Invertebrates include a great diversity of animals that do not have a notochord, as opposed to the 'vertebrates', a group which includes beings considered to be more evolved, those that do have a notochord (such as fish and mammals). They appeared in the sea many millions of years ago and evolved slowly, giving rise to beings with progressively more complex morphological characteristics, long before the appearance of vertebrates.
It is estimated that about 97% of the total number of animal species currently existing on Earth are invertebrates. So, there are more invertebrates than all other animals in existence.