Invertebrates used to be one of the primary divisions in the scientific classification of the Animal kingdom when for convinience the term was used for designating a group of phyla which lack a backbone (vertebral column) as distinct from the vertebrata which are the Animals with a vertebral column. the term invertebrata is retained to cover all the non-chordate phyla and the chordates other than the vertebrata. The primary lines of real cleavage in the Animal kingdom nowadays divide it into three unequal sections called sub-kingdoms. These are:
- subkingdom protozoa: These are invertebrates without cellular differentiation. they are for the most part unicellular (i.e made up one cell) or are said to be acellular (i.e not divisible into cells).
- subkingdom parazoa: this is a unique group among many-celled (multicellular) organisms in possessing collared flagellate cells. They are the most primitive and the simplest multi cellular Animals (metazoan). The many cells in their body are not organised into tissues and organs. they are therefore at a cellular level of organisation.
- subkingdom Metazoa: these are invertebrates with strongly marked cellular differentiation. there are two levels of organisations represented in this subkingdom. These are the diplobastic stage showing tissue grade level of organisation; where the bodily architecture is composed of two cellular layers (inner endoderm and outer endoderm) represented by the phylum coelentrata; and the triploblastic stage showing organ grade level of organisation. The other metazoan phyla which constitute the triploblastica (three-layered Animals) possess a third body layer, the mesoderm, lodged between the outer ectoderm and inner endoderm. The mesoderm is ussually more bulky than either of the other layers and forms the greater part of the body. The mesoderm contains important organs and definite systems of spaces.