Continuous formation of new pseudopodia keeps amoeba in constant locomotion .This is called amoeboid movement .It occurs in many other protozoans , in amoebocytes of sponges and in leucocytes of vertebrates, It is the simplest mode of animals locomotion;
As observed ,microscopically amoeboid movement in amoebae depends upon three basic factors:
1. Attachment to substratum : Since amoeboid movement involves crawling, it needs contact of the body with a solid substrantum . Adhesion with substratum is facilitated by glycocalyx and also by traces of inorganic ions *(Ca++, Mg++. K+) that are always present in surrounding water .
2. Continuous viscosity conversions : Conversion of plasmagel (ectoplasm) into sol at the trailing end of plasmasol (endoplasm ) into gel at the advanceing end must constantly occur.
3. There must be some force to cause forward flow of plasmasol (endoplasm) from behind into a pseudoodium.
Amoeba crawls about 0.02 to 0.03 mm per minute. A pseudopodium can form at any point on body surface. It first appears a small fingerlike bulge of homogeneous glistening fluid, called hyaline cap .As the bulge touches a sub stratum, there is a sudden rush or granular endoplasm into it ,The bulge therefore elongates and become a pseudopodium .In a roteus several pseudopodia usually being to form simultaneously in different directions, but only one ultimately elongates all other are withdrawn back into the body .Consequently there is some displacement of body in the direction of the completed pseudopodium ,soon a new pseudopodium similary forms elsewhere and the organism, therefore moves in a different direction .The movement is thus quite erratic and never in the same direction for long.
How pseudopodia form and locomotion is affected is still imperfectly understood. The following theories have been postulated in this connection.
1. Adhesion Theory : According to this earliest view amoeba moves like a drop of water streaming out in the path of greatest adhesion upon rough surfaced substratum.
2. Contraction Theory :: In early 19th century , a pseudopodium was regarded a hernia like protrusion bulged out a weak point , probably due to contraction of body mass elsewhere .In 1835, scientists believed that amoeba contains contractile strands of gel which pull the whole body mass in the direction of a advancing pseudopodium .Later when Heitzmann advocated that the body of amoeba was a 3-dimensioanl . network ( reticular theory of cytoplasmic structure ) of live contractile fibres embedded in a non living and non contractile fluid schultze (1861) postulated that a contraction at the rear end of body pushes or squezes the sol ahead .
3. Surface tension theory :Berthold (1886) postulated that constant internal tension forces the cytoplasm to shoot out from surface in the form of a pseudopodium wherever surface tension suddenly lower due to local effects. Butschli (1892) and rhumbler (1898) supported this view, surface tension does exits on a fluid surface . but its existence upon the surface of plasmalemma of amoeba is doubtful.
4. Theory of rolling movement : By dropping a particle of lamp black on the surfac of a verrucosa Jennings (1904) noted that amoeba simply rolls like a ball during locomotion ,Such a movement is possible only in unipodal forming a single pseudopodium at a time species like A verrucosa but not in bi or mulitpodal amoeba .
5. Walking movement theory :: When we observe an amoeba under the microscope , we get the impression that it is creeping in close contact with the substratum. Dellinger (1906) viewed amoebae from a side and noted that, during locomotion the main part of body remains lifted from the substratum and supported on tips of pseudopodium thus amoeba virtually walk on pseudopodia just as higher animals move on the legs .He explained that amoeba extends a pseudopodium free in water and swings it about .As the pseudopodium strikes an objects it sticks to it, this causes contraction of an endoplasmic of an endoplasmic fibrous network of contractile elements, pulling the whole body mass close to the pseudopodium .
6. Pressure or Ectoplasmic contraction or sol gel theory : When cytoplasm was finally recognized as a colloidal system Hyman (1917) schaudinn and others suggested that con version of ectoplasm into endoplasm and vice versa was the chief basis of amoeboid movement ,actually worked out by pantin (1923) and mast (1926,1931) this theory postulated that pseudopodia are formed and withdrawn due to cycle viscosity change in that colloidal cytoplasm from sol to gel and gel states. Locomotion in this way was explained by four main processes .a
a. Contact of plasmalemma lof advancing side of body with a substratum and resultant weakening of plasmagel ( ectoplasm) layer here due to its partal solation .
b. Sudden forward streaming of plasmasol ( endoplasm ) under tension which breaks the weakened plasmagel sheet and converts into gel after striking against the hyaline cap.
c. Conversion ofplasmagel into plasmasol in trailing part of body
d. Simultaneous contraction in the liquefying plasmagel at the rear end.