Sarcodina

In the 20^th century, protozoans were classified based on their modes of nutrition and locomotion. They were divided into various taxonomic groups - Mastigophora(based on locomotion with flagella), Sarcodina (based on locomotion with pseudopodia), Ciliophora(based on locomotion with cilia), and Sporozoa (parasites that do not have specific locomotory structures). 

But these divisions were not accurate from the evolutionary perspective and thus this classification is considered outdated. According to the classification given by Honigberg, protozoans are divided into 4 subphylums and superclass Sarcodina is included under the subphylum Sarcomastigophora. 

The members of the superclass Sarcodina are known as sarcodines. It compromises Amoeba and other related organisms. The key feature of the organisms of this class is the presence of pseudopodium, it has a role in trapping food for the ingestion process performed by the organisms. Pseudopod can be defined as the temporary extension of the protoplasm. They can be singular as a single blunt lobopodium with simple anatomy or they can be found in the bundle as reticulopodia which protrudes from a foraminifera shell.

Features of the Sarcodines 

Sarcodines anatomy is very simple as it only consists of a single cell, with abundant protoplasm. All the metabolic processes occur in the protoplasm. Amoeba is the key organism that is used to study the physiological and biochemical features of the superclass Sarcodina. Amoeba is also used for the phylogenetic study of the superclass. Phylogenetic history and relation are generally hypothesized using the genetic and proteomic data of the organism. Some of the general characteristics and features of sarcodines are as follows. 

1. Most of the organisms of this class are free-living carnivores.

2. They engulf food by pinocytosis.

3. Some of the strains of protozoans are parasitic. For example, amoebic dysentery is caused by the sarcodine family protozoa Entamoeba histolytica.

4. Most of them are free living.

5. Most of them are sessile, that is they are attached to some kind of surface and can not perform voluntary locomotion. Locomotion is rather achieved by external forces, such as water current, laminar flow, etc.

6. Sarcodines are further classified into three main classes that are - Rhizopodea, Piroplasmea, and Actinopodea.

7. The class differentiation among sarcodines is based on the type and feature of pseudopodia. 

Classification of Sarcodina

It is based on classification by B.M Honigberg. Sarcodina is classified under the phylum protozoa. Protozoa are further classified into subphylum Sarcomastigophora, Sporozoa, Cnidospora and, Ciliophora. The subphylum Sarcomastigophora is further classified into three superclasses, Sarcodina, Mastigophora and, Opalinata. The class Sarcodina is then further classified into subclasses of Rhizopoda, Piroplasmea, and Actinopoda.

• Rhizopoda

The class Rhizopoda includes the following groups: Lobosea, Acarpomyxea, Acrasia,  Eumycetozoea, Plasmodiophorea, Filosea, Granuloreticulosa, and Xenophyophores. The most common example of this class is the Amoeba, which belongs to the subgroup of Lobos. The common classification pattern is based on the similarities and differences among the pseudopodia of the specific group. The locomotion appendage is pseudopodia, it is known as lobopodia in Amoeba. The common features of this class are as follows:

1. They have a naked body, thus their shape is not rigid.

2. They use pseudopodia as locomotory appendage

3. The cytoplasm is classified into an endo and ecto cytoplasm.

4. Contractile vesicles are present in nonpathogenic strains while they are absent in pathogenic strains. 

• Piroplasmea

The members of this class are generally parasitic. The absence of locomotory organs has been observed in them. Example- Babesia.

• Actinopoda

They have a protrusion appendage called axopodia.  Axopodia differs from pseudopodia in the shape and the cytoskeleton arrangement as they possess axial filament. The cytoskeleton has a microtubular framework that provides permanence to the structure relative to pseudopodia found in other protozoans. The common groups that are classified under it are as follows: Acantharea, Polycistinea, Phaeodarea, and Heliozoea. Some common features are found in this class, are as follows,

1. They are free-floating in nature.

2. They are classified as platonic.

3. They have a special appendage called axopodia that serves as a locomotory tool.

4. Axopodia also performs pinocytosis.

5. Some groups of this superclass have a skeleton made up of silica, which provides them a relatively rigid structure.

6. Asexual reproduction takes place by binary fission.

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How Do Sarcodines Eat?

Sarcodines have a unique process of eating. They engulf the food particle by pinocytosis, pinocytosis is mediated by special appendages that protrude from the cell membrane of the protozoan. Pinocytosis can be explained as the process of engulfment of small particulate matter or liquid that is suspended in the extracellular fluid. The plasma membrane of the cell invaginates from the surface, encircling the target particle. 

It then completely encapsulate the particle by forming a vesicle-like structure around it, the vesicle is then carried into the intracellular compartments of the cell, where it is metabolized. Pinocytosis is also referred to as fluid endocytosis. The food particle is then digested by breaking into small monomers, this is done by enzymes of the protozoan cell. 

The major digestive enzymes are as follows acid phosphatase, acid protease, acid phosphomonoesterase, alpha-glucosidase, beta-glucosidase, and amylase. It is to be noted that pinocytosis is an active process, that is it requires ATP for the completion of the process.

Diagrammatic Representation of Pinocytosis

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Conclusion

This is all about Sarcodina and its features. Understand the features of this protozoan species and differentiate it through its specific classification cited in this article. Develop your conceptual foundation in this topic by studying this organism in detail with proper images.