Animals > Sponges
Sponges: phylum Porifera
Sponges are the simplest animals anatomically, lacking many of the features that characterize all other animals. However, close studies of both their embryology and their genomes show that sponges are closely related to other animals, and not to any other kingdom.
The apparent simplicity has led biologists to regard sponges as primitive offshoots of the animal lineage, having split off before the evolution of the more complex features that most other animals share. However, some studies have suggested that the ancestors of sponges may actually have been more complex.
All sponges live in the water; most live in the ocean.
- No true tissues. Although sponges have several types of cells, the cells do not show the level of tissue organization seen in other animals.
- No symmetry. Other animals show radial symmetry (cnidarians, for example) or bilateral symmetry (humans). Sponges have variable and irregular body forms.
- Intracellular digestion. Unlike other animals, sponge cells take in small food particles by phagocytosis.
- Spicules: hard, crystalline structures secreted outside the cells; see the description below.
Scypha is a small, tube-shaped sponge. We have two microscope slides of Scypha: a cross section and a longitudinal section.
Scypha uses the flagella on its choanocytes to draw water through its body so it can capture small suspended particles of food. The water is drawn in through the incurrent canals, passes through the radial canals, and finally out through the spongocoel.
The spongocoel is an empty space in the middle of the body. It is not comparable to the coelom of other animals, because there are no organs in it; it's just an empty tube through which water is expelled.
At higher magnification, you will be able to make out several types of cells. The choanocytes, or collar cells, perform the essential functions of pumping water by flagellar action and capturing food particles with their collar of microvilli. The incurrent and radial canals are lined with choanocytes.
Amoebocytes are cells with an amoeboid, or irregular, shape. They perform various functions and can differentiate to become other cell types. The amoebocytes are surrounded by a layer of extracellular matrix material called mesohyl.
Pinacocytes are flattened cells that form the outer layer of the sponge.
This microscope slide shows a whole mount of Leucosolenia, a small sponge. There isn't a lot of detail to see in this slide; the individual cells are hard to make out. Instead, you should look for the overall morphology (simple, with no distinct organs) and the spicules.
In Leucosolenia, the spicules are sharp, crystalline structures made of calcium carbonate, the same material that makes up the shells of many other marine animals. The spicules reinforce the body and make it more resistant to attack by other animals.
Leucosolenia is a calcareous sponge; some other sponges have spicules made of silicon dioxide, the main component of glass.
At higher magnification, the spicules are clearly visible (and, incidentally, rather beautiful). In order to see them clearly, you may need to adjust the condenser on your microscope (the wheel just under the stage).
Calcium carbonate is formed by many marine animals. However, it also dissolves fairly easily if the water's pH decreases even slightly.
Gastrulation in sponges
Gastrulation is an important embryological event that helps to define the basic embryonic tissue layers in animals; it lays the foundation for the later development of complex animal bodies. This will be covered in another lab, later this quarter.
Many sources state that sponges lack gastrulation, and thus lack defined embryonic tissue layers (see, for example, the Wikipedia article on Porifera). However, many sponge specialists believe that gastrulation does occur in sponges, although the process is quite different from that in other animal groups.
In case you're overwhelmed by curiosity about this topic (and have some spare time on your hands), here's an interesting article on this topic:
Don't worry, you don't need to know this for the test!
This page updated September 17, 2011