Food is really important to you and to the animals you'll be collecting. Getting food and digesting it provides energy for life processes. Once an animal eats something, it must be digested. Well, in most animals. Digestion is the process in which food is broken down into the elements and nutrients that can be used by cells. And as you learned earlier, cells use the process of cellular respiration to convert oxygen and food into cellular energy—the energy they use to live. Do you see the connection between digestion of food and energy for cell processes? Just remember not to feed animals your food. They have ways of getting their own.
Because animals must get their food from some outside source, most animals must move. However, not all animals need to move. Animals like the sea sponge you observed earlier are sessile; they remain fixed to one place for their entire adult lives. They are filter feeders—cells in those holes you observed create a current that draws water into the holes, bringing with it the food the sponge eats. Other cells then filter the food out of the water.
Food gathering is one of the most interesting and most studied aspects of organisms in the Animal Kingdom. The way an organism gathers its food is often related to its niche in the ecosystem. Take a look at these examples (roll your mouse over the images).
Invertebrates are animals; so all of this applies to them too. However, what's fascinating is that because invertebrates are so diverse, there are many different feeding habits.
For example: