In this article we will discuss about Phylum Rotifera:- 1. Taxonomic History of Phylum Rotifera 2. Characteristics of the Phylum Rotifera 3. Classification 4. Affinities.
Taxonomic History of Phylum Rotifera:
1. Leeuwenhock (1703) first discovered rotifers with his newly invented microscopes.
2. Linnaeus and Lamarck regarded rotifers as protozoa.
3. Ehrenberg (1838) also placed rotifers in a distinct class under Infusoria.
4. But recent workers like Remane and Myers (1933) placed the rotifers under a separate and independent phylum.
Characteristics of the Phylum Rotifera:
1. Phylum Rotifera are microscopic animals, mainly found in freshwater, rarely in marine or parasitic.
2. Body wall of Phylum Rotifera generally lacks a cuticle and thickened into stiff plates or lorica into which the head may retreat.
3. Anterior end with a ciliated organ called corona helps in swimming and feeding.
4. Posterior foot of Phylum Rotifera has two toes; foot with cement glands.
5. Cuticle secreted within epidermis and never moulted.
6. Digestive system with a highly muscular pharynx called mastax lined internally with cuticle and within mastax is a rigid structure or jaws called trophi used for grasping and grinding the prey.
7. Pseudocoelomate animals.
8. Eutelic condition is seen in Phylum Rotifera.
9. Excretory organs are protonephridia with flame cells. The protonephridia function in osmoregulation.
10. Nervous system includes cerebral ganglion with longitudinal nerve cords.
11. Sexes in Phylum Rotifera are separate (gonochoristic).
12. Parthenogenesis is largely present in Phylum Rotifera.
13. Spiral cleavage.
14. No larval stage in the life cycle.
Classification of Phylum Rotifera:
The phylum rotifera is divided into 3 classes:
(i) Seisonidea
(ii) Bdelloidea and
(iii) Monogononta.
Class 1. Seisonidea:
1. Elongated body with reduced corona.
2. Lateral antennae and toes absent.
3. Males fully developed and with little sexual dimorphism.
4. Gonads paired in both sexes.
5. Ovaries without vitellaria.
6. Epizoic marine rotifers.
Example:
Single genus Seison.
Class 2. Bdelloidea (Digononta):
1. Anterior end retractile and usually with two trochal discs.
2. Mastax adapted for grinding with ramate trophi.
3. Cylindrical body with very small forked tail.
4. Ovaries with vitellaria.
5. Males are absent.
6. Parthenogenesis only.
7. Creeping movements by the contractile body.
It includes the following families:
Family Habrotrochidae:
Trochal discs are present. Corona can be retracted into mouth Oviparous.
Example:
Habrotrocha.
Family Philodinidae:
Body is divided into several segments. The foot is retractile. The corona has trochal discs which look-like two wheels. The members are oviparous or viviparous.
Examples:
Philodina, Rotaria.
Family Adinetidae:
The cilia are not on trochal discs but are on the corona. Rostrum is imperfect. They are unable to swim.
Example:
Adineta.
Class 3. Monogononta:
1. Females with a single ovary.
2. Males occur in some species.
3. Parthenogenesis common.
4. Sexual dimorphism well-marked.
5. Body either naked or covered with a lorica.
6. Mastax mostly grinding type but not as in the bdelloids.
The class is divided into three orders:
(1) Ploima
(2) Flosculariacea and
(3) Collothecacea.
Order 1. Ploima:
1. The posterior foot with two toes.
2. Body with or without a lorica and sometimes sac-like.
3. The ciliated disc acts as locomotor organ.
4. The tail is usually forked and retractile.
5. Mostly free-swimming.
It includes the following families:
Family Notommatidae:
Cilia may be arranged uniformly on the corona or may form a ring-like around the corona. Most rotifers are either illoricate or with a fragile lorica. The foot bears two toes. The trophi is virgate type.
Example:
Scaridium.
Family Synchaetidae:
The conical shaped bodies bear either literal appendages or three to four sensory bristles. Foot may be present or absent. The trophi is of virgate type.
Examples:
Polyarthra, Synchaeta.
Family Asplanchnidae:
Rotifers are pelagic, flattened sac-like bodies. Corona has only one circumapical plate. Foot is without toes. The mastax is of incudate type.
Example:
Asplanchna.
Family Brachionidae:
They are freshwater animals and having a broad and flattened body. The foot is ringed with two toes in some (e.g., Brachionus) but absent in Keratella. The mastax is malleate or submalleate type. Other examples are Epiphanes, Macrochaetus, etc.
Family Coluridae.
Example:
Colurus.
Family Lecanidae.
Example:
Lecane.
Order 2. Flosculariacea:
1. Some are free swimming and the rest are sessile.
2. Foot is without toes.
3. Many sessile species live in protective tubes.
4. The larger bodies are provided with lobed corona.
Examples:
Lacinularia, Sinantherina, Floscularia, Hexarthra, Testudinella.
Order 3. Collothecacea:
1. They are sessile rotifers with funnel- shaped anterior end.
2. Corona is large and the mouth is situated at its centre.
3. The margin of the infundibular corona is lobed and is provided with long bundles of setae
4. The mastax is of uncinate type.
Examples:
Collotheca, Stephanoceros.
Affinities of Phylum Rotifera:
Since their discovery, rotifers have a very uncertain systematic status. Rotifers exhibit superficial similarities with many invertebrate groups, namely Arthropoda, Annelida and Platyhelminthes
A. Affinities with Arthropoda:
Resemblances:
(i) Body covered by a cuticle.
(ii) Superficial metamerism.
(iii) Presence of two jaws (trophi)
(iv) Movable bristle bearing arms of pedalia suggest the appendages of a crustacean larva.
Remark:
As the similarities are superficial, that the relationship can’t be drawn.
B. Affinities with Annelida:
The annelidan relationship of rotifers as advocated by Hatschek (1878) is based on structural resemblance between Trochophore larva of annelids and a peculiar rotifer, Trochosphaera. The ciliary girdle, bent intestine and excretory organs of Trochosphaera are similar to the corresponding parts of trochophore.
Remark:
For the above resemblances Hatschek propounded his famous ‘Trochophore theory’ which proposes that living rotifers are closely related to the ancestral Mollusca, Annelido and certain other groups. The annelid theory concludes that the rotifers are simply annelids that have remained in a larval condition.
But this theory fails because Trochosphaera is merely a peculiar rotifer with a modified girdle-type corona.
It is assumed that such similarities are regarded as a case of coincidence without having any phylogenetic significance.
C. Affinities with Platyhelminthes:
Similarities:
(i) Primitive type of corona may have been derived from a complete or ventral ciliation in turbellarians.
(ii) Formation of trophi is also common in turbellarians.
(iii) The protonephridial system with flame cells is identical with that of rhabdocoels.
(iv) The retrocerebral organ is probably homologous with the frontal organs of turbellarians.
Dissimilarities:
(i) Presence of an anus in rotifers.
(ii) Lack of sub-epidermal continuous muscles.
(iii) Lack of epidermal nerve plexus.
Remark:
The above resemblances do not suggest that they are closely related to turbellarians than that of other groups. The study of development of rotifers suggests that they are of primitive forms, not a degenerate group.
D. Affinities with Nematoda:
Similarities:
(i) A syncytial epidermis.
(ii) Pseudocoelomic body cavity in both cases.
(iii) Presence of a gut with mouth and anus.
(iv) Lacking a circulatory system in both cases.
(v) Cleavage determinate type in both.
(vi) No larval stage in the life cycle.