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In this article we will discuss about Rotifers:- 1. Shape and Size of Rotifers 2. Structure of Rotifers 3. Body Wall 4. Body Cavity 5. Digestive System 6. Respiratory System 7. Excretory System 8. Nervous System 9. Reproductive System 10. Development 11. Affinities.
Shape and Size of Rotifers:
Rotifers are commonly called as “Wheel animalcules”. The name has derived from Latin words, rota-wheel, ferre – to bear and means ‘wheel-bears’. This refers to the rapid movement of cilia on the head, producing the appearance of a rotating wheel.
Rotifers are abundant fresh water animals and are smallest metazoa. Females are common, males are rare and absent in many species. About 2000 species of rotifers have been described.
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Shape:
Extremely variable, slender and worm like, broad, flattened sacciform or even spherical.
Size:
Minute, usually microscopic, normal range between 100 to 500 u in length.
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Symmetry:
Bilaterally symmetrical.
Colour:
Usually they are transparent and colourless, some may show brown, red, and orange colours in their digestive tract
Structure of Rotifers:
The rotifer body is divided into an anterior head, a middle trunk and a posterior tail or foot. A neck may separate head and trunk.
1. Head:
Central part of head is without cilia. This is known as the apical field. The ciliated corona or trochal disc encircles the apical field. In some cases the corona is surrounded by a double ciliated ring, the outer ciliary band cingulum and inner ciliary band or trochus. Ciliary movement helps in feeding and respiration.
2. Trunk:
Cylinderical or flattened covered with lorica, made up of cuticle. Lorica may be an immovable case or divided into telescoping rings, which allow the body to extend and contract. At the anterior end of the trunk, single or paired dorsal antennae are present. Lateral antennae may be present on the posterior end of the trunk. The mid-dorsal anus is located at the union of trunk and tail.
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3. Tail:
It is also called foot and is covered by cuticular rings. It ends in an adhesive disc in sessile species. In creeping and swimming rotifers foot ends in one to four movable toes containing pedal glands. The pedal glands secrete an adhesive used to attach the animal permanently or temporarily for feeding or creeping.
Body Wall of Rotifers:
It consists of cuticle, epidermis and subepidermal muscles. Cuticle is made up of scleroprotein. It covers the body surface forming lorica, spines and other surface structures. The epidermis is syncytial containing a constant number of nuclei. Epidermis also give rise to subcerebral glands, pedal glands and other surface glands.
Muscles are not arranged in layers. Visceral muscles occur in some organs. Circular muscles are complete rings. Longitudinal muscles are well developed in creeping forms.
Body Cavity of Rotifers:
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It is the spacious pseudocoel derived from embryonic blastocoel. It lacks peritoneum and mesenteries. Pseudocoel is filled with a perivisceral fluid and a loose syncytial reticulum composed of amoeboid cells. These cells seem to be phagocytic and excretory in nature.
Digestive System of Rotifers:
The mouth usually lies in the buccal field. Sphincter and dilator muscles may open and close it. Mouth leads, by a ciliated buccal tube, into the pharynx.
The pharynx or mastax is a unique rotifer structure. It is a muscular chamber containing hard chitinous jaws called trophi. Jaw structure is complex. A trophi consists of seven main pieces. A median fulcrum and pairs of rami, unci and manubria make up the major parts.
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The fulcrum and rami together form the incus the unci and manubria compose the malleus. The posterior pointed end of manubrium is known as cauda. Jaw structure is variable according to food and feeding habits. Jaw structure is important in taxonomy.
The pharynx leads through a short, narrow and dorsal oesophagus. It is followed by a thick-walled ciliated sac or tube called stomach. Stomach leads to a thin-walled syncytial intestine. It receives the protonephridia and oviducts near the anus continuing as the cloaca. The dorsal anus often has dilator muscles. Intestine coaca and anus are lacking in pelagic forms like Asplanchna and Ascomorpha.
Rotifers feed on Protozoa, other microscopic organisms and debris swept by ciliary action.
Respiratory System of Rotifers:
No respiratory organs, and little is known of respiratory physiology. Some rotifers have ability to respire anaerobically.
Excretory System of Rotifers:
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It consists of a pair of coiled, syncytial protonephridial tubules. Fine capillary tubules terminate in flame bulbs. Sometimes a tranverse tubule called Huxley’s anastomose connects the two main tubules. Protonephridia drain into the cloaca, by the way of a common duct or a bladder. Flame-cell activity varies with the osmotic pressure of environment.
Nervous System of Rotifers:
The brain consists of a single, dorsal, bilobed supra-pharyngeal ganglion situated above mastax in head. Cephalic sense organs and dorsal antennae are innervated from the brain. Lateral antennae receive nerves from the geniculate ganglion. Caudal sense organs are innervated by nerves from the pedal or caudovesicular ganglion. A pair of longitudinal, ventrolateral nerve trunks arise from the sides of bilobed brain. There is a clear separation of sensory and motor elements in the nervous system.
Sense Organs:
They include stiff bristles (styles), ciliated pits, antennae and eyespots. Stiff bristle occur along anterior edge of the body. Paired ciliated pits, probably chemoreceptive, occur on the apical field. Most rotifers have a dorsal antenna and some have lateral antennae. Antennae are tactile organs. A retrocerebral organ of unknown function is present inside head.
Reproductive System of Rotifers:
Dioecious, sexes separate with sexual dimorphism. Most rotifers are females and all bdelloids are females producing only parthenogenetic ova.
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Males are only a quarter size of females often degenerate without digestive organs. Monogononta males have a single sacciform test is occupying greater part of body. Test is opens into a spermduct ending in the male gonopore. The last part of the spermduct is sometimes modified as a cirrus. One pair to many prostate glands open into the spermduct.
Females have a single syncytial ovary and a syncytial vitellarium. An oviduct leads from the ovary into the cloaca.
Development of Rotifers:
Fertilization is internal. Sperms penetrate the body-wall into the pseudocoel to fuse with ova. Few rotifers have copulatory apparatus and so copulate.
Monogononta females are two types, Amictic and Mictic. Amictic females lay egg which are incapable to fertilize. Amictic eggs are large, thin walled and diploid. They develop parthenogenctically into females of both types. Mictic females lay small, thin walled, haploid eggs.
If not fertilized they develop parthenogenetically into males. If fertilized they become thick walled and diploid dormant resting or winter eggs. These eggs can survive unfavourable conditions and hatch into females during spring.
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Cleavage is spiral and determinate. Gastrulation is epibolic. Stomodaeum and proctodaeum are formed by ectodermal invagination. There is no larval stage, embryo develop into adult without metamorphosis.
Affinities of Rotifers:
Rotifers are biologically peculiar organisms. Many zollogists still keep them as a class of phylum Aschelminthes.
The affinities of these animals can be studied under following heads:
1. Affinities with Platyhelminthes,
2. Affinities with Aschelminthes.
3. Affinities with Annelida.
4. Affinities with Arthropoda.
1. Affinities with Platyhelminthes:
1. Primitive corona homologous to ventral ciliation in flatworms.
2. Presence of cuticularized parts such as trophi.
3. Presence of protonephridia.
4. Retrocerebral organ homologous to frontal ganglion of turbellaria.
5. Division of female gonad into germarium and vitellarium.
However, rotifers differ from flatworms in following respects:
1. Absence of subepidermal continuous muscle.
2. Absence of epidermal nerve plexus.
3. Absence of larval stage and metamorphosis.
4. Lack of power of regeneration.
5. Presence of anus.
6. Presence of pseudocoel.
7. Exhibit cyclomorphosis.
2. Affinities with Aschelminthes:
1. Somewhat worm-like body.
2. No complete cephalization.
3. Presence of Syncytial epidermis.
4. Presence of pseudocoel.
5. Constancy of cell in each organ.
6. Lack of regenerative capacity.
3. Affinities with Annelida:
The peculiar rotifer, Trochosphaera appears almost like a sexually mature trochophore larva with a mastax. This led Hatschek to propose his famous trochophore theory. According to him living rotifers are closely related to the ancestral annelids. That is rotifers are simply annelids which have remained in a larval condition. But recent findings do not support above view.
4. Affinities with Arthropoda:
1. Body covered with cuticle.
2. Superficial metamerism.
3. Presence of jaws.
4. Moving bristles-bearing arms of rotifer Pedalia resemble the appendages of crustacean larva.
But there are a large number of differences between rotifers and arthropods.
Conclusion:
Rotifers are included in the phylum Aschelminthes by many authors. But on account of many morphological, anatomical and embryological peculiarities, it seems preferable to treat them as a separate minor phylum.
