Cnidaria and Its Features

Cnidaria

Cnidaria Phylum

The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry believed primitive for eumetazoans. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions.

  
Features:

1. Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella. 
2. Polyps, in contrast, are usually sessile. They have tubular bodies; one end is attached to the substrate, and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense.
3. Reproduction in polyps is by asexual budding (polyps) or sexual formation of gametes (medusae, some polyps). Cnidarian individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and free-swimming.
4. If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. These tiny organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned.
5. A nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death.

Cnidaria Phylum can be classified into following main groups

1. Sessile Anthozoa (sea anemones, corals, sea pens) 
2. Swimming Scyphozoa (jellyfish)
3. Cubozoa (box jellies) 
4. Hydrozoa ( Hydra)

Porifera and Its Features

Porifera

Porifera Phylum

Sponges are animals of the phylum Porifera (means "pore bearer"). They are multicellular organisms which have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. Sponges have unspecialized cells that can transform into other types and which often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. 

  

Classes  

Sponges were traditionally distributed in three classes: calcareous sponges (Calcarea), glass sponges (Hexactinellida) and demosponges (Demospongiae).However, studies have shown that the Homoscleromorpha, a group thought to belong to the Demospongiae, is actually phylogenetically well separated. Therefore, they have recently been recognized as the fourth class of sponges 

Calcarea :Its features
Cell structure :Single nucleus, single external membrane. 

Spicules: calcite may be individual or large masses, 

Spongin fibers : never
Massive exoskeleton: made of calcite if present,  

Body form:asconoid, syconoid, leuconoid or solenoid. 

Glass sponges : Its features 

Cell structure :Mostly syncytia in all species 

Spicules: SilicaMay be individual or fused 

Spongin fibers : never  

Massive exoskeleton: never
Body form: leuconoid . 

Demosponges :- Its features

Cell structure :Single nucleus, single external membrane 

Spicules: Silica 

Spongin fibers : never In many species 

Massive exoskeleton: In some species.Made of aragonite if present. 

Body form: leuconoid . 

Homoscleromorpha: Its features

Cell structure :Single nucleus, single external membrane 

Spicules: Silica 

Spongin fibers : never In many species 

Massive exoskeleton: Never
Body form: Sylleibid or leuconoid

Protozoa and Its Features

Protozoa:- Single-celled eukaryotes or organisms that possess membrane-bound organelles and nuclei are known as Protozoa. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g., photosynthesis. The word protozoa means "little animal." They are so named because many species behave like tiny animals-specifically, they hunt and gather other microbes as food.

 

Features:

1. Protozoa commonly range from 10 - 52 micrometers, but can grow as large as 1 mm, and are seen easily by microscope. 
2. The largest protozoa known are the deep-sea dwelling xenophyophores, which can grow up to 20 cm in diameter. They were considered formerly to be part of the protista family. 
3. Protozoa exist throughout aqueous environments and soil, occupying a range of tropic levels. 
4. Protozoa mainly feed on bacteria, but they also eat other protozoa, bits of stuff that has come off of other living things-what's generally called organic matter-and sometimes fungi.

Classification :- The classification of protozoa has been and remains a problematic area of taxonomy. Where they are available, DNA sequences are used as the basis for classification but for the majority of described protozoa such material is not available. They have been and still are mostly on the basis of their morphology and for the parasitic species their hosts. Protozoa have been divided traditionally on the basis of their means of locomotion.

Sub Groups of protoza
Flagellates (e.g Giardia lamblia)
Amoeboids (e.g. Entamoeba histolytica)
Sporozoans (e.g.Plasmodium knowlesi)
Ciliates ( ( e.g. Balantadium Coli)

Cell Division and Its Types

Cell Division

Cell Division

Cell division is the process by which a Parent Cell divides into two or more Daughter Cell. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct type of cell division: a vegetative division, whereby each daughter cell is genetically identical to the parent cell , and a reductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half, to produce haploid gametes . Both of these cell division cycles are required in sexually reproducing organisms at some point in their life cycle, and both are believed to be present in the last eukaryotic common ancestor Prokaryotes also undergo a vegetative cell division known as binary fission, where their genetic material is segregated equally into two daughter cells. All cell divisions, regardless of organism, are preceded by a single round of DNA replication.

Types of Cell Division

Generally there are two types of cell division

1. Mitosis cell division :- It is the process by which new cells are generated. Mitosis is the process by which a cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is a form of karyokinesis, or nuclear division. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles, and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle: the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.
2. Meiosis cell division :- It is the process by which gametes are generated for reproduction. Meiosis  is a special type of cell division necessary for sexual reproduction in eukaryotes. The cells produced by meiosis are gametes or spores. In many organisms, including all animals and land plants (but not some other groups such as fungi), gametes are called sperm and egg cells.Meiosis begins with one diploid cell containing two copies of each chromosome:one from the organism's mother and one from its father.

Basic terms to be more cleared on concept of Cell division are as follows :-
 Gene :- basic unit of heredity; codes for a specific trait   

Somatic cell :- all body cells except reproductive cells 

Locus :- the specific location of a gene on a chromosome (locus - plural loci)
Genome :- the total hereditary endowment of DNA of a cell or organism
Chromosome :-elongate cellular structure composed of DNA and protein - they are the vehicles which carry DNA in cells
Gamete :- reproductive cells (i.e. sperm & eggs)
Haploid (n) :- cellular condition where each chromosome type is represented by only one chromosome 
Diploid (2n) :-cellular condition where each chromosome type is represented by two homologous chromosomes
Chromatid :-one of two duplicated chromosomes connected at the centromere
Homologous chromosome :- chromosome of the same size and shape which carry the same type of genes
Centromere :-region of chromosome where microtubules attach during mitosis and meiosis