The simplest invertebrates, or animals lacking backbones, are sponges. The sponge’s hollow body has neither bones nor symmetry. A sponge’s body has a jellylike substance between its two-cell layers. Holes allow food and water to flow in and water and waste to flow out. It eats many small organisms, including bacteria, single-cell algae, and protists. Young sponges move around searching for a(n) place to settle. Sponges are able to grow back missing parts.