Medical Entrance Exams : NEET Aspirants
Medical Entrance exams in India class 11th and 12th passed out students preparing all important material for Biology combined with NCERT package . By Affairs Cloud Hindi
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Medical Entrance Exams in India
EARTHWORM [ INTRODUCTION ]
-- Scientific name : PHERETIMA POSTHUMA
1 . Earthworm is nocturnal burrowing (fossorial)_and brown coloured (due to porphyrin) with 100-120 segments .
2 . Prof KN Bahl (Lucknow University) wrote the ' Memoir of Pheretima ' .
3 . The dorsal surface is darker due to presence of mid dorsal line from anterior to posterior, which is actually dorsal blood vessel.
Divisions of body
4 . On the basis of clitellum, body has 3 regions:
(a) Pre-clitellar region : 1 to 13 segment.
(b) Clitellar region : 14, 15,16 seg.
(c) Post clitellar region : 17 to last segment.
5 . First segment is peristomium, which is mouth bearing seg. A fleshy lobe that overhangs the mouth is called prostomium.
Important Apertures
6 . Imp. apertures on its body are:
(a) Mouth : present on ventral side of peristomium.
(b) Female genital pore : single, midventral; 14h seg.
(c) Male genital pore : one pair, ventral; 18" seg.
(d) Male genital papillae : 2 pairs, ventral, one pair on 17" & 1 pair on 19th seg.
(e) Spermatheal pores : 4 pairs, on inter segmental grooves of seg 5/6, 6/7, 7/8 & 8/9.
(f) Dorsal pores : One pore on each groove from seg. 12/183 to last but one seg. The coelomic fluid oozes out from these pores to keep the body wall moist for respiration.
(g) Nephridiopores : numerous; are the openings of integumentary nephridia, present in all segments except first six and last segments.
(h) Anus : single slit-like aperture present on last or anal segment.
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Body wall & Body cavity
7 . Its body wall consists of :
(a) Thin non-cellular cuticle of collagen
(b) Single layered epidermis, With all columnar cells having many specialized cells.
(c) Circular & longitudinal muscle layers.
(d) Coelomic epithelium
8. S-shaped, 18-120 chitinous setae. present on each segment except 1st , last and clitellar segments. Its arrangement is perichaetine type (in ring)
The setae and contraction of muscles help in locomotion .
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9. Body cavity is true coelom (schizocoel) filled with milky white alkaline coelomic fluid. which acts as hydraulic skeleton.
10. The cavity has numerous chloragogen or yellow cells, concerned with storage of reserve food , deamination of protein and formation of urea analogous to vertebrate liver) .
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Digestion
1. Earthworm has straight alimentary canal with tube within tube body plan and includes following parts:
• Mouth : 1 seg
• Buccal chamber : 1-3 seg.
• Pharynx : 3-5 seg
• Oesophagus : 5-8 seg.
• Gizzard : 8-9 seg.
• Stomach : 9-14 seg.
• Intestine : 15- last seg.
2. Roof of pharynx contains unicellular salivary glands/chromophil cells, which produce saliva (mucus + proteolytic enzymes)
3. Gizzard is a grinding organ with thick layer of circular muscles and lined by cuticle.
4. Stomach wall has calciferous glands, which secrete chalky substance to neutralize the acidity of soil.
5. The intestine is divided into following 3 parts:
(a) Pre-typhlosolar part : 15-26 seg : a pair of intestinal caeca arise from 26" seg. and extend anteriorly up to 22 or 23 segment.
(b) Thyphlosolar part : 27 to last but 23 or 25 seg, is a large sized villuş (dorsal internal fold) hanging from intestinal wall and increases the absorptive
Surface area.
(c) Post typhlosolar part : last 23-25 seg; opens outside through anus.
6. Earthworm is nocturnal, omnivorous and feed upon organic humus, debris, insects, micro-organisms etc.
7. Digestive enzymes are secreted by chromophil cells of pharynx, glandular cells of stomach, intestine and caeca. Different enzymes with functions are as
following;
• Proteases : protein digestion
• Lipase : fat digestion
• Amylase : digestion of starch & glycogen
• Cellulase : digestion of cellulose of leaves
• Chitinase: digestion of chitin of insects..
The chief site of digestion is intestine where
digestion is completed .
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Respiration
8. Earthworm respires but it has no specialized respiratory organs.
9. It is aerobic and gaseous exchange occurs through moist skin (cutaneous respiration).
20. It Oxygen is not available; earthworm can survive up to 30 hours .
Excretion
1. Excretion is mixed amono-ureotelic.
2. Excretory and osmoregulatory organs are nephridia (ectodermal), which occur in all segments except 1,2,3 & last segments.
3. The nephridia may be of following types:
• Septal nephridia/ holonephridia / largest
• Pharynageal nephridia / peptonepridia / moderate
• Integumentary nephridia / smallest
4. The nephridia form the urine through following three processes :
(a) Ultrafiltration
(b) Selective reabsorption
(c) Chemical transformation
-- These extract water, excretory wastes and
useful substances from blood and coelomic fluid through ultrafiltration. The water and useful substances are reabsorbed through selective reabsorption and urine is excreted.
-- During Summer and winter seasons,water is conserved by increased reabsorption hence urine is hypertonic but in rainy season,urine is dilute due, to decreased water reabsortion.
(a) Septal nehridium :
• It remains attached to intersegmental septa behind 15th segment onwards (absent from 1 to 14 segments).
• 80-100 nephridia are present per segment,
arranged in 4 lines to 20-25 each.
• Each septal nephridium is composed of
(a) Nephrostome
(b) Neck
(c) Body proper
• Terminal duct >septal ex. canal - Supra intestinal ex. duct >intestine
• This is the open or holonephridium as it
contains all parts of a typical nephridium including neck & nephrostome.
(b) Pharyngeal nephridium :
• It occurs in bunches in 4th, 5h & 6h segments on both sides of pharynx and oesophagus.
• Each bunch contains 100 nephridia.
• Nephrostome is absent hence called closed nephridium .
• Septal and pharyngeal nephridia are called enteronephridia because they open into alimentary canal and help is conservation of watar.
(c) Integumentary nephridium :
• It is v-shaped,occurs in max. number; absent in first six segs.
• Each segment contains 200-250 nephridia.
• 2000-2500 nephridia in clitellum (forest of nephridia).
• Nephrostome absent hence called closed
nephridium.
-- This is called exonephridium as it opens
outside through Nephridiopores , present in all segments except first six and last segments .
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Nervous system
-- Earthworm has well developed nervous system with brain but there is no head.
-- Nerve ring or brain ring is found around pharynx in 3 & 4 segments. Its mid dorsal part is formed by a pair of cerebral/ supra pharyngeal ganglia (brain) while mid ventral part is formed by a pair of sub
phanyngeal ganglia. One pair circum phanyngeal connectives join these 2 types of ganglia.
-- Two solid ventral nerve cords arise from sub pharyngeal ganglia and extend up to posterior part.It appears externally as single because both cords are surrounded by a common perineurium. The segmental ganglia are found from 5th segments onwards.
-- Earthworm has following three types of receptors:
(a) Epidermal receptors : Tactile
(b) Buccal receptors : Chemical
(c) Photoreceptors : intensity and duration of light
-- Each photoreceptor consists of a sing optic cell having L-shaped hyaline lens. These numerous photoreceptors are concerned with the detection of intensity and duration of light but there is no image
formation hence these are not the true eyes.
Circulatory system
-- Blood vascular system closed type with neurognic heart. Blood is red due to Hb found dissolved in plasma.
-- Blood vascular system is different in first 13 segments .
-- Dorsal blood vessel is largest considered as heart.
-- It is of distributive nature , in first 13 segments while of Collecting nature in the segments 14 onwards. The flow of blood is from posterior to anterior.
-- Ventral vessel is of distributive nature, found from 2nd to last segments, below the alimentary canal . The flow of blood is from anterior to posterior.
-- Paired lateral oesophageal vessels are located in segments 1 to 13 on both sides of alimentary canal.
Atter 13th segments, these unite to torm Singie subneural vessel midventral, collecting .
-- One pair lateral hearts are found in 7th seg and one pair in 9th seg. They send blood from dorsal vessel to ventral vessel .
-- One pair anterior loops are found in 10th and one pair in 11th seg.They send blood from lateral oesophageal vessel to supra oeso phageal vessel (collecting) .
-- One pair lateral oesophageal hearts occur in 12th and one pair in 13th seg. They send blood from dorsal and supra oesophageal vessel (smallest) to ventral
vessel.
-- The blood corpuscles and Hb are probably synthesized by blood glands, located in 4th , 5th and 6th segments.
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Reproduction
-- Earthworm is hermaphroditic but due to protandy, reciprocal cross fertilization is the rule, Occurs externally inside_the cocoon (secreted by glandular Clitelum).
-- These cocoons are left on the moist soil . Cleavage holoblastic, development direct without any larval stage and no metamorphosis
-- The male reproductive system includes
• Tests : 2-pairs;. 10,11 seg.
• Testis sac : 2-bilobled; 10,11 seg
• Spermiducal funnel : 2-pairs; 10,11 seg.
• Seminal vesicles : 2-pairs; 11,12 seg.
• Vas deferentia : 2-pairs; 10,18 seg.
• Prostate gland : 1-pair;16 or 17-20 or 21 seg.
• Accessory glands : 2-pairs, 17,19 seg.
• Male genital pore : 1-pair 18 seg.
-- The male genital pore is formed by 3
openings (1 prostate + 2 vas deferentia). Two pairs of genital papillae are found associated with accessory glands. These papillae are the most prominent structures one ventral side of body. The papillae and
secretion of accessory glands help in copulation.
-- The female reproductive system includes
1. Ovaries : 1-pair; 13thseg
2. Oviducal funnel: 1-pair, 13thseg
3. Oviducts : 1 - pair , 13- 14 seg.
4. Female genital pore : single 14 seg.
5. Spermatheca : 4-pairs; 6,7,8 & 9 seg.
6. Spermathecal pores : 4-pairs ; 5/6, 6/7, 7/8 & 8/9 seg.
-- The spermathecae are used to store the sperms after copulation and their ampullae secrete a nutritive Substance for sperms.
-- Earthworm is a natural ploughman. it is called friend of the farmers because it increases soil fertility by making
the soil porous.
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