Animal Kingdom Classification and Body Plans

Criteria for Classification: The classification of animals in the Animal Kingdom is established based on several fundamental morphological and physiological features:
- Levels of Organisation
- Symmetry
- Coelom (Body Cavity)
- Circulatory System (Open or Closed types)

Protoplasmic Level of Organisation:
- Observed in unicellular organisms such as $Amoeba$ .
- All biological body functions are performed by a single cell.
Cellular Level of Organisation:
- The organism is multicellular.
- However, there is a distinct lack of coordination among cells because sensory cells or nerve cells are absent.
- Example: $Porifera$ (adjacent cells).
Tissue Level of Organisation:
- Cells are grouped together to form specific tissues.
- Organisms at this level are often Diploblastic.
- Example: $Cnidaria$ and $Ctenophora$ .
Organ Level of Organisation:
- All various tissues join together to form complex organs.
- The first phylum to exhibit the organ level is $Platyhelminthes$ .
- Characteristics: Bilateral symmetry and Triploblastic nature.
Organ System Level of Organisation:
- Organs are integrated into functional systems to perform physiological tasks.
- Example: $Nematode$ / $Aschelminthes$ .
- Characteristics: Triploblastic and Bilateral symmetry.

Definition: Symmetry refers to the geometrical arrangement of body parts. When an animal is cut into two equal halves through a plane of division passing through the center (principal axis), the resulting halves are termed antimeres, and they are identical/symmetrical.
Types of Symmetry:
- Asymmetry:
  - Occurs when an organism cannot be cut into two equal halves through any plane passing through the center.
  - This is typically due to a lack of a definite shape.
  - Example: $Porifera$ (Sponges).
- Radial Symmetry:
  - When an organism is cut through the central axis (principal axis), the two resulting halves are identical in any plane.
  - Examples: $Cnidaria$ , $Ctenophora$ .
  - Biological Characteristics: These organisms are often marine, sessile (fixed in one place), planktonic, or sluggish.
  - Starfish ( $Asterias$ ) exhibit a specific type called Pentamerous Radial Symmetry.
  - Symmetry in these organisms is often determined during the larval stage.
  - Limitation: Unlike bilateral symmetry, radial symmetry has no cephalization (cephalochord); however, the organism can receive stimuli from every side.
- Bilateral Symmetry:
  - The animal can be cut into identical left and right halves only through a single plane: the median sagittal plane.
  - Example: From phylum $Platyhelminthes$ to $Chordata$ .
  - Advantage: Associated with cephalization (presence of a cephalochord/head region).

Definition: The coelom is the body cavity located between the ectoderm and the endoderm.
History: The term "Coelom" was coined by the scientist Ernst Haeckel.
Types of Coelom:
- Acoelom (Acoelomate):
  - There is no body cavity between the ectoderm and the endoderm.
  - The space is entirely filled with mesenchyme, which is derived from the mesoderm.
  - Animals with this structure exhibit a "Solid Body Plan."
  - Example: $Platyhelminthes$ .
- Pseudocoelom (Pseudocoelomate):
  - The body cavity is present but is not lined with mesodermal epithelium.
  - The cavity is filled with pseudocoelomic fluid.
  - This fluid acts as a hydrostatic skeleton, providing structural support through fluid pressure.
  - Example: $Nematoda$ and $Rotifers$ .
- Eucoelom (True Coelom):
  - The body cavity is present between the ectoderm and endoderm.
  - The gut (alimentary canal) is lined by the Visceral Peritoneum.
  - The body wall is lined by the Parietal Peritoneum.
  - The true coelom is the cavity localized between these two peritoneal layers.
  - Clinical Note: The kidney is described as a retroperitoneal organ (located behind the peritoneum).

Schizocoelom:
- Formation: Developed by the splitting of mesodermal cells/blocks.
- Cleavage Type: Spiral and Determinate cleavage.
- Classification: These organisms are Protostomians.
- Protostome Development: The blastopore (the first opening of the embryo) becomes the mouth. The opposite opening becomes the anus.
- Examples: $Annelids$ , $Arthropoda$ , and $Mollusca$ .
Enterocoelom:
- Formation: Mesodermal pouches are formed from the archenteron (the primitive gut). These pouches eventually join to form the enterocoelom.
- Cleavage Type: Radial and Indeterminate cleavage.
- Classification: These organisms are Deuterostomians.
- Deuterostome Development: The blastopore (the first opening) becomes the anus. The second opening becomes the mouth.
- Examples: $Echinodermata$ , $Hemichordata$ , and $Chordata$ .

Tube within Tube Body Plan:
- The body consists of two tubes: the outer body wall (derived from ectoderm) and the inner gut/alimentary canal (derived from endoderm).
- Example: $Pseudocoelomates$ and $Coelomates$ .
- Advantages:
  1. Allows for continuous body activity.
  2. Facilitates organ specialization.
Blind Sac Body Plan:
- The gut has only one single opening that serves as both the mouth and the anus.
- The gut is considered incomplete.
- Example: $Platyhelminthes$ , $Coelenterata$ , and $Ctenophora$ .
Solid Body Plan:
- The cavity between the Ectoderm ( $Pinacoderm$ ) and the Endoderm ( $Choanoderm$ ) is entirely filled with $Mesophyl$ .
- There is no internal cavity for organ systems.
- Example: $Porifera$ .