Re: Does a living sponge have a digestive system?

Hi Tara,

You are definitely in the right category! Your class is covering life science, which is really just biology.

Your teacher is right -- sponges don't have a digestive system. This just means that they don't have a stomach, intestines, and organs like that. In fact, sponges don't have any organs at all. Their bodies consist of aggregations of cells, but these cells are not organized into tissues or organs.

Sponges do, however, eat. They feed by drawing water through their bodies and filtering out the tiny particles, which they then digest. Because they eat such small food, they don't need things like teeth or a sophisticated digestive system. To get an idea of how a sponge's body is put together, take a look at this diagram (Pearse and Buchsbaum, 1987):

A sponge's body is riddled with little pores. The inner cavity of the sponge is lined with a layer of flagellated cells called choanocytes. These choanocytes beat their flagella and create a current that draws water into the body through the pores. The choanocytes are also the cells that pick up the tiny food particles that come in the water. The filtered water (i.e., from which food has been removed) is expelled from the sponge's body through a larger opening; in the diagram above, this excurrent opening is at the top of the sponge.

As you can see, there are no organs or digestive system. And yet, sponges can feed on particles as small as bacteria, and are extremely efficient filterers – an actively feeding sponge can filter up to 1200 times their body volume in a day!

I hope this answers your question!

Allison J. Gong
MAD Scientist

References:

Pearse and Buchsbaum, 1987. Living Invertebrates. Blackwell Scientific Publications, Inc.
Brusca and Brusca, 2003. Invertebrates. Sinauer Associates, Inc.