(i) Cell level structure: The body structure in which some cells combine to perform specific functions is called cellular level structure. Cells perform reproductive functions, nutrient collection, metabolism etc. For example, Porifera animals.
(ii) Cell-planar structure: The structure in which some similar cells are arranged in a specific layer to form a tissue is called cell-planar structure. E.g. Cnidaria animals. Dr. Siddiq Publications
(iii) Organ-level structure: The body structure in which multiple tissues combine to form organs is called organ-level structure. E.g. Platyhelminthes.
(iv) Structure of organ-tantra level: The body structure in which more than one organ forms a system is called structure of organ-tantra level. Such structures first appeared in Nemertean sea creatures. Such as Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata and Chordata.