Sponges have cellular-level organization, which means that their cells are specialized such that distinct cells perform different activities, but comparable cells are not grouped into tissues, and bodies are a loose aggregation of various cell types. In contrast, animals with complex organs composed of discrete units often have no more than three or four distinct cell types. Sponges are the most ancient multicellular organisms; they date back over 500 million years. Although they lack nerves or muscles, they are still able to respond to stimuli through their surface receptors. Sponges also contain small quantities of hormones and other signaling molecules that are released when needed. In addition to being one of the first multicellular organisms, sponges are also among the simplest ones: they are composed only of two basic cell types (filamentous cells and epithelial cells), and have a simple structure without tissues or organs. However, this does not mean that they are lacking in features that would be found in more advanced organisms. For example, they are capable of regenerating themselves if injured or removed. This ability is probably what allowed them to survive for so long before evolving into more advanced species. The sponge's cellular simplicity may also explain why it is so resistant to disease. There are many diseases that infect humans and other mammals that would kill a fish or a bird in a matter of days.