This group of animals is probably considered as the oldest animal group. They are also called as Sponges. These are by far the simplest multicellular animals. Even though they are multicellular, they do not have any tissues or organs. Sponges live in an aquatic habitat as they have to have intimate contact with water. Water plays a major role in the feeding, exchange of gases and as well as excretion. The body of the sponges has many holes or pores called Ostia. The body structure of sponges is designed in such a way that water moves through the body, where it can filter out food and also absorb the dissolved oxygen, along with eliminating waste material.
Sponges are only just classed as animals and are believed to be one of the first animals to have evolved. A single sponge cell can survive and reproduce to create a colony of sponge cells that work together as a multicellular animal.