the first few segments become thinner and extend forward. This is due to contraction of the circular muscle fibres and relaxation of the longitudinal muscles in that region. The longitudinal muscles of the anterior segments now contract and the circular muscles relaxes due to which anterior end becomes shorter and thicker. The seta of these segments then protrude out and get gripped into the soil. The contraction of anterior segments pull the posterior body of the worm forward. The contraction of longitudinal muscles passes backwards like a wave. Again a wave of contraction of circular muscles starts from the anterior end making it thinner and extended forwards. These alternating series of waves of extension and contraction bring about the locomotion of worm. Setae always protrude during longitudinal contraction and retract during circular contraction. During locomotion, the coelomic fluid serves as a kind of hydraulic skeleton. When compressed due to contraction of circular muscles, it provides stiffness to the body and aids in the relaxation of the longitudinal muscles. The earthworm moves at the rate of 25 cm per minute. The worms can move backward also. When the direction of wave is reversed, the worm crawls backwards.

6) Digestive System:
Alimentary Canal: The alimentary canal is a straight tube throughout the length of the body from the mouth to the anus. It consists of the following regions:
a) Mouth and buccal chamber: Mouth is a cresentic opening situated below the prostomium, on the ventral surface of the buccal segment or the peristdigestive system earthworm pheretima posthumaomium. It opens in a short narrow thin-walled buccal chamber which extends up to the middle of third segment. The walls of buccal chamber are slightly folded and is surrounded by muscle strands. The buccal chamber can be protruded through the mouth with the help of special muscles and it acts as an organ of ingestion of food.

b) Pharynx: The buccal chamber leads into a pear-shaped muscular pharynx. it is a spacious structure, extending up to the fourth segment. It is separated from the buccal cavity by a groove. Its anterior end is marked by a nerve ring placed in the groove. Its cavity is dorsoventrally compressed because of the pressure of the large glandular pharyngeal mass present on the roof of the pharynx. The lateral walls are pushed inwards forming a narrow horizontal shelf on either side. The two shelves divide the pharyngeal cavity into a dorsal or salivary chamber and a ventral or conducting chamber. The pharyngeal mass or bulb produces mucus and proteolytic enzymes that are lodged in to the salivary chamber before use. The ventral conducting chamber of the pharynx serves as the passage for the ingested food. The pharynx acts as a pump in feeding. The mouth is pressed against bits of humus or mud, then the pharynx undergoes a series of contractions resulting into the sucking of the food into the buccal chamber and then pumping the same backwards into the oesophagus.

c) Oesophagus:
The pharynx opens in a short narrow, thin walled tube, the oesophagus or the gullet. It extends up to the seventh segment.

d) Gizzard: The oesophagus leads into a prominent, oval, hard and thick-walled highly muscular organ, the gizzard. The muscular wall consists of circular muscle fibres internally lined by a tough cuticle. It is found in eight segment and grinds the food into a fine state.

e) Stomach: The stomach region is a short narrow, highly vascular region of the gut. It extends from 9th to 14th segments. Each end of stomach is sphinctered. It is rich in glandular cells and its inner epithelium is thrown into transverse folds. The gland cells secrete proteolytic enzyme. The stomach is highly folded and contains additional calciferous glands. The glands secrete calcium and CO2. The calcium probably neutralizes the contents of the alimentary canal. The calciferous glands are also excretory in function, as they remove excess ions of calcium and carbonate from the blood. These ions are excreted as calcite into the stomach from where it passes out with mud through the anus.

f) Intestine:
The stomach is followed by a long, wide and thin walled tube, the intestine. It extends from the 15th segment to the last. It has a beaded appearance due to circular constrictions corresponding to septa. Its internal lining is ciliated and glandular. The lining of the intestine is folded to form villi, one of these villi becomes larger than the others and is known as typhlosole. The typhlosole runs mid-dorsally from the 26th segment to last 23 or 25 segments. Presence of typhlosole distinguishes the intestine into three regions:
1) Pre-typhlosolar Region: The first part of the intestine lying between 15th to 26th segments is called the pre-typhlosolar region. The lining of the intestine is folded internally to form villi and is highly vascular. The 26th segment contains two short and conical outgrowths, the intestine caeca on either side of the intestine. The intestinal caeca run forward up to 22nd or 23rd segment. The caeca are highly vascular and internally thrown into villi-like processes. These are believed to be digestive glands and they secrete an amylatic enzyme which digests starch.
2) Typhlosolar Region: The second part of the intestine is called typhlosolar region. It extends from 26th segment to last 23 or 25 segments infront of the anus. This region is characterised by the presence of a median dorsal internal fold, the typhlosole along its whole length. The typhlosole is highly glandular and vascular longitudinal ridge. It increases the absorptive area of the intestine.
3) Post-typhlosolar Region: This region of the intestine is also called the rectum. It extends in last 23 or 25 segments. It is internally marked by the presence of longitudinal folds. There is no typhlosole in this part. It opens outside by a terminal opening, the anus situated at the last or anal segment.

g)Anus: The last segment of the earthworm contains a small circular opening in the centre, which is known as anus.

7) Histology of Alimentary Canal:
The alimentary canal consists of following four layers:
a) Peritoneum: It is an outermost layer consisting of tall and narrow cells. Some of these cells in the intestine and stomach are modified into chloragogen cells containing yellow refractive granules. Chloragogen cells manufacture glycogen and store it as reserve food. They are excretory and take up waste material from blood and store it as yellow granules. When these cells become filled, they drop off into the coelom and are got rid off through nephridia or dorsal pores. Deamination of proteins, formation of ammonia and synthesis of urea also takes place in chloragogen cells.
b) Muscles: Muscles form the second layer of alimentary canal. It consists of an outer layer of longitudinal muscle fibres and an inner layer of circular muscle fibres. Both these layers are well developed in pharynx and oesophagus. In the gizzard, the main bulk of musculature is formed by circular muscles, while longitudinal muscles are absent. They are poorly developed in intestine. All the muscles are non-stripped or involuntary.
c) Internal Epithelium: It consists of a single row of tall columnar cells, which is ciliated on the roof of pharynx, glandular in the stomach and absorptive and glandular in the intestine. It is internally thrown into folds in oesophagus, stomach and intestine.
d) Cuticle: It forms a thin lining in the buccal cavity and thick lining in the gizzard.

8) Food and feeding mechanism:
The earthworm feeds on dead organic matter present in the soil. The proportion of organic matter in ordinary soil is very small, therefore, the earthworms ingest a large quantity of soil. They also feed directly on leaves, grass and other vegetation. The food is ingested by the pumping activities of pharynx. The pumping or sucking activities is brought by the action of muscle fibres. It presses its mouth against soil and the contractile sucking action of pharyngeal wall draws fragments of soil into buccal chamber.

Physiology of Digestion:
The swallowed food passes into the pharynx through the buccal chamber. In pharynx digestive enzyme containing mucin and proteolytic enzyme is poured on the food. The mucin lubricates the food and proteolytic enzyme digests the proteins. The digestive enzymes are secreted by gland cells of the pharyngeal bulb. In gizzard the food particles are pulverized into a fine state. In the stomach the secretion of calciferous glands neutralizes the humic present in the soil. Final digestion of food occurs in the intestine. The intestinal digestive juice contains pepsin, trypsin, amylase, lipase and cellulase. The pepsin hydrolyses proteins into peptones and proteases and the trypsin hydrolyses peptones into amino acids. The amylase converts starch into sugar and lipase hydrolyses fat into glycerol and fatty acids. While cellulase digests the cellulose into cellulobiose. The digested food is absorbed by the intestinal villi and typhlosole. The absorbed food is passed into the blood stream for distribution. The coelomic fluid and blood transport food to the tissues. Undigested food and soil are eliminated in the form of worm castings through the anus.

9) Nervous System:
A well developed nervous system is present in earthworm. It consists of central nervous system, peripheral nervous system and sympathetic nervous system.

Central Nervous System: The central nervous system comprises an anterior nerve ring (brain ring) and a posterior nerve cord.
a) Nerve Ring: It comprises paired cerebral ganglia or supra pharyngeal ganglia, circumpharyngeal connectives and subpharyangeal ganglia. The nerve ring is found in third and fourth segment around the pharynx. A pair of white pear-shaped cerebral ganglia is fused together to form a brain dorsally in the third segment of the body. It lies in the groove separating the buccal chamber from the pharynx. From the brain a pair of thick stout circum-pharyngeal or pripharyngeal connectives arise one on each lateral side. They encircle the pharynx and ventrally meet with a pair of fused subpharyngeal ganglia lying in the fourth segment beneath the pharynx. Thus, a complete nerve ring or nerve collar is formed around the pharynx.
b) Ventral nerve cord: The ventral nerve cord arises from the subpharyngeal ganglia and runs posteriorly along the mid-ventral line up to the posterior end. It consists of two longitudinal cords fused together. In each segment from the fifth to the last, the nerve cord shows a swelling the segmental ganglion. Each segmental ganglion id formed by the fusion of a pair of ganglia, one belonging to each cord of the double ventral nerve cord. The nerve cord consists of nerve fibres and nerve cells. The fibres form the core of the cord. The nerve cells or neurons are present on the sides and below the cores of nerve fibres all along the length. Nerve cells occur more in the ganglia. Nerve cells and nerve fibres lie embedded in a mass of connective tissue called neuroglia. Four giant fibres (one median, one sub-median and two laterals) are found dorsal to the nerve cord along the entire length. They are tubular, filled with homogemous plasma like matter and surrounded by epineurium. These giant fibres are responsible for rapid conduction of impulses throughout the nerve cord. They conduct impulses to the longitudinal muscles of the body wall which contract almost simultaneously. The double nerve cords are enclosed in a common sheath called perineurium consisting of three layers: the outer visceral peritoneum, middle longitudinal muscle layer and inner thick fibrous layer of epineurium.

Peripheral Nervous System
The nerves originating from the central nervous system to supply the different parts of the body constitute the peripheral nervous system. Eight to ten pairs of nerves arise from the central ganglia and go to the prostomium, buccal cavity and pharynx. The circumpharyngeal connective also gives two pairs of nerves to the peristomium and buccal cavity. From the sub-pharyngeal ganglia three pairs of nerves go to the 2nd, 3rd and 4th segments. The segmental ganglion of the nerve cord give out three pairs of minute nerves in each segment, which supply the various parts of the segment. The nerves system has both sensory or afferent and motor or efferent neurons. Stimuli from the skin are transmitted by sensory fibres to the nerve cord. Ganglia change the sensory impulse into motor impulse which passes on to the muscle cells.

Sympathetic Nervous System:
An extensive nerve plexus lying beneath the epidermis, within the muscles of the body wall and on the alimentary canal constitute the sympathetic nervous system. The nerve plexuses are connected with the circumpharyngeal connective.

10) Sense Organs:
The sense organs are quite simple in structure, consisting of either single or of small groups of specialized ectodermal cells. They receive a number of stimuli from the outside world. Pheretima has three types of sense organs: the epidermal receptors, buccal receptors and photoreceptors.
a) The Epidermal Receptors: These are found all over the epidermis but are more numerous on the lateral and ventral surface of the body. Each epidermal receptor consists of a group of tall, slender and columnar cells which cause little elevation of the cuticle. Each cell has a nucleus in the middle and its lower end is supplied with nerve fibres. The outer end of each cell contains sensory hair like process which penetrate the cuticle and project beyond it. The inner end of receptor cell remains connected with the nerve fibres. They are tactile in function i.e. relating to touch and also receive thermal and chemical stimuli. Hence earthworms are very sensitive to touch and to vibrations transmitted through solid objects, though they have no hearing organs.
b) Buccal Receptors: They are found in large numbers in the epithelium of buccal cavity. The buccal receptor consists of group of tall cells which project beyond the epithelial cells. They possess sensory hair-like processes. These are olfactoreceptors (relating to smell) and gustatoreceptors (concerning to the taste). Thus the earthworms can recognise different types of leaves that can be used as food.
c) Photoreceptors: A photoreceptor is a single ovoid cell with a nucleus and clear cytoplasm, The cytoplasm contains a network of neurofibrillae and a small,transparent L-shaped lens or optic organelle. The lens focuses light rays from all directions on the neurofibrils. They are found in the inner parts of the epidermis. The photoreceptors are restricted to the dorsal surface and are more numerous on the prostomium and first segment. They gradually reduce in number towards the posterior end. They are not found in clitellum and on the ventral surface. The photoreceptors enable the worm to detect changes in the intensity of light. The worms avoid strong daylight.