https://www.sciencearena.in/p/education-resources.html
Euglena
Ø Euglena is free-living and solitary. It occurs in stagnant water of ponds, pools and ditches with decaying nitrogenous organic matter. During rainy season it is so abundant that it imparts green colour to the water.
Ø In Euglena excess of water is
eliminated out by the contractile vacuole. The accessory vacuoles collect
excess of water from the endoplasm and release their contents in the main
vacuole which gradually increases in size and finally contracts to force fluid
into the reservoir. From it fluid escape through the gullet. Along with this water-soluble
wastes are also thrown out of the body.
EXTERNAL FEATURES
1. Shape and size: The body is
fusiform or spindle-shaped with blunt anterior and pointed posterior end, the
size if microscopic varying from 53-100 μ.
2. Anterior end: The blunt anterior end of the body has a funnel shaped
depression. It represents reservoir or flagellar
sac. Its openings is called cytoplasm
or cell mouth. It leads into a spherical reservoir through a short tube,
the cytopharynx or gullet.
3. Flagella: A long thread-like flagellum
arises from the base of reservoir and projects out of cytostome. A small
flagellum remains within the reservoir. The two flagella arise from two tiny
granules, the kinatosomes or blepheroplasts lying at the base of the reservoir.
The long flagellum has a lateral swelling at its base within the reservoir It
is called the paraflagellar body. It acts as a photoreceptor, and contains
lactoflavin as sensitiser.
4. Pellicle: The body is enclosed in a
tough but flexible pellicle or periplast. Its electron microscopic structure
reveals that pellicle consists of helically arranged thin and elastic strips. These fuse at both the ends of
the body. Each strip has a ridge and a groove. The ridge of mine strip fits
into the groove of adjacent strip. Under higher magnification these
articulating edges appear as striations and are called myonemes.
Pellicle lies
inside the plasma membrane and is formed of clastic fibrous protein. Due to
toughness pellicle gives a definite form to the body Its elasticity permits
slight changes in body form.
Under the
pellicle, parallel to its strips are present muscus secreting muciferous bodies and bundles of microtubules.
INTERNAL STRUCTURE
1. Cytoplasm
The
cytoplasm is distinguished into:
(a) an outer
thin, clear layer of ectoplasm.
(b) an inner
granular and more fluid-like central mass of endoplasm. Oblique and
longitudinal myonemes extend in the ectoplasm.
2. Cytoplasmic Inclusions
Nucleus: It is a large oval or spherical
vesicular body. It lies in the broad posterior part of the body.
The position of Euglena is stilla debatable
question as it is considered an animal by zoologists and plant by botanists.
Its inclusion in the kingdom Animalia
is based upon the following facts:
1. The
proteinous nature of pellicle and absence of cellulose covering over the body.
2. Presence
of contractile vacuole.
3. Presence
of photosensitive stigma or eyespost.
4.
Saprophytic and even holozoic mode of nutrition among some species of Euglena.
5. Movement
from place to place.
6.
Longitudinal division of the body.
NUTRITION
Euglena
exhibits mixotrophic type of
nutrition as it feeds in more than one ways:
Ø Holophytic or autotrophic nutrition (Plant-like) In bright sunlight Euglena synthesises its
food from CO2, and water with the aid of chlorophyll (photosynthesis
as found in plants). The synthesised food is a variety of starch, the
paramylum, which is stored in the pyrenoid
bodies or is found scattered as paramylum bodies.
Ø Saprozoic nutrition or saprophytic nutrition-In
the absence of sunlight Euglena absorbs decaying organic matter by the general
body surface. Euglena secretes certain digestive enzymes which are typically
animal-like.
Ø Holozoic nutrition Some species of Euglena are
recorded to feed on solid food particles but it occurrence is doubtful.
Euglena progresses in two different
ways:
1. Flagellar movement: The long highly contractile flagellum acts as a locomotory
organ. It remains directed obliquely backwards towards the side bearing
stigma. It undergoes spiral undulations which are passed from the base to the
tip. The active lashing movements of flagellum produce waves of motion which
propel the body forward with a spiral rotation or gyration along the axis.
Paraflagellar
body is a photosensitive structure. It is located near the base of long
flagellum. It contains lectoflavour, a photo sensitive substance. Due to its
presence the euglena moves towards light.
2. Euglenoid movements: The pellicle being flexible permits the organism to
perform worm-like wriggling movements of the body. A peristaltic expansion
passes along the entire length of the body from the anterior to the posterior
end and the animal progresses very slowly. This is known as euglenoid movement.
The contraction of body is brought about by the contraction of the myonemes of
ectoplasm.
Reproduction
1. Longitudinal binary fission: Under
favourable conditions, Euglena reproduces by longitudinal binary fission. The
nucleus divides longitudinally into two with the duplication of cytopharynx,
reservoir, eye spot, blepheroplasts and contractile vacuole. A constriction
arises at the anterior end and passes backwards resulting in longitudinal
splitting of the body. One of the two daughter euglenae retains the old
flagellum, while a new one is developed by the other.
Some aspects
of Euglena are said to divide in an inactive stage within a mucilaginous
covering around its body.
2. Multiple fission and palmella stage: To
tide over the unfavourable conditions Euglena secretes a gelatinous cyst around
its body. Prior to this it throws off its flagellum, stops swiraming and gets
rounded. In the encysted condition, Euglena divides longitudinally into two but
these may divide further forming 4, 16 or 32 daughter individuals. All of them
remain extangled in a common cyst forming the so called palmella stage. On
commencement of favourable conditions the cyst and daughter euglenae are set
free, which develop flagella and start free existence.
3. Encystment: Under unfavourable
environmental conditions, Euglena secretes a thick, spherical, yellowish-brown
cyst formed of a special carbohydrate. The cyst may be thick, stalked or
operculated. The cyst assists to tide over the adverse conditions and in the
dispersal of organism. On the commencement of desirable conditions the cyst
ruptures and the animal starts an active free life.
FOLLOW
THE INSTRUCTIONS FOR DOWNLOAD THE ASSIGNMENT FOR CLASS IX TO XII:
1. SEARCH WWW.SCIENCEARENA.IN
2. OPEN EDUCATION AND RESOURCES IN MENU BAR.
3. SINGLE CLICK DOWNLOAD AVAILABLE IN DOWNLAOD SECTION.
OR DIRECT OPEN THE ASSIGNMENTS WITH THIS LINK GIVEN BELOW:
https://www.sciencearena.in/p/education-resources.html