PHYLUM ROTIFERA (rota, wheel ± (era, to bear)
Characteristics of the phylum Rotifera are:
- 1. The rotifers derive their name from characteristic ciliated organ called corona. Corona is present around lobes on the head. The cilia of the corona do not beat simultaneously. Rather each cilium beat earlier than the next cilium. A wave 01 beating cilia appears. It passes around the periphery of the ciliated lobes. It gives the impression of a pair of spinning wheels. That is why the rotifers were earlier called wheel animalcules.
- 2. Rotifers are small animals. They are abundant in most freshwater habitats. A few species are marine. There are two thousand species of rotifera.”They are divided into three classes.
- 3. The body has a thousand cells. The organs are eutely.
- Rotifers are solitary, free- swimming animals. Some are colonial. Others occur between grains of sand.
- They are triploblastic, bilateral, unsegmented, pseudocoelomate
- They have complete digestive system with specialized organs.
- Anterior end has a ciliated organ called a corona.
- Posterior end has toes and adhesive glands.
- Well-developed cuticle is present.
- Protonephridia with flame cells are present
- Males are reduced in number or absent. Therefore, parthenogenesis is common in them.
1. Cuticle: A cuticle covers rotifers external surface. In many species, the cuticle thickens to form lorica. Lorica is case that covers the body. The cuticle or lorica provides protection. The fluid in the pseudocoelom provides hydrostatic support. But cuticle is the main supportive element.
2. Epidermis: The epidermis is syncytial. In this cac plasma membranes are absent between the nuclei.
3. Head: The head contains the corona. mouth, sensory organs and brain. The corona surrounds buccal field. Buccal field is a large ciliated area.
4. Trunk: The trunk is the largest part of a rotifer. It is elongated and saclike. The anus occurs dorsally on the posterior end of trunk.
5. Foot: The posterior narrow portion of rotifers is called foot. The terminal portion of the fbot hears one or two toes. At the base of the foot are many pedal glands. Their ducts open on the toes. The foot is attached to substratum with the secretions of these glands.
FEEDING AND THE DIGESTIVE SYSTEM
Most rotifers feed on small microorganisms and suspended organic material. The corona! cilia create a current of water. This current brings food particles into the mouth. Digestive system is composed of:
1. Pharynx: Pharynx contains a structure called the mastax (jaws). The mastax is a muscular organ. It grinds food. The inner walls of the mastax contain several sets of jaws called trophi. The trophi have different structures.
2. Stomach: Food passes from the mastax into a ciliated esophagus. It then enters into the ciliated stomach. Salivary and digestive glands secrete digestive enzymes into the pharynx and stomach. Digestion is extracellular and absorption of food takes place in the stomach.
3. Intestine and cloaca! bladder: In some species, ciliated intestine forms cloacal bladder. It receives water from the protonephridia and eggs from the ovaries. It also receives digestive waste. The cloacal bladder opens to the outside by anus. Anus is present at the junction of the foot and trunk.
OTHER ORGAN SYSTEMS
All visceral organs lie in a pseudocoelom. Pscudocoelom is filled with fluid and interconnecting amoeboid cells. Protonephridia opens into cloaca! bladder. The function of protonephridia is osmoregulation. Rotifers exchange gases and remove nitrogenous wastes through body surfaces. The nervous system is composed of two lateral nerves and t bilobed brain. Brain is present on the dorsal surface of the mastax. Numerous ciliary :lusters and sensory bristles act as sensory structures. They are concentrated on short antennae or the corona. One to five photosensitive eyespots are present on the head.
REPRODUCTION AND DEVELOPMENT
Some rotifers reproduce sexually. But parthenogenesis occurs in most species. Smaller male may be produced in some classes. But in other classes no male are produced. In some classes, fully developed males and females are present.
Female reproductive organs
Mostrotifers have a single ovary. A syncytial (multinucleate) vitellarium is attached Nit it. It produces yolk for eggs. The ovary and vitellarium fuse to form a single germovitellarium. After fertilization, each egg travels through a short oviduct. It enters into cloaca bladder and passes out through its opening.
Male reproductive organs
The mouth, cloacal bladder, and other digestive organs are absent in males. A single testis produces sperm. Sperms travel through a ciliated vas deferens and enter into the gonopore. Male rotifers have an eversible penis. It injects sperm into the pseudocoelom of the female. It is called hypodermic impregnation.
1. In one class (Seisonidea), the females produce haploid eggs. This egg is fertilized to develop into males or females.
2. In another class (Bdelloidea), all females are parthenogenetic. They produce diploid eggs. These eggs hatch into diploid females.
3. The third class (Monogononta) produces two different types of eggs: Amictic and mictic.
(1) Amictic cycle: Amictic females produce amictic eggs by mitosis. These eggs are diploid. These cannot be fertilized. Therefore, they develop directly into amictic females. Therefore, first amictic cycle starts. These female develops large populations quickly. Some other eggs become dormant. Another amictic cycle starts more dormant eggs are produced. It occurs before the yearly cycle is over. Winds or birds disperse dormant eggs.
Mictic cycle: Some amictic females produce mictic eggs in summer by meiosis. These are thin-shelled eggs. Mictic eggs are haploid. Some mictic eggs are not fertilized. These develop parthogenetically into a male. Some mictic eggs are fertilized and secrete a thick, heavy shell. These become dormant or resting winter eggs. Dormant egg is hatch in melting snows and in spring rains. They develop into amictic females.
Most females lay amictic or mictic eggs but not both. The physiological condition during of the female oocyte development determines whether her eggs will be amictic or mictic.