Classification of Annelida

Classification of Annelida:

Class 1. Polychaeta

(Gr., poly = many + chaete = bristles):

1. Polychaeta are marine and carnivorous.

2. Body is elongated and segmented.

3. Head consists of prostomium and peristomium and bears eyes, tentacles, cirri and palps, etc.

4. Setae are numerous and are borne up on lateral prominences of the body wall known as Para podia.

5. Clitellum is absent.

6. Cirri or branchiae or both may be present for respiration.

7. Coelom is spacious usually divided by inter-segmental septa.

8. Alimentary canal is provided with an eversible buccal region and protrusible pharynx.

9. Excretory organs are segmentally paired nephridia.

10. Sexes separate, fertilisation external, free swimming larval stage trochophore.

11. Asexual reproduction by lateral budding

Order 1. Errantia:

1. Free-swimming, often pelagic, while some living in tubes.

2. All body segments are similar except at anterior and posterior ends.

3. Parapodia, provided with cirri, are equally developed throughout.

4. Head with distinct prostomium which is provided with eyes, tentacles and palps.

5. Pharynx is usually protrusible and armed with chitinous jaws and teeth.

Examples:

Nereis, Aphrodite, Glycera, Polynoe, Syllis.

Order 2. Sedentaria:

1. Burrowing and tube dwelling forms.

2. Body is divisible into two or more regions with unlike segments and Para podia.

3. Head is small or much modified, without eyes and tentacles, prostomium indistinct.

4. Pharynx is non-protrusible devoid of jaws or teeth.

5. Gills, when present, localised to the anterior segments.

6. Feeding on plankton or organic detritus.

Examples:

Chaetopterus, Terebella, Sabella, Arenicola, Serpula, Amphitrite, Spirorbis.

Class 2. Oligochaeta:

(Gr., oligos =few + chaete = setae):

1. Mostly terrestrial or some freshwater forms.

2. Body with conspicuous external and internal segmentation.

3. Distinct head, eyes and tentacles are absent.

4. Parapodia are absent.

5. Setae are usually arranged segmentally.

6. Clitellum is usually present.

7. Pharynx is not eversible and without jaws.

8. Hermaphrodite, i.e., sexes united.

9. Development is direct and takes place within cocoons secreted by clitellum; no free larval stage.

Order 1. Archioligochaeta:

1. Mostly freshwater forms.

2. Body consists of few segments.

3. Setae are present in bundles.

4. Gizzard is poorly developed, non- muscular or absent.

5. Clitellum is simpler, consists of single layer of cells and situated far forwards.

6. Eye spots are frequently present.

7. Male reproductive openings lie in front of the female reproductive openings.

8. Reproduction asexual and sexual.

Examples:

Tubifex, Aelosoma.

Order 2. Neooligochaeta:

1. Usually terrestrial forms.

2. Body is large and many segmented.

3. Setae are arranged in lumbricine manner.

4. Gizzard is well developed.

5. Clitellum is composed of two or more layers of cells and never begins before twelfth segment.

6. Female genital apertures are always on the fourteenth segment and the male pore lies a few segments behind them.

7. Vasa deferentia are elongated extending over the three or four segments.

8. Eye spots are never developed.

9. Reproduction sexual. Asexual reprodu­ction is not known

Examples:

Pheretima, Eutypheus, Megascolex, Lumbricus.

Class 3. Hirudinea:

(L., hirudo = a leech):

1. Mostly ectoparasitic and freshwater forms, while few are marine feeding upon fishes and other animals.

2. Body is elongated usually flattened dorsoventrally or cylindrical.

3. Body consists of definite number of segments, each segment breaks up into 2 to 4 rings or annuli.

4. Parapodia and setae are absent.

5. Body is provided with an anterior and a posterior sucker, both situated ventrally.

6. Mouth opens on the ventral surface in the anterior sucker, while anus opens dorsal to the posterior sucker.

7. Hermaphrodite, i.e., sexes united; repro­duction sexual.

8. Asexual reproduction is not known.

9. Eggs are usually laid in cocoons.

10. Development is direct without free swimming larval stage.

Order 1. Acanthobdeilida:

1. Mostly parasitic on the fins of salmon fishes.

2. Body comprises thirty segments only.

3. Anterior sucker is absent but posterior sucker is well developed and composed of four segments.

4. Anterior five segments are provided with double rows of setae.

5. Proboscis is short.

6. Body cavity is spacious and incompletely divided by septa.

7. Vascular system consists of dorsal and ventral vessel.

8. Nephridial opening situated on the surface between the segments.

9. Acanthobdeilida forms a connecting link between Oligochaeta and Hirudinea.

Example:

Acanthobdeilida.

Order 2. Rhvnchohdellida:

1. Parasites on snails, frogs and fishes, marine and freshwater forms.

2. Each typical body segment consists of 3, 6 or 12 rings.

3. Mouth is small median aperture situated in the anterior sucker.

4. Proboscis is prostrusible; jaws are absent.

5. Blood is colourless.

6. Goelom is reduced to sinuses without botryoidal tissues.

Examples:

Pontobdella, Glossiphonia, Branchellion, Piscicola.

Order 3. Gnathobdetlida:

1. Freshwater and terrestrial forms.

2. Each typical body segment consists of five rings or annuli.

3. Anterior sucker with three jaws, one median dorsal and two ventro-lateral.

4. Proboscis is absent.

5. Blood is red coloured.

6. Botryoidal tissue present.

Examples:

Hirudo, Haemopis,

Hirudinaria, Herpobdella.

Class 4. Archiannelida

(Gr., archi = first):

1. Exclusively marine forms.

2. Body elongated and worm-like.

3. Setae and Para podia are usually absent.

4. External segmentation is slightly marked by faint, while internal segmentation is marked by coelomic septa.

5. Prostomium bears two or three tentacles.

6. Unisexual or hermaphrodite.

7. Larva is typical trochophore.

Examples:

Polygordius, Protodrillus, Nerilla, Saccocirrus

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Origin of Mammals

Archaic Mammals:

A sudden survey of the obsolete mammals will be informative to profit a sure idea approaching the ancestry of mammals. Mammals became adroitly flourished at the arrival of Cenozoic era but have started their real career in the Mesozoic era. So the appear in of some Mesozoic and Cenozoic mammals will abet the strive for.

Mesozoic Mammals:

The physical-like reptiles were quite abundant in Permian epoch. But at the at the forefront Triassic times, those forms became gradually extinct and ultimately they became approximately utterly absent by the fade away of Triassic. But again in the Jurassic and subsequent periods fossil mammals became more abundant.

Mesozoic mammals possessed some basic characteristics:

(a) little size,

(b) cunning-barbed teeth,

(c) teeth were generally heterodont and

(d) habits were varied.

Pantodonta:

The fossil forms were discovered in upper Triassicperiod. Dentition was more taking into account that of reptile than mammalian. But the easy jaw consisting of a single bone customary their mammalian affinity. The fossil archives of this group arc rather incomplete.

Allotheria:

They were every one important organization amid the extinct mammals and their fossils were recorded in the upper Triassic rocks of Germany. The most important mammalian characteristic was the flora and fauna of the molar teeth which had two to three longitudinal rows of tubercles. The presence of these tubercles gave the say Aluitituberculata to this group.

Triconodonta:

These fossils were not far-off off fromcorded in Jurassic times. The molars had three cusps established in a single longitudinal row.

Symmetrodonta:

This intervention was confined to the upper Jurassic epoch of England and North America. The molars had three main cusps chosen in a triangular fashion.

Pantotheria:

The symmetrodonta was obscurely united to this action. The molars had four or more cusps. The three principal cusps assumed the form of a trigon. Their fossils were recorded throughout the Jurassic era.

Cenozoic Mammals:

Besides the antiquated mammals, a few important Cenozoic mammals dependence mentioning in relationship when the ancestry of mammals. They membership three groups, Creodonta, Condylarthra and Amblypoda. Creodonta had little reptile-in the middle of brain, carnivore-once teeth and terminal phalanges taking into consideration claws.

Condylarthra represented the primitive ungulates and structurally formed a sort of member happeninging connection together in the midst of the hoofed and clawed mammals. The cranial part was located astern the orbit. Teeth were suited for herbivorous exaggeration. Molars were low crowned. Amblypoda was curt footed and hoofed forms of approaching elephantine size.

Fate of Archaic Mammals:

With the unventilated of reptilian age, the Mesozoic mammals became adeptly flourished in the Mesozoic times and exhibited broad adaptive radiation. But completely speedily they became extinct and were replaced by modernised mammals which were adequately equipped to slope all eventualities of nature. The cause of total destruction of old mammals is primarily due to their incapabilities to respond to added environmental conditions.

The antique mammals had a brief evolutionary career and they were subjected to racial obliteration. The challenges of computer graphics were customary by modernised mammals who flourished dexterously in become out of date and environment. Some of the old forms transmutated into sophisticated types even if some became emigrant. Marsupials passed greater than the limits of South America and crossed the Antarctic house-bridge to submit Australia where they yet persist.

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Reproduction in Annelida

In Polychaeta sexes are separate. The gonads are patches of coelomic epithelium and are repeated in most of the segments. The gonads become conspicuous during the breeding season and they proliferate a very large number of germ cells which detach and fill coelom where they undergo maturation in the coelomic fluid.

When ripe the germ cells pass to the exterior either through segmental organs or by rupture of the body wall. Fertilisation occurs in sea water. In many forms a phenomenon of swarming occurs, the crawling or burrowing worms rise to the surface to discharge their sex cells, then they sink to the bottom.

Swarming habit is an adaptation for securing fertilisation of the greatest possible number of eggs. Swarming is usually during definite periods and often coincides with lunar periods. Discharge of gametes is nearly always followed by death of the sexual individuals. Fertilised egg gives rise to a trochosphere larva.

In syllids gonads are usually confined to the posterior part of the body which is detached as a free-swimming zooid which develops a head but no jaws or pharynx, it lives for a time to produce gametes. Many annelids have the ability to regenerate lost parts, this is accompanied by a capacity to reproduce asexually.

Some forms reproduce asexually by budding, but in Autolytus there is a proliferating region at the end from which a chain of sexual zooids is budded off which detach one by one. Syllis forms many branches by budding, some of which form a head, develop sex organs, notopodia are formed to reconstruct the parapodia, these sexual forms may remain attached to the parent for long or they may separate from the colony.

In Oligochaeta certain features of reproductive organs are salient characters, they are almost all hermaphrodite.

The sex cells are discharged into the coelom or into seminal vesicles which are special parts of the coelom separated from the rest, they are large coelomic sacs varying in number in different genera; often a pair of seminal vesicles may coalesce to form a median sperm reservoir into which ciliated funnels of vasa deferentia open.

Testes may be several but ovaries are never more than two. Spermathecae are usually present to receive spermatozoa of another worm during copulation.

The clitellum is a glandular development of the epidermis for formation of cocoons and albumen for nourishment of the embryo.

The clitellum may be permanent, as in earthworms, or it may develop only during the breeding season. Some Oligochaeta possess special copulatory setae. In some Oligochaeta asexual reproduction occurs, e.g., Nais and Chaetogaster multiply by proliferation of segments at the posterior end forming a chain of zooids which eventually separate and acquire sex cells.

Hirudinea are hermaphrodite with several pairs of testes but only two ovaries, gonads are completely shut off in closed coelomic vesicles, but they are continuous with their ducts in distinction from other Annelida. The spermatozoa unite in bundles to form permatophores.

Generally copulation occurs, though in some hypodermic impregnation takes place. The clitellum appears during breeding season, and eggs are laid in cocoons formed by clitellar glands.

Archiannelida are generally hermaphrodite, the ovaries occurring in anterior segments and testes behind them, so that gonads are restricted to a few segments. In Polygordius sexes are separate, the ovaries or testes develop in a few posterior segments, there are no ducts.

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Life history of Salim Ali

Timeline

1896: Born on November 12 in Mumbai

1913: Completed matriculation from Bombay University
1914: Admitted to St. Xavier's College and went to Burma
1917: Returned to India
1918: Married distant cousin, Tehmina in December
1926: Employed as guide lecturer in Prince of Wales Museum, Bombay
1928: Left the job and went to Germany
1930: Came back to India
1939: Wife Tehmina died
1941: Wrote first book "The Book of Indian Birds"
1953: Awarded with Joy Gobinda Law Gold Medal by Asiatic Society of Bengal
1958: Received doctorate degree from Aligarh Muslim University
1958: Honored with Padma Bhushan Award
1970: Bestowed with Sunder Lal Hora Memorial Medal from INSA
1973: Received honorary doctorate from Delhi University
1976: Conferred upon with Padma Vibhushan Award
1978: Received honorary doctorate from Andhra University
1985: Penned autobiography "The Fall of a Sparrow"
1987: Died on July 27 in Mumbai from prostate cancer, aged 90
1990: Salim Ali Centre for Ornithology and Natural History established at Coimbatore

Early Life
Salim Moizuddin Abdul Ali, or Salim Ali as he is better known as, was born as the ninth and youngest child in a Sulaimani Bohra Muslim family. He was born in Mumbai to Moizuddin and Zeenat-un-nissa. Losing his father at the age of one and mother at three, Salim Ali and other kids were brought up by his maternal uncle, Amiruddin Tyabji, and childless aunt, Hamida Begum. He was also surrounded by another maternal uncle, Abbas Tyabji, a prominent Indian freedom fighter. He attended primary school at Zanana Bible Medical Mission Girls High School at Girgaum and was later admitted to St. Xavier's College at Mumbai.

However, due to his frequent chronic headaches, he was forced to drop out of school every now and then since he was 13 years old. He was sent to Sind to stay with his uncle with hopes of the dry air making an improvement in his health. Thus, on returning, he just managed to clear his matriculation examination from Bombay University in 1913. Since childhood, Salim Ali gained an interest in observing birds closely and had a hobby of shooting birds with his toy air gun. With the help of W.S. Millard, secretary of Bombay Natural History Society (BNHS), the bird was identified as Yellow-throated Sparrow, which further increased his seriousness towards ornithology.

Life in Burma and Germany
After spending a difficult first year in Xavier's College, Mumbai, Salim Ali dropped out of college and went to Tavoy, Burma to care of his family's Wolfram mining and timber business. The forests surrounding the area helped him further develop his naturalist and hunting skills. He developed good relations with J.C. Hopwood and Berthold Ribbentrop who worked with the Forest Service. On returning to India in 1917, he decided to complete his studies. Hence, he studied commercial law and accountancy from Davar's College of Commerce. He used to attend morning classes at Davar's College and go to St. Xavier's College to attend zoology classes to complete his course in zoology. Apart from his interest in birds, Salim Ali was also fascinated by motorcycles and hence, owned his first motorcycle, 3.5 HP NSU while he was in Tavoy.

He later went on to possess Sunbeam, Harley-Davidson (three models), Douglas, Scott, New Hudson, and Zenith, amongst other models. He went further to get his Sunbeam shipped to Europe on being invited to the 1950 Ornithological Congress at Uppsala, Sweden. While touring France, he even injured himself in a minor accident and cobbled several times in Germany. He was rumored to have ridden on his bike all the way from India, when he finally reached Uppsala. Coming back to his interest in ornithology, he was rejected a position at the Zoological Survey of India due to lack of a formal university degree. With this, he began studying further when he was hired as a guide lecturer in the newly opened natural history section at Prince of Wales Museum in Mumbai in 1926 with a salary of Rs. 350 per month.

Being fed up with the monotony of the job, he decided to go on a break and went to Germany in 1928 on a study leave. He worked under Professor Erwin Stresemann at Zoological Museum of Berlin University. He was also required to examine the specimens collected by J.K. Stanford, a BNHS member. Stanford was supposed to communicate with Claud Ticehurst at the British Museum who did not like the idea of involving an Indian in his work. Hence, he kept distance with Stresemann. Salim Ali then moved to Berlin and associated with popular German ornithologists, such as Bernhard Rensch, Oskar Heinroth, and Ernst Mayr. Apart from his usual ornithology experience, he also gained knowledge in ringing at the Heligoland observatory.

Contribution to Ornithology
After studying ornithology in Germany, Salim Ali returned to India in 1930 and started looking for a job. However, to his surprise, the position of a guide lecturer had been dropped off from universities due to lack of duns. Left with no option, Salim Ali, along with wife Tehmina, moved to Kihim, a coastal village near Mumbai. This place gave him another opportunity to observe and study birds very closely, including their mating system. He then spent a few months in Kotagiri on being invited by K.M. Anantan, a retied army officer who served in Mesopotamia during World War I. He also met Mrs. Kinloch and her son-in-law R.C. Morris, who lived in the Biligirirangan Hills.

Gradually, on traveling places, Salim Ali got an opportunity to conduct systematic bird surveys in the princely states of Hyderabad, Cochin, Travancore, Gwalior, Indore, and Bhopal. He was financially supported by Hugh Whistler who had previously conducted surveys in various parts of India. Although Whistler initially resented Salim Ali for finding faults and inaccuracies in the early literature, he later re-examined his specimens and accepted his mistakes. With this, began a close friendly relationship between Ali and Whistler. He introduced Ali to Richard Meinertzhagen and the two went on an expedition to Afghanistan. Initially, Meinertzhagen was also critical of Ali's views but later, the two became close friends.

Salim Ali was more attracted towards studying birds in the field rather than getting into the details of bird systematics and taxonomy. However, he did show some interest in bird photography with the help of his friend Loke Wan Tho, a wealthy businessman from Singapore. Ali and Loke were introduced by JTM Gibson, a member at BNHS and Lieutenant Commander of Royal Indian Navy, who had also taught English to Loke in Switzerland. Hence, Loke provided financial support to both Ali and BNHS. Ali talked about the history and importance of bird study in India in Sunder Lal Hora memorial lecture in 1971 and again in Azad memorial lecture in 1978.

Literary Career
Salim Ali was not only passionate about studying birds in general; he also showed equal interest in capturing his views on them in words. With the help of his wife Tehmina, a learned scholar from England, Ali improved his English prose. Thus, began Ali's writing career, particularly journal articles for Journal of the Bombay Natural History Society. One of his most popular articles was "Stopping by the woods on a Sunday morning" in 1930 which was reprinted again in Indian Express on this birthday in 1984. He penned several books as well, the most prominent of them being "The Book of Indian Birds" in 1941, which was inspired by Whistler's "Popular Handbook of Birds". It was later translated into several languages and saw more than 12 editions. However, his masterpiece was the 10 volume "Handbook of the Birds of India & Pakistan", written along with Dillon Ripley and was often known as "The Handbook".

The first edition began in 1964 and was completed in 1974. The second edition came from contributions by J.S. Serrao of BNHS, Bruce Beehler, Michel Desfayes, and Pamela Rasmussen. This was completed after Ali's death. Besides the national and international bird books, Ali also authored several regional field guides, like "The Birds of Kerala" (first edition was titled "The Birds of Travancore and cochin" in 1953), "The Birds of Sikkim", "The Birds of Kutch" (later renamed as "The Birds of Gujarat"), "Indian Hill Birds", "Field Guide to the Birds of Eastern Himalayas". He penned his autobiography "The Fall of a Sparrow" in 1985 where he included his vision for BNHS and the importance of conservation related activities. One of his last students, Tara Gandhi, published a two-volume compilation of his shorter letters and writings in 2007.

Personal Life
On his return from Burma, Salim Ali was married off to his distant relative, Tehmina, in December 1918 in Bombay. She accompanied him to all his expeditions and surveys. But his life came to a halt when she suddenly died following a minor surgery in 1939. Ali then started living with his sister Kamoo and brother-in-law. 

Death
After battling with prostate cancer for a very long duration, Salim Ali died on July 27, 1987 in Mumbai at the age of 90.

Honors & Memorials
Salim Ali was honored and credited with several honorary doctorates and awards during his lifetime, though this journey began late. Starting with "Joy Gobinda Law Gold Medal" in 1953 by the Asiatic Society of Bengal, he went on to receive numerous accolades. It was based on the appreciation he received from Sunder Lal Hora. Thus, in 1970, he was conferred upon with the Sunder Lal Hora Memorial Medal of the Indian National Science Academy. He was bestowed with honorary doctorate degrees from Aligarh Muslim University in 1958, Delhi University in 1973, and Andhra University in 1978. On receiving the Gold Medal from the British Ornithologists' Union in 1967, Salim Ali became the first non-British citizen to be bequeathed with such an honor. He received the John C. Philips Memorial Medal of the International Union for Conservation of Nature and Natural Resources in the same year.

In 1973, he received the Pavlovsky Centenary Memorial Medal from the USSR Academy of Medical Science and was made the Commander of the Netherlands Order of the Golden Ark by Prince Bernhard of Netherlands. He was honored with Padma Bhushan Award in 1958 and Padma Vibhushan Award in 1976. The Government of India established the Salim Ali Center for Ornithology and Natural History (SACON) in Coimbatore in 1990. Further, Salim Ali School of Ecology and Environmental Sciences has been established by Pondicherry University. The Salim Ali Bird Sanctuary in Goa and Thattakad Bird Sanctuary near Vembanad, Kerala have been set up in his honor. 

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History of Annelida

History of Annelida:

The annelids were by early zoologists included with other worms in the group Vermes but were separated by Cuvier in 1798 from the un-segmented worms. The name Annelida was first of all used by Lamarck in 1809 for the higher segmented worms.

Annelida (L., annellus = little ring or F .anneler = to arrange in rings) are triploblastic, bilaterally symmetrical, coelomate and segmented Metazoa. Body is covered by a thin cuticle having chitinous setae in most. Body wall is covered with glandular epidermis, below which are muscles forming an outer layer of circular muscles and an inner layer of longitudinal muscles.

The body is divided for the first time in the animal series into metamerically arranged segments or metameres arranged in a linear series, but the segments are integrated into a single functional unit. Perivisceral cavity is a schizocoelic coelom between two layers of mesoderm. The coelom contains a coelomic fluid which is incompressible, consequently it acts as a hydraulic skeleton.

There is a single pre-oral segment called prostomium and a similar post-segmental region posteriorly known as a pygidium. The nervous system has a pair of pre-oral ganglia or brain, and paired ventral nerve cords ganglionated in each segment. There is a closed circulatory system.

The digestive tract is more or less straight but differentiated into well-defined regions; digestion is entirely extracellular. Organs of excretion are metameric ectodermal nephridia, besides which there are tubular mesodermal coelomoducts used for passage of reproductive cells. The larva, if present, is a trochosphere.

The members of the phylum are modified for sedentary, active, or ectoparasitic life, they occur on land, in freshwater, or in the sea.

The phylum contains over 8,600 known species and is divided into four classes called Polychaeta, Oligochaeta, Himdinea, and Archiannelida. But in some systems of classification, Echiuroidea, Sipunculoidea, Priapulida, and Myzostomaria have also been included as classes, while in other systems these classes have been considered as appendix to phylum Annelida or to some of its classes.

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Nephridia and Nephromixia of annelida

Nephridia and Nephromixia:

Some Polychaeta have nephridia of closed tubes, their blind ends project into the coelom, this is a primitive condition. The blind end is fringed with solenocytes lying separately or in groups. The solenocytes are round ciliated cells connected to the nephridium by a thin tube each, in the lumen of the tube is a long vibratile flagellum; solenocytes resemble flame cells of Platyhelminthes.

Such nephridia are known as protonephridia, e.g., Phyllodoce, Vanadis. But in many Polychaeta and Oligochaeta the nephridia are of the open type, each having a ciliated nephrostome opening into the coelom, they are called metanephridia, e.g., Neanthes, Lumbricus. Some Polychaeta have compound excretory organs formed by the union of nephridia and coelomoducts, they are called nephromixia.

In these the functions of excretory organs and genital ducts become combined.

They are of three types:

(a) Protonephromixium, the coelomoduct has combined with a closed protonephridium, e.g., Aliciopidae, Phyllodoce.

(b) Metanephromixium, coelomoduct is attached to an open metanephridium, e.g., Hesione.

(c) Mixonephrium or Nephromixium, coelomoduct is attached to a nephridium in such a way that they form a single organ, the funnel being coelomoduct and the duct is nephridial, e.g., Capitellidae, Arenicola. There is no hard-and-fast boundary between metanephromixium and mixonephrium.

In some Polychaeta, e.g., Neanthes, a part of the coelomoduct separates from the metanephromixium and becomes attached to the dorsolateral muscles as a dorsal ciliated organ, this keeps the coelomic fluid in circulation.

In some tubicolous worms, e.g., Serpula there is a division of labour, the nephridia of the anterior region are large and carry on excretion, and those of the posterior region are small and act as genital ducts exclusively.

In Oligochaeta and Hirudinea nephridia and coelomoducts are separate. There is generally one pair of metanephridia in each segment, but coelomoducts are restricted only to a few reproductive segments. Their nephridia may open to the outside of the body and are called exonephric nephridia (e.g., Lumbricus) or they may open into the gut and are known as enteronephric nephridia (e.g. Pheretima).

In a majority of earthworms there is a pair of original large-sized metanephridia in each segment, they are called holonephridia or meganephridia, e.g., Lumbricus. But in Pheretima there is large number of small-sized nephridia in each segment, they are called meronephridia or micronephridia. It is assumed that the original pair of holonephridia has broken up to form a large number of meronephridia.

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