The simplest sponges resemble vases or clusters of tubes with irregular shapes. Each sponge has a large central opening and hundreds of pores in the body wall. Cells lining the cavity remove and digest suspended food particles and water. Waste exits through the central opening.
Sponges are generally without symmetry, and they have a relatively simple cellular organization. The body wall contains a protective layer of flat cells on the outside, an inner layer of flagellated cells, and a gelatinous filling between the two layers. This gelatinous layer contains a group of wandering cells called amoebocytes. A sponge is supported by protein that may contain spikes of silicon or calcium compounds. This protein “scaffolding” is typical of the bath sponge.
The cells in the sponge act independently, and there is no evidence of tissue organization. Reproduction occurs by sexual or asexual means. It is possible that sponges evolved from protozoa living in colonies.