Limnology (2+1)

Practical No. : 6

Collection and identification of Freshwater Plankton
Plankton
The term ‘plankton’ was coined by Victor Henson in 1887 to designate the heterogeneous assemblage of suspended microscopic materials, minute organisms and detritus in water which wander at mercy of winds, currents and tides. However, the use of the term has been confined to designate only the microscopic, free-floating organisms; which depending on their nature are divided in two major groups, namely, phytoplankton and zooplankton.
Based on the size, the plankton have been classified as ultra (0.5 to 10µm), nano (10 to 50µm), micro or net (50 to 500µm) and macroplankton (>500µm).
Phytoplanktons are chlorophyll bearing suspended microscopic organisms consisting of algae with representative from all major taxonomic phyla; the majority of members belong to Chlorophyceae, Cyanophyceae and Bacillariophyceae. Their unique ability to fix inorganic carbon to build up organic matter through primary production makes their study a subject of prime importance. The quality and quantity of phytoplankton, and their seasonal succession patterns have been successfully utilized to asses the quality of water and its capacity to sustain heterotrophic communities.
Collection of plankton
Collection of nano plankton
The nano plankton compared to net plankton has less number of species flagellates diatoms which have a size range of 5-20µm contribute more than 90% in the phytoplankton biomass.
Methods of collection
1. Bottle sampler
2. Pump and hose
Bottle sampler
Bottle samplers are ideal for small quantitative phytoplankton collection. It is mainly used for the collection of water samples from any desired depth of shallow ecosystem from a stationary vessel – near shore waters, estuaries and mangroves. Surface water can be obtained by gently scooping water in to a container of suitable size from the leeward side of the ship. Subsurface water can be obtained by using sampler like Mayer’s Water Sampler, Friedenger’s Water Sampler, Nansen reversing water sampler, Vaan Dorn water sampler, Niskin water bottle, NIO water bottle, Universal water sampler, etc. Samplers are sent to a desired depth on the rope in an open condition. Messengers are sent to close the lids to secure and lift the sample onboard.

Pump and hose
Pump and hose is used to collect water sample for quantitative plankton analysis from the desired depth. An electrically operated rotary pump or centrifugal pump having flexible inlet and outlet hose pipes is suitable. The water is taken on to the vessel from the desired depth and transferred to sedimentation chamber or poured through one or several hand nets of various mesh sizes. The plankton collected from different bag nets is preserved for further analysis.
Methods of concentration
1. Mesh filtration
2. Centrifugation.
3. Settling sedimentation
4. Membrane filtration
Mesh filtration
1. Fixed volume of water is passed through a mesh of about 200µm mesh size
2. Then the micro zooplankton in the water is concentrated with small conical nets of fine gauge (20-25µm mesh size)
3. Small flagellates, ciliates, diatoms, etc. pass through the 20-35µm mesh
4. This fraction of the sample can be concentrated by centrifugation, settling or settling filtration methods
Centrifugation
1. 10-20ml of the water sample is centrifuged for about 15-30min at 1500-2000rpm with centrifuge.
2. The supernatant water is removed until the volume reduced to 1/10-1/30 of the initial.
3. The plankton is re suspended in the remaining volume of water.
4. The sample is fixed and preserved in neutralized formalin or Lugol’s solution for subsequent analysis.
Sedimentation
1. After the filtration, the organisms are allowed to settle and undisturbed for 1-2 days
2. Supernatant is carefully siphoned out
3. This process may be repeated until the sample is concentrated to 10-25ml
Membrane filtration
1. A known volume of water is filtered through a membrane filter of 0.5-1.0 µm porosity
2. The filtrate with filter is dehydrated by passing through the concentrated ethanol stained with alcohol (0.1% in 95% ethanol) and finally washed with ethanol.
3. The filter is then cleared with creostone or immersion oil and mounted on a slide using Canada balsam or filtrate may be subjected to immersion oil and taken for utermohl counting.
Collection of net (micro) plankton
Plankton of more than 50µm size can be collected by ordinary net sampling. This method could preferably used for quantitative plankton collections, as large quantity of water is filtered. Net is towed vertically, horizontally, or obliquely. Shape of nets commonly used is conical, conico- cylindrical and conical with a mouth reducing cone. Rectangular shaped nets have been designed. The net is attached to the wire directly with a bridle. At the cod end of the plankton net, a sampling bucket is attached.

Sampling methods
Vertical haul
In a vertical haul the entire water column is filtered through from the bottom to the surface, or only a top part is filtered. The net is lowered to the determined depth from the anchored research vessel and" slowly hauled up. The hauling speed is determined principally by the mesh width of the net (1 m/sec for a 300 µ size net), a weight is attached to the net bucket.
A net is lowered to a fixed depth from a stationary boat and immediately pulled upward at a speed of 0.7-1.0m/sec. When a net is lowered from a research vessel, the mouth rings of the net is folded vertically and descended smoothly with the help of weight (10-15kg).

Horizontal haul
The horizontal haul is used to obtain plankton samples from a particular water layer. A weight holds the net while it is being towed in the depth. The depth position of the net can be determined
from the wire angle and the length of the cable paid out. Net is hauled horizontally behind the vessel at a speed of 1m/sec.
Oblique haul
The net is lowered to a particular depth from a stationary vessel. Afterwards the vessel is slowly moved forward. The net would come up to the surface filtering' an oblique column of water. The advantage of this method is that the water column is more intensively filtered in this manner than in a vertical haul. In another type of oblique haul, the ship will be in motion while the net is being lowered as well as hauled up. The net were towed obliquely from a desired depth towards water surface during a boat is moving. It is done by slowly releasing plankton net from water surface to the given depth and then tow at that depth for a while before pulling the net towards the water surface.

Preservation of plankton
After the collection of plankton, the samples should be preserved immediately. For determination of chemical composition, specimens should be fixed within 10 min after collection. For taxonomic studies samples should be narcotized, fixed and preserved in this order. Otherwise autolysis, bacterial action, cannibalism or chemical deterioration will set in.
a. Norcotisation (relaxation)
Norcotisation is a vital process before fixation and preservation. It is to prevent from contraction and distortion of organisms at the time of fixation and ensuring easy identification of the organisms.
Preparation of Norcotics :
1) Osmic acid : Dissolve 200mg osmium tetroxide in 10ml of distilled water
2) Formalin (20%) : Dilute 200ml of 40% formaldehyde solution to 800ml of filtered water in a beaker. Add 0.5g borax (sodium borate +6.5g disodium hydrogen sulphate). Allow the solution for 24 hrs for settlement. The supernatant solution is siphoned out and used.
3) Menthol crystal
4) Magnesium chloride
5) Acetone-chloroform
6) MSS-222 etc
Procedure
1. Menthol - Small flakes of menthol are dropped on the surface of the sample. The crystal will gradually dissolve and anaesthetize the plankton.
2. Formalin - Animals are placed into a volume of water 50-500 times their own volume. Add 3 drops of 2.5% formalin to each 100ml of sample for 15min for the first hour. Add 6 drops in the second hour and 12 drops in the third hour at 15min intervals.
3. Propylene and phenoxytol - Add 3 drops of concentrated propylene phenoxytol into 1 litre of water containing plankton.
I. Fixation
Fixation is the application of a chemical (fixative) to kill an organism but to retain its morphological characteristics as far as possible.
Preparation
Buffered formalin : Add 30g of borax (sodium tetraborate ) to 1000ml of 40% formaldehyde and mix well. Allow the solution to stand for 1-2 months. Filter the solution to remove the borax precipitate.
Steedman’s fixative : Dissolve propylene phenoxytol in propylene glycol in 0.5:4.5(v/v). To this solution add 5 volume of Buffered formalin (undiluted)
Glutaraldehyde : Dissolve 8g of glutaraldehyde in 100ml distilled water and neutralize the solution with 1N Na OH solution and preserved at 4oC. Supernatant solution is used as fixative.
Rodhe’s iodine solution: Add 10g of iodine, 20g of potassium iodide and 20ml of acetic acid to 200ml of distilled water and mix well.
Procedure
1. Micro plankton
• Flagellates : add 1-5 % (v/v) of buffered formalin.
• Micro zooplankton: add 5-20ml of glutaraldehyde to 100ml of sample.
• Ciliates: add 5ml of Rodhe’s iodine solution to 100ml of sample.
• Phytoplankton: add 5 drops of osmic acid to 100ml phytoplankton sample.
2. Net plankton
• Sample is transferred in to a 500ml container.
• Add buffered formalin in the ratio of 5-90 parts (v/v)
• Invert the sample bottle after adding fixative for dispersing
• Add 5/ (v/v) formalin solution.
• In case of steedman’s fixative –the sample to fixative ratio should be 10:90 (V/V)
• The pH of both solution should be maintained around 7.6 to 8.3
II. Preservation
Preservation is the maintenance of fixed condition for extended periods of time. Specimens after one week fixation are used for preservation after thorough washing with distilled water.
Preparation
1. Lugol’s solution
Dissolve 10 gm of potassium iodide, 5 gm of doubly sublimed iodine in 20 ml of distilled water. To this add 50 ml of distilled water and 5gm of sodium acetate or 5 ml of 10% of acetic acid. Then the solution is made up to 100ml with distilled water.
2. Ethanol solution
Prepare 30 %, 50 % and 70 % with distilled water
3. Steedman’s preservative
Buffer formalin (undiluted) – 2.5 ml + Propylene phenoxytol – 1 ml + Propylene glycol- 10ml + Filtered water (88.5ml)
Procedure
1. Buffered formalin – 2.5 – 5 % formalin is used in ratio of 1:9 (sample to preservative). pH should be maintained at 7
2. Ethanol – One week after fixation wash thoroughly in distilled water before transferred into ethanol. The specimen is then immersed in 30 % ethanol for 10 min; move to a 50 % solution and after 1 hr transfer to 70 % solution.
3. Lugol’s solution – Add 1- 2 ml of preservative to 1000ml of phytoplankton sample.
4. Osmic acid – Add 3-6 drops in 100 ml phytoplankton sample.
5. Glutaraldehyde – Preserve phytoplankton in the ratio of 1:1
Note : To preserve the natural colour of the plankton, fish and crustaceans may be preserved in phinolic antioxidant such as 40 % emulsifiable concentrate of butylated hydroxyl toluene or butylated hydroxyl anisole (BHA).
Storage of sample
Container such as glass bottles with wide mouth, polypropylene with plastic screw on lid, polycarbonate containers, whirl pack – polypropylene bag are commonly used.
• Store the sample in a dust free dark and cool place
• Maintain the pH between 6.5 and7.5
• Periodic checking for colour and pH is required
• The preservative should be changed if necessary.
Labels :
Water resistant papers are used for external and internal labeling with following information in each container. External label should have bottle no, station no, date of sampling, day/night, sky, time, depth of sampling, type of net, mesh aperture, type of haul, flowmeter reading, collector name etc. and internal label should have station no, date of sampling, sampling depth, type of net, mouth size and mess size, type of haul, number of turns in flowmeter, collector’s name, etc.
Identification of phytoplankton
1. Chlorella
Cells spherical to ellipsoidal, solitary or aggregated, small smooth walled, chloplast single parietal, cup-shaped or laminate, with or without a pyrenoid, reproduction by autospores, free living or symbiotic.
2. Pediastrum
Colonies stellate to disc-shaped, monostromatic disc entire or perforate, cells 4 to 128, polygonal, marginal cells mostly with one, two or four processes, chloplast single parietal with a pyrenoid, diffuse in mature cells with one or more pyrenoid, cells multinucleate, reproduction by zoospores and isogamets, planktonic.
3. Coelastrum
Colony hallow sphere, rarely polygonal to pyramidal, cells 4 to 128, radially arranged, spherical, ovoid or pyramidal, closely adjoined and interconnected by narrow processes forming intercellular spaces, chloroplast cup-shaped to diffuse with a pyrenoid, reproduction by autocolonies, planktonic.
4. Ankistrodesmus
Cells acicular or cresent shaped, solitary or in small loose groups, usually not enclosed in a mucilaginous envelope, cells straight or curved, often twisted around one another, wall smooth with gradually tapering ends, spines lacking, chloroplast single, parietal with or without pyrenoid, reproduction by autospores, planktonic.
5. Clostridium
Cells solitary or in loose aggregates, semicircular to lunate orcylindrical and involved with a short stout spine at either pole, gelatinous sheath absent, cell wall relatively thick, chloroplast single, large and usually with a pyrenoid, reproduction unknown, planktonic.
6. Selenastrum
Colonies without an outer mucilaginous envelope, consist of 4, 8 or 16 cells, cells arcuate to lunate with convex faces apposed, apices acute, chloroplast single, parietal, lying along the convex wall, with a pyrenoid, reproduction by autospores, planktonic.
7. Kirchneriella
Colonial, gelatinous, envelope homogeneous, cells lunate to sickle shaped with pointed ends or irregularly spirally curved cylinders with rounded ends, cells usually in even numbers, chloroplast single, parietal usually with a pyrenoid, reproduction by autospores, planktonic.
8. Scenedesmus
Colonies flat plate like, 2-4-8 (rarely 16-32) celled, cells in one plane, multiplies of two, cells acicular, ellipsoid, ovoid or cylindrical (never globose), cells arranged in a single or double series of alternating cells with long axis parallel to one another, cell wall smooth or granulate, with or without lateral ridges, teeth or spines, chloplast single, laminate with a pyrenoid, cells uninucleate, autospores form autocolonies, planktonic.
9. Crucigenia
Colony enclosed by a thin inconspicuous gelatinous envelope, cells flattened, spherical to rhomboidal, quadrately arranged with a large or small open space in the centre, frequently form multiple colonies of 16 or more cells, cell wall without ornamentation and spines, chloplasts 1=4, parietal or disc-shaped and usually with a pyrenoid, reproduction by autocolonies and akinetes, planktonic.
10. Tetrastrum
Colonies flat plate like, always 4 celled, cells triangular, cruciately arranged with or without an open space at the centre, lie in a thin gelatinous matrix, angles rounded (never lunate), with one or more setae, chloplast 1 to 4, parietal with or without pyrenoid, reproduction by 4 autospores which form autocolonies, planktonic.
11. Ulothrix
Simple unbranched filaments of indefinite length, basal cell present, cells uninucleate, mostly cylindrical, often broader than long and never in pairs, terminal cell never pointed, chloplast single, riddle shaped parietal band, partially or fully encircling the protoplast, pyrenoids one or several, reproduction by fragmentation, bi- or quadriflagellate zoospores or biflagellate gamets, epiphytic or planktonic.
12. Microspora
Filaments unbranched, usually sessile when young or free floating, protoplast enclosed by the conjoined halves of two successive H-pieces, each protoplast surrounded by thin cellulose layer, interpolation of H-pieces in diving cell, H-pieces impregnated with silica, outer most layer of filament is made up of pectin, cells uninucleate with a central vacuole, chloroplast in young cells irregularly expanded and perforate and reticulate sheet, without pyrenoid, zoospores bi-or quadriflagellate.
13. Cladophora
Filaments repeatedly branched with basal-distal differentiation in the habit of branching, lateral branches arise close to the upper septa and often appear to be bi or trichotomous (due to evection), cells 5-20 times longer than breadth, multinucleate, cell wall thick and stratified, reticulate chloroplast with pyrenoids at intersections, reproduction by quadriflagellate zoospores and biflagellate gamets, mostly attached rhizoidal branches when young.
14. Pithophora
Filaments branched, lateral branches arise a short distance from the upper septa, cells cylindrical, coenocytic, cell wall thick but without lamellation, apical cell prominent, cylindrical akinetes common, intercalary or terminal, only half cell takes part in their formation, free floating.
15. Mougeotia
Filamentous, cells conspicuously longer than breadth, chloroplast laminate, axial plate like, mostly one, pyrenoids many, conjugation usually scalariform, zygospore always formed in conjugation tube, adjoined by two or four cells, gametangia with cytoplasmic residues and without gerlatinous material after zygote formation.
16. Zygnema
Unbranched filaments of short or long cylindrical cells, chloroplast two, stellate, each with a central pyrenoid, gametangia not filled with gelatinous material after zygote formation, zygospore either in the conjugation tube or in one of the gametangium
17. Spirogyra
Filaments long, unbranched, cells as long as broad or several times the breadth, chloroplast 1 to 16, parietal, ribbon shaped with half to 3 or rarely 8 left hand spirals, pyrenoids many in a single series, lying equidistant from one another, gamets physiologically anisogamous, aygopore in either of the conjugating cells.
18. Closterium
Cells solitary, elongate without a median constriction, poles distinctly attenuated but without spines, chlorplasts two, sickle shaped, with or without longitudinal ridges, pyrenoids mostly few.
19. Cosmarium
Cells compressed, oval to spherical with a deep median constriction, length slightly greater than the breadth, cell wall smooth or ornamented, semicells without spines, apex not incised, each semicell with single axial chloroplast with four radiating plates, pyrenoids axial in position.
20. Staurastrum
Cells strongly compressed, bilaterally or radially symmetrical, deeply constricted with acute-angled sinus, cell wall smooth or ornamented, apex of semicells extended into 2 or more divergent arms, chloplast axial pyrenoids one to many.
21. Desmidium
Filamentous unbranched, spirally twisted in gelatinous envelope, cells broader than long, triangular or quadrangular in vertical view, without deep median constriction, cell wall smooth, chloroplast axial, with a lobe (deeply incised) and a pyrenoid in each angle of a semicell.
22. Tetmemurous
Cells solitary, cylindrical to fusiform, median construction with open sinus, length 2 to 8 times the breadth, apex of semicells rounded with a vertical incision, cell wall smooth, punctuate or minutely scrobiculate, without transverse rings of spines or verrucae, chloroplast single, axial with many longitudinal radiating plates, pyrenoids many, axial and arranged in a row.
23. Xanthidum
Cells solitary, compressed, median constriction deep, semicells with 1 to many simple lateral spines, middle front of semicells wall thickened, semicells not incised at apex and with two laminate axial or four parietal chloroplast, pyrenoids usually one.
24. Arthrodesmus
Cells solitary, length and breadth about equal, strongly compressed, deeply constricted, sinus widely open to linear, cell wall uniformly thick and smooth, with straight or strongly curved spines at angles, chloplast axial laminate, pyrenoids 1 to 2.
25. Penium
Cells solitary, cylindrical with parallel sides and rounded or truncated poles, long axis straight, mostly without a median constriction, more than one gridle piece in each semicell, striae or puncate in regular longitudinal rows, semicell with axial chloroplast, pyrenoids on to many.
26. Gymnozyga
Unbranched filamentous, often with a gelatinous sheath, cells barrel shaped, length double the width, median constriction very slight, semicells inflated at the base and flattened at the apices, base and apices with or without longitudinal striae, chloroplast axial, pyrenoid single.
Euglenophyta (Euglenoids)
27. Euglena
Cells uniflagellate, fusiform to acicular, flexible, constantly change their shape, posterior end more or less pointed, gullet and eye spot anterior, contractile vacuole 1 to many. Chloroplasts many, discoid to band shaped, pyrenoids may be present, flagellum bifurcated at base, neuromotor apparatus present, planktonic.
Pyrophyta
28. Peridinium
Cells slightly flattened dorsoventrally, hypotheca with 5 postcingular and 2 antapical plates, epitheca with 6 to 7 precingular, none to 8 inercalary and 3 to 5 apical plates, plates usually ornamented with spines or reticulum of small ridges, sutures broad with striations which are often longitudinal or transverse, planktonic.
29. Ceratium
Cells broadly fusiform, hypotheca with 5 postcingular and 2 antapical plates which terminate in posterior horns, epitheca with a series of 4 precingular and 4 apical plates, apical plates form an apical horn, girdle transverse, ventral plate large, membranaceous and anticulated with pre-and postangular plates, planktonic.
Bacillariophyta (Diatoms)
30. Melosira
Cells united to form long unbranched filaments, gridle sculptured, valves circular in vertical view, ornamentation in two parts, concentric, cylindrical in gridle view, polar margins with denticulations, chromatophores many, discoidal, planktonic.
31. Cyclotella
Cells usually solitary, gridle unsculptured, valves circular, ornamented in two concentric regions, outer peripheral radially costate, inner smooth and irregularly finely punctuate, intercalary bands absent. Chromatophores many and discoidal, planktonic.
32. Stephanodiscus
Cells solitary, gridle unsculptured, valves circular with radiately alternating punctuate and smooth areas, peripheral portion with short spines which extend beyond the edge of the valve and multiseriate punctuate, rectangular in gridle view, planktonic.
33. Coscinodiscus
Cells solitary, gridle unsculptured, valves circular to elliptical in valve view, irregularly ornamented with minute forking rows of punctuate to coarse areolae, valve surface without radiate hyaline areas, denticulation at margin may be present, rectangular in gridle view, planktonic.
34. Tabellaria
Cells generally in free floating zigzag chains, sometimes semistellate, septa more than two, longitudinal, straight, perforate, present between gridle and intercalary bands, valves enongate, inflated laterally in the middle and at the poles, transverse finely punctuate striae lateral to median pseudoraphe, planktonic.
35. Diatoma
Frustules lanceolate to linear in valve view, often subcapitate poles, valves bilaterally symmetrical in both axis, septa transverse, many run across the valves to the inercalary bands of the gridle, pseudoraphe faint, valve and gridle ornamented, planktonic.
36. Fragilaria
Cells attached side by side to form ribbon shaped colonies (rarely flat stellate), linear to fusiform in valve view and rectangular in gridle view, bilaterally symmetrical in both axis, pseudoraphe present, valves with transverse striae or punctae, planktonic.
37. Synedra
Frustules usually narrow, many times longer than broad, solitary or in radiate fan shaped free-floating or epiphytic colonies, needle shaped in both views or with slightly capitates poles, valves linear to lanceolate, straight to curved, pseudoraphe and transverse ornamentation present, apices truncate in gridle view, bilaterally symmetrical in both views, epiphytic or planktonic.
38. Asterionella
Colonies stellate, cells in one plane, ends of valves flat, dissimilar in size, broader ends joined by gelatinous cushions, indistinct pseudoraphe and transverse ornamentation present, planktonic.
39. Navicula
Frustules symmetrical, rectangular in gridle view, raphe and axial filed straight, latter is narrow and without any expansion, lateral to axial field striae or punctuate in transverse rows, stauros absent, planktonic.
40. Pannularia
Frustules symmetrical, axial field broad and expanded next to the central and polar nodules, raphe with somewhat sigmoid or straight outer fissure, valves with smooth transverse costae, rectangular in gridle view, gridle smooth and without intercalary bands, planktonic.
41. Gyrosigma
Valves convex, sigmoid, gradually attenuated, poles acute or rounded axial field raphe sigmoid, punctae in transverse and longitudinal rows making a pattern of intersections, planktonic.
42. Pleurosigma
Similar to Gyrosigma, valves with one transverse and two oblique rows of punctae to the axial field.
43. Gomphonema
Frustules transversely asymmetrical in both views, raphe straight with central and polar nodules, valves with transverse rows of delicate or coarse punctae, frustules borne on tips of dichotomously branched gelatinous stalks, epiphytic or planktonic.
44. Nitzschia
Frustules with tranverse septa, keel single, excentric, on lateral margin of valve, raphe lies within it, keeled margin of one valve faces the unkeeled margin of the other valve, rapheal fissure with uniseriate row of circular pores (carinal dots), planktonic.
45. Surirella
Face of valve flat or spirally twisted, valves with keel (containing raphe) on both margins and a pseudoraphe in the middle, costae prominent, transverse, planktonic.
46. Microcystis
Colonies many celled, microscopic or macroscopic, spherical to irregular or net like, cells spherical, densely aggregated, without evident sheaths and even distribution, pseudovacuoles often present, mostly planktonic.
47. Spirulina
Trichomes unicellular, without sheath and lack dissepiments, cells terminals round, usually not tapering, trichome, regularly spirally coiled, spirals broad or narrow, planktonic.
48. Anabaena
Trichomes solitary or aggregated in a soft amorphous mucilaginous mass, never contorted like Nostoc, thickness of trichome usually the same throughout, trichome with watery and inconspicuous sheath, heterocysts intercalary akinetes single or in series.
49. Anabaenopsis
Filaments unbranched, short, spirally coiled, hetercysts terminal, at both the ends of the trichome and usually at different stages of development heterocyst in pairs when intercalary, akinetes remote from the heterocysts, planktonic.
50. Nostoc
Trichomes within a definite sheath, contorted, colonical matrix firm with a definite shape, heterocyst, intercalary, akinete solitary or in chains.
51. Oscillotoria
Trichome unbranched and without a distinct sheath, solitary and scattered or form expanded masses, trichomes may dissociate easily, mostly straight or in irregular spirals, ends distinctly marked, attenuated, rounded bent or coiled and with or without calyptras, cells discoidal or cylindrical, homogonia may have a thin sheath.
52. Lyngbya
Trichomes many celled, cylindrical, occur singly or interwoven, with thin but firm colourless or brownish sheath.
53. Phormidium
Single trichome within a thin watery or less sheath, cells barrel shaped, sheath of filaments agglutinated with one another and trichomes do not dissociate easily, never grow in erect tubes, mostly subaerial, morphology and apical cells much similar to Oscillotoria.
Protozoa (Rhizopoda)
54. Amoeba
Body shape irregular, asymmetrical and changes constantly, pseudopodi and body with ecto and endoplasm, lobopodia many, indeterminate a directing locomotion, without shell or pellicle, covered with plasmalemma usually uni or binucleate, single contractile and food vacuoles conspicuous.
Verticellidae
55. Vorticella
Body inverted bell shaped, solitary (never colonial), usually in groups stalk unbranched, long and contractile, it is secreted by aboral tip, oral groove, circular around the edge of the cup and extends inward, plugged by a raised disc, two circles of cilia on the disc, endoplasm with a long and horse-shoe shaped macronucleus and a small micronucleus, colourless, green or blue.