What are the Major Groups of Myriapods?

The main feature of Rhizopoda is that the cytoplasm of the worm body can be extended to form pseudopods, which are organelles for movement and feeding.

Rhizopoda

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The body surface has a thin layer of cell membrane, which makes the worm body very elastic. It can change the shape of the worm body and perform amoeboidmovement. Most types of camps live freely, minority groups live, freshwater and seawater are distributed, and very few species are parasitic.

Rhizopoda has a simple structure with few organelles, and seems to be the most primitive protozoa. However, many species have complex "skeletal" structures, all of which are heterotrophic. Flagellar gametes appear in life history. It may be more evolved than flagellates.

Many animals of the Rhizopoda are not fixed in shape, and there is a thin cell membrane on the surface. For example, the naked amoeba, Amoebaproteus, lives on the surface of ponds, streams, and silt. The shape of the insect body can be extended to form a pseudofoot. The direction of the pseudofoot's extension represents the temporary front end of the body. Because the new pseudofoot can be continuously extended, the body shape is not fixed. It is short and stiff, and contains flowing cytoplasm. This pseudofoot is called lobopodium. Under the light microscope, the worm body can be clearly divided into a colorless and transparent ectoplasm and an endoplasm with granular opacity. The ectoplasm contains telescopic vesicles, food vesicles, and particulate matter of varying sizes. The amoeba's nucleus is disc-shaped, usually in the endoplasm of the center of the body.

The structure of many species of Rhizopoda is more complex than that of naked amoeba. The surface of the worm body can be formed with different shapes of outer shells or inner shells. Some types of cells can secrete mucus and adhere to fine sand particles, forming a Sandy shells, such as Difflugia; or chitin secreted by the cytoplasm, forming a chitin shell, such as Arcella, etc .; or calcium carbonate secreted by the cytoplasm, forming a single or multi-chamber Calcareous shells, such as Foraminiferida; this calcareous shell is arranged in various forms. Other species can form siliceous shells, or siliceous shells, such as Euglypha; or siliceous inner shells, located in the cytoplasm, called central sacs, such as Haliozoa; also Some species can extend long bone needles outside the body, such as radiodiolia (Radiolaria).

The shape of the pseudofoot is also different. The large amoeba is a leaf-type pseudofoot, which is formed by the outer and inner substance. Some crusty amoeba (Testacea) pseudofoots are slender, pointed at the tip, and consist only of exoplasm. These pseudopods are called filopodium. Foraminiferous pseudopods are also slender as silk, but then the pseudopods branch again, and the branches are connected to each other to form a net or root. This pseudofoot is called rhizopodium. Sunworms and radiolariae pseudopods are also slender as silk. There is a bunch of microtubules in the pseudopod to form a shaft, which supports the surface. The surface of the pseudopod is a thin layer of protoplasm, often sticking some particles. Shortened, or retracted, so the shaft is not a skeletal structure. This pseudofoot is called axopoidium.

The organelles of rhizopods are pseudopods. Due to the different structures of pseudopods, their movements are also different. Generally speaking, "deformation movement" refers to the movement mode of leaf-type pseudofoot. This kind of movement is very clear in the big amoeba. The surface of the big amoeba has a very thin cell membrane. The cytoplasm is divided into exoplasm and endoplasm. The endoplasm can be divided into solid gelatin (plasmamagel) and liquid plasmasol (plasmasol). During exercise, the gelatinous substance at the rear end of the worm body causes liquid pressure due to the contraction of the protein, forcing the solaceous substance to flow forward while extending the pseudofoot. The top of the pseudofoot forms a transparent layer (hyalinelayer). After flowing to the front end, the pressure is reduced, the gelatinous material becomes thinner, the solitary substance in the transparent layer region flows back from front to back, and the solitary substance becomes gelatinous. Deformation movements are repeatedly formed. Regarding the mechanism of deformation movement, different opinions still exist. One view believes that the motive force during exercise comes from the contraction of the gelatinous substance at the back end of the body; the other opinion holds that the motive force comes from the contraction of the solitary substance at the front end to drag the cytoplasm toward the pseudofoot, because the gelatinous substance has smaller Stickiness. Recently, someone observed an amoeba section with an electron microscope and found that it contains two kinds of thick and thin microfilaments, which are 16nm and 7nm in length, which are similar to the thick myosin filaments and thin actin filaments of vertebrate striated muscle The muscle contraction is provided by ATP for energy. It moves by sliding the actin filaments on the myosin filaments, and the movement of the amoeba may also rely on the slipping of the muscle filaments in the pseudofoot. For silk-type, root-type and axial-type pseudofoot, because it is composed of ectoplasm, or with a shaft in the pseudo-foot, its movement is different from leaf-type pseudofoot. Under the light microscope, with the flow of particles in the pseudofoot, it can be seen that the protoplasm flows in the pseudofoot in two opposite directions, and flows from the base to the end on one side of the pseudofoot; on the other side, the end is opposite Flow towards the base.

The type of benthic life, crawling forward by dragging the body with pseudofoot. The fully floating species' vertical motion in water is adjusted by increasing or decreasing the foaming of the outer substance and the change of oil droplets in the inner substance. The movement in the horizontal direction is aided by water current or wind. Its pseudofoot is not mainly used for movement, but as an organelle for feeding. The extension and contraction of the axial pseudofoot only plays an auxiliary role in the movement.