Annelida Study Notes PDF

PHYLUM: ANNELIDA FIRST EXAMPLE OF ANNELIDA PHERETIMA CLASSIFICATION Phylum-Annelida Class-Oligochaeta Genus-Pheretima Species- posthuma Habit and Habitat Pheretima is the common Indian earthworm found in moist places. They are abundantly found during rainy season. The presence...

PHYLUM: ANNELIDA FIRST EXAMPLE OF ANNELIDA PHERETIMA CLASSIFICATION Phylum-Annelida Class-Oligochaeta Genus-Pheretima Species- posthuma Habit and Habitat Pheretima is the common Indian earthworm found in moist places. They are abundantly found during rainy season. The presence of earthworms is guessed by seeing their characteristic coiled castings. They live in burrow during day and come out at night in search of food. External Features It has a long, cylindrical body (length 15-20 cms; breadth 0.3-0.5 cm). The dorsal surface is more brownish in colour than the ventral surface. This colouration is due to the presence of a special pigment called porphyrin which is formed from the chlorophyll of the leaves etc. taken by earthworm as food.  The body consists of 100-120 segments. At the anterior end is the mouth lying below a fleshy a structure called prostomium. Surrounding the mouth is the first segment peristomium. The dorsal surface is marked by the presence of dorsal blood vessel and the ventral surface by ventral blood vesel.  The groove between any two segments is the intersegmental groove. In these grooves after 12 segments is found a series of dorsal pores. On the ventro-lateral side in intersegmental grooves 5/6, 6/7, 7/8 and 8/9 are found spermathecal pores. In the middle of each segment is found a series of locomotory structures called setae formed from a setigerous cell. When fully formed lies in a setal sac and is curved in the form of 'S' with a thicker middle part called nodule It is attached with protractor and retractor muscles which help in the movement of seta. Setae are absent in first and last segment as well as in the clitellum.  Except the first two segments a large number of minute apertures, the nephridiopores are present on the body wall through which integumentary nephridia pour their excretory products to the outside. The segments 14-16 are deep brown and thick walled. This region is the clitellum. On the ventral side of the 14th segment is found the female genital opening. The male genital openings are present on the ventral side of 18th segment and a pair of genital papillae are found on the ventral surface of each 17th and 19th segment. The anus is at the extreme end of the last segment. DIGESTIVE SYSTEM OF EARTHWORM A. Alimentary Canal It is a straight tube running from the mouth at the anterior end to the anus at the posterior end. It consists of ; (i) mouth (ii) buccal cavity (iii) pharynx (iv) oesophagus (v) gizzard (vi) stomach (vii) intestine (viii) anus. It is concerned with the digestion and absorption of food in earthworm.  The mouth is situated as a crescentic aperture on the ventral side of the prostomium in the first segment and opens into a small muscular and eversible buccal cavity which extends upto the third segment. Next to buccal cavity is the pharynx extending from segment three to segment fourth. The wall of the pharynx is thick and muscular. The roof is provided with pharyngeal glands, the secretion of which lubricates the food and helps in the protein digestion. Internally the roof is lined by a single layer of ciliated epithelium. The lateral walls form two horizontal shelves dividing the lumen of the pharynx into a dorsal (salivary) and a ventral (conducting) chamber. Behind the pharynx comes the oesophagus which extends upto the eighth segment. It is a straight, narrow, thin walled tube. The oesophagus leads into the gizzard which occupies the 8th and 9th segment. It is thick walled and highly muscular: Internally it is lined by tough cuticle below which is single layer of epithelial cells. The gizzard opens into a straight, short narrow tube, the stomach extending from 9th to 14th segments.  After stomach the alimentary canal extends as intestine. It can be divided into three regions: (i) pre-typhlosolar region (ii) typhlosolar region (iii) post-typhlosolar region. The pre-typhlosolar region extends from 15 to 26 segment. The wall of this region is internally folded and highly vascular. At the 26 segment two conical outgrowths, the intestinal caeca are given out which extends upto 22nd or 23rd segment. They are vascular and internally folded forming villi-like processes.  Their secretion takes part in digestion. The typhlosolar region extends from 26th segment to 23 or 25 segments in front of anus. The internal lining forms the villi, one of these villi in the mid-dorsal line, in this region of the intestine, becomes larger and forms the typhlosole. This increases the internal absorptive surface of the intestine. The third part of the intestine is the rectum or post-typhlosolar region extends in the 23 to 25 posterior most segments. It is devoid of typhlosole. The anus is situated in the last segment of the body (anal segment). Digestion The earthworm eats leaves seeds and the soil containing organic sub stances. It everts the buccal cavity and the food is drawn into the mouth by the sucking action of the muscular pharynx from where digestion starts. (a) The pharyngeal glands of the roof of pharynx secrete a juice containing mucin and proteolytic enzyme. The mucin lubricates the food and the enzyme acts on protein. (b) The food then reaches the gizzard where it is further grinded by the muscular action of its wall.  (c) In the intestine the food comes in contact with the digestive juice. (i) Proteolytic enzyme converts protein into peptones. ii) Diastase converts starch into sugar. iii) Lipase splits the fat. (iv) Amylase and invertin act on sugars. Absorptions:The end products of digestion are absorbed by the epithelial lining of the intestine and passed on to the blood for distribution to the various parts of the body. Egestion: Undigested food reaches the post-typhlosolar region and expelled out through anus at intervals. a 13/5/2021 THURSDAY Excretory System 1) Septal nephridia  It is connected with the body by a narrow neck. The body has two parts (i) a short straight lobe (ii) a long spirally twisted loop with a proximal and a distal limb which are spirally twisted upon themselves. The proximal limb is connected with the neek and is also continued as terminal duct. The lumen of the nephridium has ciliated canals. There are four such canals in the straight lobe, one in each of the neck and the terminal duct, three in the lower and two in the upper parts of the twisted loop.  In each segment behind the septum the terminal ducts of these nephridia open into a septal excretory canal which runs parallel to the commissural vessel. a Each septal excretory canal opens into the supra-excretory canal of its side which in turn opens by ductules in each segment into the alimentary canal. 2) Pharyngeal nephridia They are found in 3 paired groups, one pair on either side of the alimentary canal in each of the segments 4, 5 and 6. The pharyngeal nephridia are devoid of nephrostome. The terminal limbs of each nephridia of each group join to form a common duct. Thus in each segment a pair of such thick walled ducts are formed. The ducts of the 6th segment open into buccal cavity while the ducts of 5th and 4th segments open into the pharynx. 3) Integumentary nephridia They are also like the septal nephridia but like pharyngeal nephridia are devoid of nephrostome. Each is a small V-shaped nephridium with a small straight and twisted loop. They are attached in the body wall from 7th segment to the last segment. Each of them opens directly by a small nephridiopore to the out side. There are about 200-300 nephridia in each segment but in the clitellar region the number reaches 2,000-2500 and are often called the forests of integumentary nephridia. a Septal and pharyngeal nephridia are called enteronephric as they pour their excretory contents in the alimentary canal. The integumentary nephridia are exonephric type as they pour out their contents directly to the outside.  Glandular cells of nephridia extract water and nitrogenous wastes from the blood. Septal nephridia also collect urea, ammonia, amino-acids etc. from the coelomic cavity. Septal and pharyngeal nephridia pour their contents in the alimentary canal where water is reabsorbed and other products are excreted out with the faecal matter to the outside. 20/5/2021 THURSDAY Nervous System It consists of ; I) Central Nervous System II) Peripheral Nervous System III) Sympathetic Nervous System Nervous System Central Nervous System This system includes; I) A nerve ring II) A double ventral nerve cord with arranged gangila. The nerve ring or the brain is formed by a pair of dorsally placed cerebral or suprapharyngeal ganglia lying in the third segment and a pair of circum or peripharyngeal connectives, one on each side of the pharynx, connecting the paired sub pharyngeal ganglia lying beneath the pharynx in the 4th. segment. The double ventral nerve cord arises from the sub-pharyngeal ganglia and runs backwards in the mid-ventral line upto the posterior end of the body. In each segment it is swollen and forms a ganglion. Peripheral Nervous System The nerves coming out of the central nervous system constitute the peripheral nervous system: (i) 8 to 10 pairs of nerves from cerebral ganglia (ii) Nerves from the peripharyngeal connective go to the prostomium and buccal cavity. (iii) Nerves of the sub-pharyngeal ganglia innervates the structures of the II, III and IV segments. (iv) Each ganglion of the ventral nerve cord gives rise to three pairs of nerves which supply to the body wall, gut wall and other internal structures of the segment concerned. Histology of the nerve cord Before sympathetic nervous system is described we should know the histology of the nerve cord. It is covered externally by peritoneum, longitudinal muscle fibres and fibrous epineurium which divides the nerve cord into two halves. On the ventro-lateral region are found bipolar and tripolar nerve cells. In addition in the upper part of the nerve cord are present four giant fibres-median, sub-median and two lateral giant fibres which are surrounded by epineurium. They are very sensitive and conduct impulses very quickly. The quick and sudden movement of the entire body by stimuli at one point is caused by these giant fibres. Sympathetic Nervous System Extensive nerve plexus present below the epidermis and on the alimentary canal is connected with peripheral nerves forming the sympathetic nervous system. Nerves are made of both afferent and efferent fibres. From a receptor organ of epidermis start the afferent fibres which run to the ventral nerve cord. It is connected with muscles by efferent fibres forming a simple reflex arc which functions as shown in the next slide. 27/05/2021 THURSDAY Reproductive Organ Reproductive System Earthworm is hermaphrodite. Male reproductive organs.: I) Testes- There are two pair of testes of which one pair in each of the 10th and 11th segments. Each testis consists of 4-8 finger like processes and lies inside the testis sac. II) Seminal vesicle- One pair in each of the 11th and 12th segments. Testis sacs of the 10th segment communicate with the seminal vesicles of the 11th segment and that of the 11th segment with the seminal vesicles of 12th segment. III) (iii) Spermatic funnel- Behind each testis lies a spermatic funnel for receiving the sperms.  IV) Vas deferens- Spermatic funnel leads into a slender vas deferens which extends on each side upto 18th segment. Here both the ducts of each side are incorporated into a thick muscular duct, the common prostatic and spermatic duct. V) Prostate gland- These are a pair large irregular 16-17 to 20-21 segments. glands lying on either side of the gut from VI) Male genital pores- There are two male genital pores on the ventral side of the 18th segment. In segments 17th and 19th are found a pair of rounded accessory glands which open by ducts on two pairs of genital papillae situated on the ventral side of 17th and 19th segment. The sperm mother cells of the testis fall in testis sac and from there reach the seminal vesicles where they mature into spermatozoa (sperms). These travel back into testis sac and enter the spermatic funnel. They travel all along the vas deferens and come out through the male genital aperture to the outside. Female Reproductive Organs (i) Ovaries- There are two small ovaries lying attached on the posterior side of the septum 12/13 segments. Each ovary consists of several finger like processes with ova in their different stages of development. (ii) Oviducts These are two short tubes, each with a large oviducal funnel and both unite at the distal end to open in the female genital pore on the ventral side of the 14th segment. The ova reaches the oviducal funnel and passing through oviduct come out through female genital pore. (iii) Spermatheca-There is one pair of bag-like spermatheca at the ventro-lateral side of 6th, 7th, 8th and 9th segments. Each has a pear-shaped body, the ampulla, a short neck and a round elongated diverticulum. The function of the spermathecae is to receive sperms of the other earthworm during copulation and to store it in the ampullae.  (iv) Spermathecal pore- a There are one pair of these in each intersegmental groove on the ventro-lateral side of segments 5/6, 6/7, 7/8 and 8/9. Copulation Cocoon formation and fertilization. Clitellar glands secrete some secretion which hardens and forms a girdle. Now the animal withdraws itself from it. While doing this first of all ova and then sperms are deposited in the girdle. When the animal completely withdraws itself from the girdle both ends of the girdle are closed and it forms a roundish yellowish structure called cocoon. In this the sperms fertilize the eggs. This type of fertilization outside the body is called external fertilization. The cocoon is deposited at some cool and moist place and from it after 2 to 21/2 months young earthworm comes out. Economic Importance (i) As plougher-Earthworms deposit undigested food with the soil in the form of definite castings on the earth. The burrows permit the penetation of the air and water. This helps the downward growth of the root and the castings above make the soil suitable for seed germination. (ii) As manurer-The castings contain digested leaves and other things rich in nitrogen. The soil is thus enriched with nitrogenous wastes which is a good plant food. (iii) Used as medicine. During ancient times earthworms were used as medicines to cure bladder stone, piles, gout, diarrhoea and sexual impotency.  (iv) Used as food Maories and other uncivilized people used them as food. Centipedes fishes, frogs, lizards and birds feed on earthworms. (v) Use as baits As bait pieces of earthworms are used in fishing. (vi) As laboratory material As these are easily available and easy to handle, they are used in laboratories for study purposes. (vii) As hosts In the seminal vesicles of earthworm are found protozoan parasite, Monocystis. Some earthworms act as intermediate hosts of chicken-tapeworm and lung nematode of pigs. (viii) As parasites-Some of them are external parasites of crabs and cray-fishes. THANKYOU!