Discuss the various levels of organization in the animal kingdom.

(N/A) $\rightarrow$ All members of $Animalia$ are multicellular,heterotrophic eukaryotes.
$\rightarrow$ However,they do not all exhibit the same pattern of cellular organization.
$\rightarrow$ The cells in their bodies are of several types and are organized into functional units of increasing complexity.
$\rightarrow$ The animal body shows four basic levels of structural organization:
$(1)$ Cellular Level:
$\rightarrow$ In this level,the body shows some division of labor among cells.
$\rightarrow$ It is found in sponges $(Porifera)$.
$\rightarrow$ The body consists of many cells arranged as loose cell aggregates,but the cells do not form tissues.
$(2)$ Tissue Level Organization:
$\rightarrow$ In coelenterates $(Cnidaria)$,the arrangement of cells is more complex.
$\rightarrow$ Cells performing the same function are arranged into tissues; hence,it is called the tissue level of organization.
$(3)$ Organ Level:
$\rightarrow$ In $Platyhelminthes$ and other higher phyla,tissues are grouped together to form organs,each specialized for a particular function.
$(4)$ Organ System Level:
$\rightarrow$ In animals like annelids,arthropods,molluscs,echinoderms,and chordates,organs have associated to form functional systems,each concerned with a specific physiological function.
$\rightarrow$ This pattern is called the organ system level of organization,which exhibits various patterns of complexity across different animal groups.