Cellular level - Level of body organization
The next higher level of organization is the cellular level. This is really an aggregation of cells that are functionally differentiated. A division of labour is evident so that some cells are specialized for reproduction some for nutrition.
Figure: (a) Cellular level of body organization in mesozoan – Rhopalura (b) a placozoan (c) L.S. asconoid sponge
Among the metazoans, Placozoa and Mesozoa are said to belong to the cellular level of body organization some authorities place the sponges (porifera) among this group too because they have several cell types differentiated for various functions but there is no true tissue organization yet.