By Joseph Enea
Background
The members of the Porifera phylum make up some of the oldest animal fossils found on Earth which date back to the Late Precambrian Era. Over 5,000 species are classified into this phylum. Members of Porifera are commonly called sponges and are one of the most unique types of animals. Sponges do not have mouths; instead, sponges have tiny pores in their outer wall. These pores draw in water, allowing the sponges to filter-feed. Water flows through the sponge in one direction. Sponges are classified into three main groups: the Hexactinellida, the Demospongia, and the Calcarea. These classes are based on the strength of their skeletons. All sponges are sessile when they reach adulthood. They are usually found on the ocean floor. Sponges can reproduce in two ways: either a part breaks off of it to form a new sponge, or by using gemmules, collections of amoebocytes within protective outer layers.
Tube sponges
Tube sponges are more advanced than other sponges. These sponges filter-feed to obtain oxygen. When water is absorbed, it does not circulate because the sponges does not have a massive body to support this process. Instead, oxygen is absorbed from the water. The rush of the current helps push the water into the sponge.
Sea Sponges
Sea sponges are clssified into three different groups. Sea sponges have no organs or tissues. Without the use of respiratory organs, sea sponges use a different method of gas exchange. The sponges are so simple that there is no special area where gas exchange takes place. Water containing oxygen is distributed throughtout the body of the sponge. The water enters small pores, called ostia to enter the body of the sponge. The water circulates through the sponges body with the help of cells called choanocytes. These cells contain flagella, which move the water in the sponge. As the water is removed, food and oxygen are brought to the sponge.
Barrel Sponges
Barrel sponges breathe very similar to other sea sponges. They circulate water within themselves to produce oxygen. Barrel sponges can grow so big that a person could fit inside them.