It is believed that all animals may have evolved from the same group of protistans. They then evolved into distinct branches, one of which produced the sponges. Since no other animals are believed to have evolved from the sponges, they are considered to be an evolutionary dead end. Members of the phylum Porifera are among the simplest animals. They are little more than loose aggregations of cells with little or no tissue organization. There is some division of labor amongst the cells, but there are no organs.
The basic body form of all sponges is a sac-like structure consisting of three layers - an outer layer of epidermal cells; an inner layer of cells, many of which are flagellated cells called choanocytes; and a middle layer of amoeboid cells that form skeletal structures of various sorts. These layers are perforated by a large number of small pores (thus the name Porifera). The cavity of this sac is called the spongocoel and has at least one opening to the outside, called an osculum.
The sponges are taxonomically classified based on the type of skeletal materials produced calcareous spicules, siliceous spicules, or proteinaceous spongin 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 leuconoids 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 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) adhere to the cells and are eventually taken into food vacuoles for intracellular digestion.
In this lab, we will examine examples of each of the three body types characteristic of sponges.
The Asconoid Sponges
example: Leucoselenia (Class Calcispongiae)
Asconoid sponges have the simplest organization. Choanocytes line the spongocoel, drawing water through small ostia and expelling it through the osculum.
1. Examine a preserved specimen of this organism using a dissecting microscope. Draw the specimen, labeling the osculum, and the spicules protruding from the body wall.
The Syconoid Sponges
example: Scypha (Class Calcispongiae)
Syconoid sponges have a tubular design similar to the ascon sponge, but the body wall is folded. The "folds" form radial canals. Choanocytes line the radial canals rather than the spongocoel.
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 and the apopyles (the openings into the radial canals); the ostia and the incurrent canals they open into; and the prosypyles (the small openings connecting the radial canals to the incurrent canals). You may need to use high power to see the prosopyles.
The Leuconoid Sponges
example: 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.
3. Examine demonstration materials showing the leuconoid body form.
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