Locomotory Organelles in Protozoa | Microorganisms | Zoology

Four major types of locomotion organelles occur among protozoa; and usually each type of them is characterized by a class:  1. Pseudopodia are characteristics of Sarcodina 2. Flagella are characteristics of Flagellata (Mastigophora) 3. Cilia are characteristics of Ciliata 4. Myonemes are characteristics of Sporozoa.

Type # 1. Pseudopodia:

Pseudopodia are temporary extension of cytoplasm from any part of the body. They occur in those protozoa which are “naked” or have a very thin pellicle. Pseudopodia are formed from ectoplasm but also have a core of endoplasm.

Among Sarcodina, the following kinds of pseudopodia are found:

(i) Lobopodia:

These are relatively broad, finger like or lobe ­like and sometimes branched pseudopodia, typically with rounded tips. They are usually composed of both the ectoplasm and the endoplasm. They are quickly formed and equally quickly withdrawn.

Lobopodia are characteristic of amoeba, although they are also formed by certain flagellates and testaceans (Arcella). Several lobopodia may be given out from the body surface in different directions, as in Amoeba proteus. But in others, like A. limax, the whole body flows into a single lobopodium.

(ii) Filopodia:

It is filamentous and tapering at the tip. It is formed of only ectoplasm. Sometimes they may be branched. Example- Euglypha.

(iii) Reiticulopodia (Rhizopodia or Myxopodia):

These are also filamentous, branching and anastomosing in form a network. Reticulopodia are formed of ectoplasm. Example, Foraminifers.

(iv) Axopodia:

These are long and single radiating pseudopodia. These are formed of an axial filament surrounded by cytoplasmic sheath. Cytoplasm is fluid in nature. Example, Actinophrys and Actinosphaerium.

Type # 2. Flagella:

Flagella are extremely fine fibres having a central axoneme made of two longitudinal fibrils, and an enveloping protoplasmic sheath having nine double longitudinal fibrils forming a ring. All 20 fibrils lie in a matrix of dense cytoplasm and they fuse at the base to join a basal granule or kinetosome.

The kinetosome may be joined to the nucleus by a rhizoplast. The basal granule is often synonymous with a centriole because it initiates nuclear divisions. If it does not act as a centriole then it is connected by a rhizoplast to a centriole or to the nucleus.

Structure:

A flagellum is a thread-like cylindrical or flattened, band-like structure typically consisting of a stiff, elastic axial filament or axoneme, surrounded by a protective contractile outer sheath. The axoneme may be straight or spirally coiled. It consists of nine longitudinal paired peripheral fibres, at equal distance from each other, forming a cylinder and two longitudinal central fibres enclosed by a membranous inner sheath. Each peripheral pair bears a double row of short arms.

All the fibres remain embedded in a fluid matrix. Nine accessory fibres mostly occur between the central and peripheral fibres.

Mastigonemes:

In some flagella, the outer sheath may also contain fibrils which become frayed out laterally under certain conditions. These fibrils are known as mastigonemes or Flimmer. But their real nature is uncertain. Their arrangement seems to be constant within various groups and may prove helpful in studies on taxonomy and phylogeny.

On the basis of mastigonemes, flagella may be of— Stichonematic, Pantonemati, Acronematic, Pentachronematic, and Simple type. Stichonematic type flagella bear a single row of lateral mastigonemes (e.g., Astasia, Euglena) while Pantonematic type flagella bears neither terminal filament nor mastigonemes.

Flagellar Attachment:

Flagellar attachments are generally complex. A flagellum commonly springs from the anterior body-end, either directly (Trichomonas, Eudorina) or from a groove, pit or cytopharynx (Euglena). The axoneme of each flagellum arises from a basal granule or blepharoplast or kineto some.

It is a cylindrical body formed by the bases of the peripheral fibres. It may occur in the cytostome, attached to the nuclear membrane or inside the nucleus. The basal granule is usually connected to the nucleus by fibrils, called rhizoplast, which may also connect it with a parabasal body of unknown function, especially in some parasitic flagellates (Trichomonas, Giardia, Trypanosoma, etc.).

Number of Flagella:

The number and arrangement of flagella varies greatly in Mastigophora. They may be one to eight in number; several flagella occur in parasite forms (e.g., Giardia intestinalis). Free-living forms (e.g., Euglena) are usually with one to two flagella. Flagella are often divided into two types – tractella and pulsella. The tractellum is situated at the end that is anterior when the organism is in motion and drags the body along; the pulsellum is generally situated posteriorly and pushes or propels the body forward.

In Bodo and Aesonema there are two flagella. One is directed forward, drags the body and is called tractellum, while the other is directed backwards along the side of the body and acts as an organ of propulsion, it is known as trailing flagellum.

Type # 3. Cilia:

The cilia are highly vibratile small ectoplasmic processes. They arise from the basal granules (or blepheroplasts or kinetosomes) in the ectoplasm. Cilia form a characteristic feature of ciliates but they are also found in the larval stages of Suctoria. In primitive forms (e.g., Opalina, Paramecium etc.) cilia cover the entire body, but in more specialized forms (e.g., Vorticella) cilia are restricted to certain regions of body only.

They are commonly arranged in longitudinal, diagonal or spiral rows. Cilia may be of equal length all over the body or may be longer at definite spots (e.g., cilia of caudal tuft of Paramecium are longer than other body cilia).

The infraciliary system of ciliates differs from that of flagellates in the following respects:

(i) Cilia are shorter and numerous than flagella.

(ii) In case of ciliates, the infraciliature is not joined to the nucleus not are kinetia inter-connected. In flagellates rhizoplasts join the kinetosomes to the nucleus and the kinetia may be inter-connected.

(iii) In case of ciliates, during cell division the cleavage is perkinetal because it cuts across all kinetia. The upper halves go to one daughter cell and the lower halves to the other. Such type of division is known as homothetigenic in which the daughter cells are duplicates of each-other. In cell division of flagellates the cleavage is interkinetal because it is longitudinal and parallel to kinetia.

So that kinetia are not cut but are shared by daughter cells. Such type of division is known as symmetrigenic in which the daughter cells are not duplicated for mirror images of each other. The normal number of kinetia of an animal is restored by division of kinetosomes.

(iv) Like Flagella, cilia have no mastigonemes. Usually the cilia remain arranged in longitudinal diagonal or spiral rows, springing from either ridges or furrows. The manner of arrangement is rather constant within a species. The cilia may be found all over the body (like Opalins, Paramecium) or may be restricted to certain parts/of the body, referred to as ciliary fields or ciliary zones (like Vorticella). The term pectinella is sometimes used for such a small row of close-set cilia.

Structure:

A cilium is morphologically similar to a small flagellum and consists of an axial filament or axoneme surrounded by an elastic sheath continuous with the plasma membrane of the cell surface. Electron microscope shows that the axoneme consists of 9 paired peripheral fibres and 2 central fibres, the later enclosed within a delicate inner sheath, and all present in a fluid matrix.

One microfibre of each peripheral pair gives out a double row of short projections, called arms, all pointing in the same direction. In between the outer and inner fibre rings is present, nine spoke-like radial lamella. The cilium arises from a basal granule or bleplaroplast situated deep in the ectoplasm.

It is a tubular body formed by the basal ends of the 9 peripheral fibres, each made of a triplet of 3 microfibres arranged in a twisted fashion. According to Lenhssek and Henneguy (1898), the basal granules are centrioles or their derivatives. In Coleps, Stentor and many others, besides vibratile locomotory cilia, are also found stiff, motionless, sensory cilia, equipped with special fibrils and basal granules.

Specialized Organelles:

The cilia may form the following composite motile organelles which are found in several protozoan groups. These are membranelles of Ciliata (Paramecium), undulating membrane of Spirotricha and Peritricha and cirri of Hypotricha. Cirri are tuft-like brushes of functionally fused somatic cilia. Their kinetosomes form a basal fibre plate. Undulating membranes are formed of a row of oral cilia. Membranelle is flat plate of oral cilia with a basal fibre plate. They are arranged in a longitudinal series and beat in a coordinated fashion.

Differences in Cilia and Flagella:

Cilia are similar to flagella in structure, and function but differ in following ways:

i. They are short in length than flagella.

ii. They are larger in number.

iii. Cilia possess no mastigonemes as found in flagella.

iv. No connection of basal granule with nucleus in cilia. In flagella the rhizoplast join the basal granules to the nucleus.

Type # 4. Myonemes:

The myonemes are extremely fine and highly contractile structures running in various directions in the pellicle or ectoplasm of various Protozoa. They serve chiefly to alter the shape of the body. Some Protozoa, like Amoeba, which do not possess myonemes, can also change their shape, but this is due to the general contractility of the protoplasm.

The myonemes may be in the form of ridges and grooves (e.g. Euglena), or microtubules (e.g. Trypanosoma), or myofibrils (e.g. ciliates). The myonemes find their greatest development in Ciliata. They are band-like and cross-striated in Stentor. In certain stalked ciliates (Vorticella) the longitudinal myonemes of the body proper converge, basally into the stalks forming the spasmonemes.

In some ciliates (Stentor) and larger gregarines (Monocystis agilis), the myonemes lie inside individual hyaline ectoplasmic canals. In many Gregarinida, they are arranged longitudinally, transversely and spirally and apparently used in locomotion. In certain Radiolaria (Acanthometron) 10-30 short, thick myonemes (myophrisks) are connected basally to each radiating spicule. These serve as hydrostatic organelles since their contractions and expansions cause variations in the volume of the body, which is thus enabled to rise or sink in the water.