Animal Kingdom - Basis of classification

Levels of Organisation: Animals exhibit different levels of complexity: Cellular level ($Porifera$), Tissue level ($Coelenterata$ and $Ctenophora$), Organ level ($Platyhelminthes$), and Organ-system level ($Aschelminthes$ to $Chordata$).

Symmetry: Animals can be Asymmetrical (sponges), Radial (any plane through the central axis divides the body into identical halves, e.g., $Coelenterata$, $Ctenophora$, and adult $Echinodermata$), or Bilateral (body can be divided into identical left and right halves in only one plane, e.g., $Annelida$, $Arthropoda$, $Chordata$).

Germ Layers: Diploblastic animals possess two embryonic layers: external $ectoderm$ and internal $endoderm$, with an undifferentiated $mesoglea$ in between. Triploblastic animals possess a third germinal layer, the $mesoderm$, between the $ectoderm$ and $endoderm$.

Coelom (Body Cavity): The presence or absence of a cavity between the body wall and the gut wall is vital. $Coelomates$ possess a true coelom lined by $mesoderm$. $Pseudocoelomates$ have $mesoderm$ present as scattered pouches ($Aschelminthes$). $Acoelomates$ lack a body cavity ($Platyhelminthes$).

Metamerism: In some animals like $Annelida$, the body is externally and internally divided into segments with a serial repetition of at least some organs. This phenomenon is known as metamerism.

Notochord: A mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with a notochord are $Chordates$, and those without are $Non-chordates$ ($Porifera$ to $Echinodermata$).