There are several ways of grouping animals. In all these methods the basic Taxon remains without any change. However the taxa are rearranged in different groups. All these groupings are mostly provided for the convenience in identifying similar taxa.
I. One of the earliest method of grouping the animals could be dividing the Animal kingdom into two assemblages called Invertebrata and Vertebrata. This scheme was provided initially by Aristotle. This scheme does not have a place for the Prochordates.
II. Animals can also be grouped as single celled and multicellular. The single celled organisms are called the Protozoans. The multicellular could be called the Metazoans. In this arrangement among the metazoans the unique nature of the sponges in not having a tissue grade of body constuction is not mentioned.