PHYLUM PORIFERA AMIMALS WITHOUT TISSUES
Members of the kingdom Animalia, which are all multicellular eukaryotes, are thought to have evolved from single-celled eukaryotes (protistans). Among the simplest of all animals are the members of the phylum Porifera (sponges). Sponges are little more than loose aggregations of cells with no true tissue organization. There is some division of labor amongst the cells, but there are no organs.
The basic body form of all sponges consists of a sac-like structure with an outer layer of epidermal cells, an inner layer of flagellated choanocytes, and a middle mesohyl layer containing amoeboid cells that produce skeletal structures of various sorts. These layers are perforated by a large number of small pores, which give the group the name Porifera (= pore bearer). The cavity of the sac is called the spongocoel and it has at least one opening to the outside, the osculum.
Figure 1. Cutaway diagram of a simple sponge. After Margulis & Schwartz, 1998.
The phylum Porifera is commonly divided into 3 classes based on the type of skeletal materials produced: the class Calcarea has calcareous (calcium carbonate) spicules, the Hexactinellida has siliceous (glass-like) spicules, and the Demospongiae has predominately spongin (protein) fibers. Within each class, the sponges can be further differentiated by body type. In asconoid sponges the body wall is not folded; in syconoid sponges the body wall is folded into canals; and in leuconoid sponges the canals formed by the folded body wall are extensively branched. The term ostia is used to mean the openings into the pores of asconoid sponges, and the openings into the canals of syconoid and leuconoid sponges.
In all sponge types, the body is designed to facilitate filter feeding. Water is pulled into the pores and canals by the beating of the choanocytes' flagella. The water moves into the spongocoel and is eventually forced out through the osculum. As the water passes across the choanocytes, food particles (microscopic algae, bacteria, and organic debris) are trapped by the collar and taken into food vacuoles via phagocytosis for intracellular digestion. The more complex internal structure of leuconoid sponges increases surface area relative to volume, slowing the flow of water through the canals and making it easier for the choanocytes to trap food.
In this lab, we will examine examples of each of the three body types characteristic of sponges, and also representatives of the 3 classes of sponges.
Asconoid Sponges Leucoselenia (Class Calcarea)
Asconoid sponges have the simplest organization. Choanocytes line the spongocoel, drawing water through small ostia and expelling it through the osculum.
Figure 2. Diagram of an asconoid sponge, Leucosolenia. From Van De Graaff & Crawley, 1994.
1. Examine preserved whole specimens or preserved specimens on slides. Draw the specimen, labeling the osculum (which may be hard to see, but is at the apex), and the spicules protruding from the body wall.
Syconoid Sponges Scypha (Class Calcarea)
Figure 3. Diagram of a syconoid sponge, Scypha. From Van De Graaff & Crawley, 1994.
2. Examine a cross section of Scypha (= Grantia) using a compound microscope at low power. Draw the cross section, labeling the spongocoel, the radial canals that radiate from the spongocoel, the ostia and the incurrent canals they open into, and the osculum.
Leuconoid Sponges Euspongia, the "bath sponge" (Class Demospongiae)
Leuconoid sponges represent the most complex body form. The canal system is extensively branched. Small incurrent canals lead to flagellated chambers lined by choanocytes. Flagellated chambers discharge water into excurrent canals that eventually lead to an osculum. Usually there are many oscula in each sponge. The "bath sponge" is an example of a leuconoid sponge. The skeleton of this sponge is made of a soft protein, called spongin, rather than calcium carbonate or silica.
Figure 4. Diagram of a leuconoid sponge, Euspongia. From Van De Graaff & Crawley, 1994.
3. Examine demonstration materials showing the leuconoid body form. Draw a specimen showing the numerous ostia and the osculum.
4. Examine the Venus's flower basket specimen that is on demonstration. This is an example of a member of the class Hexactinellida, and it has fiberglass-like supporting fibers.