Excretion

Excretion

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Excretory System of Nephridia

Excretory System of Nephridia (Earthworm)

These are of three types according to their location in the body:

                 1. Septal nephridia

                 2. Integumentary nephridia 

                 3. Pharyngeal nephridia.

1. Septal Nephridia:

These are found situated on the inter-seg­mental septum between 15th and 16th segments to the posterior side of the body.

Each septum bears nephridia on both the surfaces arranged in semicircles around the intestine, two rows in front of the septum and two behind it. Each septum has about 40 to 50 nephridia in front and the same number behind, so that each segment possesses 80 to 100 septal nephridia except the 15th segment which has only 40 to 50 nephridia. These are not found in the segments up to 14th.

Structure:

The septal nephridia may be considered typical of all the nephridia of Pheretima. Each septal nephridium (Fig. 66.22) consists of nephrostome, neck, body of nephridium and the terminal duct.

(i) Nephrostome:

It is also known as ciliated funnel or nephridiostome. It is the proximal flattened funnel-shaped structure of the nephridium lying in the coelom.

It has an elliptical mouth-like opening leading into an intracellular canal of the large central cell, the margins of the opening are surrounded by a large upper lip and a smaller lower lip. The lips are provided with several rows of small ciliated marginal cells and the central canal is also ciliated.

(ii) Neck:

The nephrostome leads into a short and narrow ciliated canal forming the neck. It joins the nephrostome to the body of nephridium.

(iii) Body of Nephridium:

The body of nephridium has two parts a short straight lobe and a long twisted loop. The loop is formed by two limbs— the proximal limb and the distal limb.

Both these limbs are twisted spirally around each other, the number of twists varies from nine to thirteen. The neck of nephridium and the terminal duct join together and remain connected with the proximal limb of the twisted loop, while the distal limb becomes the straight lobe.

Internally the nephridium is made of a connective tissue matrix having long coiled neph­ridial duct forming loops. There are four such canals in the straight lobe, three in the lower part and two in the upper part of the limbs of twisted loop. Two canals of the straight lobe out of the four are ciliated like the ciliated canal of the neck.

(iv) Terminal Duct:

It is short and narrow with a terminal excretory duct. It joins the nephridium with septal excretory canal.

Relation of septal nephridia with intestine:

The nephridia hang freely in the coelom and are attached only by their terminal ducts. They open by their terminal ducts into two septal excretory canals lying on the posterior surface of the septum, one on each side of the intestine, each begins ventrally but dorsally it opens in the supra-intestinal excretory duct of its own side.

The supra-intestinal excretory ducts are two parallel longitudinal canals lying above the gut and below the dorsal vessel (Fig. 66.24). These excretory ducts begin from the 15th segment and run to the last segment, they communicate- with each other for a short space behind each septum, then either the right or the left duct opens by a ductule into the lumen of the intestine near the septum.

Thus, each segment has one such opening into the intestine of either the left or the right supra-intestinal excretory duct. The waste collected by the nephridia is discharged through the excretory canals and ducts into the lumen of the intestine. Such nephridia opening into the intestine are called enteronephric nephridia.

2. Integumentary Nephridia:

In each segment of the body from 7th to the last segment, numerous nephridia are found attached inside the lining of the body wall. These are called integumentary nephridia which are about 200-250 in each segment except the segment of the clitellar region where they number 2,000-2,500 in each segment.

These nephridia are small-sized, without nephrostome and without any opening into the coelom.

Hence, they are called closed type of nephridia. Each integumentary nephridium is V-shaped with a short straight lobe and a twisted loop, its lumen has two ciliated canals. Each nephridium opens by a nephridiopore on the outer surface of the body wall directly. Since the integumentary nephridia discharge the excretory wastes directly outside, hence, they are called exonephric nephridia.

3. Pharyngeal Nephridia:

These nephridia lie in three paired tufts, one on either side of the anterior region of the alimentary canal in the segments 4th, 5th and 6th. The tufts of pharyngeal nephridia also contain blood glands.

Each pharyngeal nephridium is about the size of a septal nephridium but it is of the closed type having no funnel or nephrostome. It has a short straight lobe and a spirally twisted loop, its lumen has ciliated canals. Ductules arise from each nephridium and unite to form a single thick- walled duct on each side in each segment.

The two ducts of nephridia of segment 6th open into the buccal cavity in segment 2nd and the paired ducts of nephridia of segments 4th and 5th open into the pharynx in segment 4th.

These nephridia also discharge their wastes into the alimentary canal and are, therefore, enteronephric but such enteronephric nephridia which open into the anterior region of the alimentary canal (buccal cavity and pharynx) are called peptonephridia because they may have taken the function of digestive glands.

Recently it has been reported that the pharyngeal nephridia of P. posthuma produce a variety of enzymes like amylase, chimosin, prolinase, prolidase, dipeptidases, aminopeptidase, lipase, etc., which hydrolyse various foodstuffs. Thus, such nephridia work like the salivary glands.

Physiology of Excretion:

Like other animals, in earthworms also, the protein catabolism results in the formation of nitrogenous waste substances like certain amino acids, ammonia and urea.

Uric acid is not found in the earthworms. However, the amino acids are degraded to form free ammonia and the urea is synthesised in the chloragogen cells which are released into the coelomic fluid and also in the blood for its removal. Free amino acids are not excreted but traces of creatinine occur in the urine.

Moreover, the nitrogen excreted in different forms in a well fed worm is about 72% NH3, 5% urea and remaining other compounds, while in a starved worm NH3 8.6%, urea 84.5% and remaining being other compounds. But generally, the excretion is 42% NH3, 50% urea, 0.6% amino acids and remaining being other compounds.

So, we can say that in a well fed earthworm, NH3 predominates the nitrogenous excretory wastes, hence, it is ammonotelic, while a starved one is ureotelic.

An earthworm excretes the nitrogenous wastes in the form of urine which generally contains urea, water, traces of ammonia and creatinine. Nephridia excrete these substances from the body of earthworm. The various excretory wastes from the coelomic fluid are drawn into the nephrostomes of septal nephridia or into the excretory canals of other nephridia along with some other useful substances.

These products are either discharged into the intestine (by enteronephric nephridia) or outside by the nephridiopores (by exonephric nephridia). The body of nephridia also absorbs some wastes. However, the useful substances are reabsorbed and the passing out waste remains concentrated for various nitrogenous compounds.

The excreted waste substances are removed out from the body with faeces. The nephridia, in addition to excretory, are also osmoregulatory in function.

The nephridia help in conserving water by reabsorption from the excreted products during summers and winters, so they pass hypertonic urine in relation to blood. During rainy season, the urine is dilute due to lesser reabsorption of water. The enteronephric nature of nephridia provides another device for conserving water.

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CYCLOMORPHOSIS IN ROTIFERS

CYCLOMORPHOSIS IN  ROTIFERS

Introduction

The Morphological change those occur in certain species of invertebrates in accordance to

environmental conditions is called Cyclomorphosis. The term Cyclomorphosis was coined by Lauterborn (1904) but the actual concept came to lime light only after Coker (1939). The phenomenon has been noted in the dinoflagellates, cladocerans, and to a much striking degree in the copepods. Organisms that reproduce during most of the year by asexual or parthenogenesis methods appear to exhibit this phenomenon. The degree to which the Cyclomorphosis is developed within different populations of the same species is variable. Although the seasonal incidence of the change is clearly determined by environmental factors, there may also be inherited diversity in the capacities of different races of a species to react to these factors. The seasonal changes of form are so striking that the summer and winter forms of the same species would certainly be supposed to represent different species by an observer, unacquainted with the facts. It involves the alternation of different morphological units in a species in accordance to the climatic as well as environmental changes. Total body size may increase, decrease or remain the same throughout the cycle, depending on the species.

Rotifers are one such group of invertebrates found abundantly in any fresh water body throughout the globe. Popularly known as “the wheel animalcules”, these are although small in number but

large populations are found in a particular environment. Among the Rotifers the monogonants especially that of order Ploima having the genus Brachionus are unique for their polymorphic forms and exhibition of Cyclomorphosis. Several investigation carried by earlier workers also reveal the same fact. Cyclomorphosis in rotifera was described first by Weisenberg-lund (1926) and later by several workers like Beauchamp (1952), Gallagher (1957), Hutchinson (1967), Gilbert (1973), Dodson (1974) also by Indian contributors like Arora (1965), Nayar (1968) and Dhanapathi (1980).

Place of investigation

To understand the phenomenon of Cyclomorphosis in Rotifers in different seasons as well as in

different ecological conditions, present work was carried out in two different perennial ponds in the city of Vizianagaram belonging to the state of Andhra Pradesh, India. Of these two ponds, one is highly polluted (Pond-1) and the other a bit clean and fresh water in nature (Pond-2) since no municipal or domestic wastages are allowed to pass in to this pond. Plankton samples were collected periodically (weekly once) from both pond-1 and pond-2. Species related to Brachionus are collected separately and measurements were being taken. Three species of Brachionus were only investigated, they are e.g., Brachionus calyciflorus, B.caudatus and Keratella cochlearis.

Brachionus calyciflorus Pallas

Brachionus calyciflorus is an extremely variable species, the variability is pronounced in the size,

length of posterolateral spines as well as occipital spines. Examination of all the data from the localities surveyed, the data from Big Pond (Pond-I) was found continuous enough to explain the phenomenon of Cyclomorphosis, in B. calyciflorus. Although this species was observed in the samples collected between April, 2009 and January, 2010, the continuous data could be obtained only from 7th October, 2009 to 9th January, 2010. On 23rd September, 2009 when the surface temperature was 32.4oC there were few forms of this species in the samples. On 7th October the number of specimen appeared a bit increased when the temperature was 28.5oC. The quantitative study of the rotifer revealed that there were two peak periods of abundance when the temperature was 21.0oC ON 26-11-09 and the second peak when it was 20.0oC ON 02-01-10. The rotifer stated disappearing from the samples when the temperature rose to 23.0oC on 16th January, 2010 and found very few on 23rd January 2010.During the period of occurrence of these species, the hydrogen-ion concentration (pH) fluctuated between 8 and 9 and turbidity between 11 and 72 ppm.

Morphological variations:

  Measurements were taken for thirty specimens from each sample collected once in a week. Length

of lorica (TBL), Maximum breadth (B) of lorica and the length of Postero-lateral spines (PLS) were

measured and their mean values were given in table. The respective points used in making the

measurements were shown in Graphs. The lengths of the lorica ranged from 230 to 328 microns while the breadth from 200 to 228 microns. In case of Postero-lateral spines the length of the right posterolateral spine which was more variable was taken. Its length ranged from 21 to 53 microns while the length of left posterolateral spine ranged from 23 to 50 microns.Thus we can conclude that the right spine is elongated than the left one.

The specimens with and those without posterolateral spines were almost equal in the samples of 7th

and 14th October. The specimens without posetrolateral spines gradually decreased and they disappeared completely in the samples collected from 4th November to 18th December. Again few specimens without posterolateral spines had re- appeared on 25th December and continued till 9th of January 2010. In the earlier collections the specimens without posterolateral spines (var. dorcas) are larger in size than those with posterolateral spines (var. dorcas f. spinosa). In the samples from 4th November till 18th December only var. dorcas forma spinosa was present. But the lateral Collections showed an assemblage of varied forms (var. dorcas, var. pala and var. dorcas f. spinosa ). Some specimens without posterolateral spines were found smaller than those with posterolateral spines in the collection during 25th December, 2009 to 9th January, 2010. Increase in the length of the postero-lateral spine (right) with the decrease in the size of the lorica has been observed.

Brachionus caudatus Barrios and Daday

 Although this species occurred for most part of the year in both the ponds but for morphological

variances, samples were collected from Pond – 1 were taken for present study. Only the measurements of the population collected from 05th March to 21st May 2009 were only analyzed for drawing the following conclusions.

Morphological Variation

 This rotifer showed a variation in length of posterior spines, arising on either side of the foot. The

measurements were taken for twenty specimens from each weekly sample. The length of lorica (TBL), maximum breadth of lorica (B) and the length of each posterior spine (PS) were measured and the mean values were presented in the Table. The respective points used in the making the measurements were shown.

 The lengths of Lorica ranged from 98 to 128 microns while the

Breadth from 87 to 112 microns. The Posterior spines are almost

equal except in very few. As the right one showed much variation,

measurements of this spine were taken for illustration. Its length varied from 12 to 63 microns.With the increase in the temperature the length of lorica increased and there was also a corresponding increase in the length of spines. But the rate of increase in the length of posterior spine is greater than the rate of increase in the length of lorica.

Conclusion

 The proximal causes including Cyclomorphosis can be mooted with the environmental conditions

those change within no time along with the climate. A relatively warm or warming temperature, turbulence presence of light, female principle and predation are some of the prime reasons. Exuberant forms of rotifers have been correlated with starvation for Branchious or with dense food or cold water. Gilbert had the opinion that incase of Branchious the abundance of Asplanchana (a predator) in the same locality also influence Cyclomorphosis. Turbidity appears to be also an important factor influencing the reproduction and abundance of both the species. B.calyciflorus was observed in abundance when the turbidity was low. Another important factor which was observed in the present observation was dissolved oxygen. With the increase in the dissolved oxygen content there was a corresponding increase in the abundance of both the species. Nayar (1956) was of the opinion that these physico-chemical factors may not have direct influence on the rotifer B.calyciflorus. This may be true to certain extent but in the present investigation it is clear that the occurrence of Cyclomorphosis is basically due to the physico-chemical changes in the environment.

  In both these ponds the maximum size of the lorica was observed for all the species during the

period of high temperature. In B.calyciflorus with the decrease in the temperature, there was a

corresponding decrease in the length of the lorica and increase in the length of postero-lateral spine. But in B.caudatus with the increase in temperature, there was a corresponding increase in the length of lorica and length of posterior spines. It may be concluded that no single factor can account for this seasonal polymorphism but a combination of many factors like temperature, turbidity and hydrogen-ion (pH) Concentration, dissolved oxygen and feeding behavior, etc. are all responsible for these variations which might act in a cumulative manner.

Brachionus is typical for its polymorphic forms and specialized spiny outgrowths. It was also

noticed that the species of Brachious examined from pond-I are much enlarged, healthy in look and much spinous comparatively to the samples brought and examined from pond-II. One of the biggest reasons for such variance may be attributed in the form of domestic sewage pollution which is opened into the pond-I in large accounts.

 After a fresh spell of monsoon, the distribution of phytoplankton is plenty. The malleate or malleo-

ramate mastex of herbivorous rotifers such as Brachionus and Keratella is specialized to capture such

nanno planktons, while the incudate trophy of Asplancha are specialized for capturing small rotifers. This suggests that prey of the right size but wrong shape is never accepted in predation, hence many be rejected after being caught. For example, the presence of long spines on B.calyciflorus does not affect the rate of collision with Asplancha but decreases the probability of being eaten once caught. Hence, a spine or any such thing at the right place at right time makes the prey unsuitable for predation.

n the present investigation, a correlation between the distribution and species composition is also observed during the period of elongation of outer ornamentation. More the spine elongations less was the species composition. It can be finally be said that the morphological structures, the lorica , abundance as well as the size and population of one species is inter related to the other especially to that of predator

CYCLOMORPHOSIS IN ROTIFERS (PDF Download Available). Available from: https://www.researchgate.net/publication/301635397_CYCLOMORPHOSIS_IN_ROTIFERS [accessed Feb 28 2018].

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Flagella and Cilia: Structure and Functions

Flagella and Cilia: Structure and Functions

Structure of Flagella and Cilia:

They are fine hair like movable protoplasmic processes of the cells which are capable of producing a current in the fluid medium for locomotion and passage of substances.

Flagella are longer (100-200 µm) but fewer. Only 1-4 flagella occur per cell, e.g., many protists, motile algae, spermatozoa of animals, bryophytes and pteridophytes, choanocytes of sponges, gastro dermal cells of coelenterates, zoospores and gametes of thallophytes. Cilia are smaller (5-20 µm) but are numerous.

They occur in group ciliata of protista, flame cells of worms, larval bodies of many invertebrates, epithelium of respiratory tract, renal tubules, oviducal funnel, etc. Cilia present on the tracheal and bronchial epithelial cells are specialised to send back dust particles into the pharynx so that the lungs remain unharmed.

However, cigarette smoking reduces/stops ciliary activity so that air borne dust particles pass into the lungs of smokers causing irreparable harm. Both cilia and flagella are structurally similar and possess similar parts— basal body, rootlets, basal plate and shaft (Fig. 8.46).

(i) Basal Body or Kinetosome:

It is also called basal granule or blepharoplasty. Basal body occurs embedded in the outer part of the cytoplasm below the plasma membrane.

It is like a micro cylinder which has a structure similar to a centriole with nine triplet fibrils present on the periphery without a central fibril, though a hub of protein is present here. Only sub-fibre A is complete (having 13 protofilaments) while sub-fibres В and С are incomplete as they share some of their protofilaments.

(ii) Rootlets:

They are striated fibrillar outgrowths which develop from the outer lower part of the basal body and are meant for providing support to the basal body. The rootlets are made of bundles of microfilaments.

(iii) Basal Plate:

It is an area of high density which lies above the basal body at the level of plasma membrane. In the region of basal plate, one sub-fibre of each peripheral fibril disappears. The central fibrils develop in this area.

(iv) Shaft:

It is the hair-like projecting part of flagellum or cilium. The length is 5- 20 µm in case of cilium and 100—200 µm in case of flagellum. The shaft is covered on the outside by a sheath which is the extension of plasma membrane. In whiplash flagellum, the sheath is smooth.

In tinsel flagellum, the sheath contains a number of thick hairy outgrowths called flimmers. Internally, it contains a semifluid matrix having an axoneme of 9 peripheral doublet fibrils and 2 central singlet fibrils (Fig. 8.46).

This arrangement is called 9 + 2 or 11-stranded. However 9 + 1 (e.g., flatworm) and 9 + 0 (e.g., eel, Asian Horseshoe Crab) arrangements have also been observed. The two central singlet fibres are covered by a proteinaceous central sheath. They are connected by a double bridge. Each peripheral fibril consists of two microtubules or sub-fibres В and A.

The sub-fibre A is slightly narrower. It bears two bent arms, the outer one having a hook. They are about 15 nm long and made up of protein dynein with ATPase activity. Such activity is also present in central fibrils. Movement of flagella or cilia occurs due to sliding motion in which dynein arm establishes temporary connection with sub-tubule В of adjacent doublet fibre.

The pe­ripheral doublet fibrils as well as central singlet fibrils are made up of tubulin. Each sub-fibre or central singlet fibril contains thirteen protofilaments. The peripheral doublet fibrils are interconnected by A-В linkers of protein nexin between B-sub-fibre of one and inner side arm of A-sub-fibre of adjacent fibril.

Each of their A sub-fibres sends a radial proteinaceous column to the centre. It is called spoke. The spokes are broader internally to form heads or knobs. Head is connected to central proteinaceous sheath through transition junction.

The cilia and flagella move by sliding of the doublet fibrils against one another. Energy is provided by ATP.

Flagella perform independent undulatory movements while cilia show rowing type of sweeping motion either simultaneously (isochronic or synchronous) or one after the other (metachronic). In a flagellum, several symmetrical undulatory waves pass from base to the tip. This pushes the cell along. Undulations passing from tip to base pull the cell through water.

In tinsel flagellum having a number of flimmers, the undulatory wave moving down from base to tip also pulls the cell along instead of pushing it. There is always a power stroke and a recovery or return stroke (Fig. 8.48).

The power stroke is able to move the fluid with a jerk in the direction of the stroke. The cell moves in the opposite direction, if it is motile. The recovery or return stroke is slow and without much force. Therefore, it does not cause much disturbance in the fluid medium.

Rate of cili­ary and flagellar movements is 10-40 strokes per second. Flagellate Monas stigmatica swims at the rate of 260 pm or 40 cell length/sec. It has the maximum speed per body length. Paramoecium caudatum has a speed of 1500 µm or 12 cell lengths/sec.

Functions of Cilia and Flagella:

1. They help in locomotion in flagellate and ciliated organisms.

2. They create current for obtaining food from aquatic medium.

3. In some protists and animals, the organelles take part in capturing food.

4. The canal system of porifers operates with the help of flagella present in their collar cells or choanocytes.

5. In coelenterates, they circulate food in the gastro vascular cavity. In tunicates and lancelets, the cilia help in movement of food and its egestion.

6. In aquatic organisms cilia create currents in water for renewal of oxygen supply and quick dif­fusion of carbon dioxide.

7. In land animals the cilia of the respiratory tract help in eliminating dust particles in the incoming air.

8. Internal transport of several organs is performed by cilia, e.g., passage of eggs in oviduct, passage of excretory substances in the kidneys, etc.

9. Being protoplasmic structures they can function as sensory organs.

10. Their tips secrete sticky substance to help in conjugation and fusion of gametes.

11. In certain protistans, cilia fuse to form undulating membrane.

12. Cilia and flagella show sensitivity to changes in light, temperature and contact.

13. Ciliated larvae take part in dispersal of the species.

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