Unit Five. Evolution of Animal Life
19.4. Sponges: Animals Without Tissues
Sponges, members of the phylum Porifera, are the simplest animals. Most sponges completely lack symmetry, and although some of their cells are highly specialized, they are not organized into tissues. The bodies of sponges consist of little more than masses of specialized cells embedded in a gellike matrix, like chopped fruit in Jell-O. However, sponge cells do possess a key property of animal cells: cell recognition. For example, when a sponge is passed through a fine silk mesh, individual cells separate and then reaggregate on the other side to re-form the sponge. Clumps of cells disassociated from a sponge can give rise to entirely new sponges.
About 5,000 species exist, almost all in the sea. Some are tiny, and others are more than 2 meters in diameter (the diver in figure 19.4a could almost crawl inside the sponge shown). The body of an adult sponge is anchored in place on the seafloor and is shaped like a vase (as you can see in figure 19.4b). The outside of the sponge is covered with a skin of flattened cells called epithelial cells that protect the sponge.
Figure 19.4 Diversity in sponges.
These two marine sponges are barrel sponges. They are among the largest of sponges, with well-organized forms. Many are more than 2 meters in diameter (a), while others are smaller (b).
The Phylum Facts illustration on the facing page takes you on a tour through a sponge. The body of the sponge is perforated by tiny holes. The name of the phylum, Porifera, refers to this system of pores. Unique flagellated cells called choanocytes, or collar cells, line the body cavity of the sponge (see the enlarged drawing of the choanocyte). The beating of the flagella of the many cho- anocytes draws water in through the pores (indicated by the black arrows) and through the cavity. One cubic centimeter of sponge tissue can propel more than 20 liters of water a day in and out of the sponge body! Why all this moving of water? The sponge is a “filter-feeder.” The beating of each choanocyte’s flagellum draws water through its collar, made of small, hairlike projections resembling a picket fence (you can see this in the enlarged view). Any food particles in the water, such as protists and tiny animals, are trapped in the fence and later ingested by the choanocyte or other cells of the sponge.
The choanocytes of sponges very closely resemble a kind of protist called choanoflagellates, which seem almost certain to have been the ancestors of sponges. Indeed, they may be the ancestors of all animals, although it is difficult to be certain that sponges are the direct ancestors of the other more complex phyla of animals.
Key Learning Outcome 19.4. Sponges have a multicellular body with specialized cells but lack definite symmetry and organized tissues.