Between the outer layer and the feeding chambers of the sponge is a jelly-like substance called the mesohyl, which contains collagenous fibers. In other sponges, ostia are formed by folds in the body wall of the sponge. In some sponges, ostia are formed by porocytes, single tube-shaped cells that act as valves to regulate the flow of water into the spongocoel. These pores have given the sponges their phylum name Porifera-pore-bearers. Scattered among the pinacoderm are the ostia that allow entry of water into the body of the sponge. For example, epithelial-like cells called pinacocytes form the outermost body, called a pinacoderm, that serves a protective function similar that of our epidermis.

While sponges do not exhibit true tissue-layer organization, they do have a number of functional “tissues” composed of different cell types specialized for distinct functions. In some sponges, multiple feeding chambers open off of a central spongocoel and in others, several feeding chambers connecting to one another may lie between the entry pores and the spongocoel.

However, we should note that sponges exhibit a range of diversity in body forms, including variations in the size and shape of the spongocoel, as well as the number and arrangement of feeding chambers within the body wall. Water entering the spongocoel is expelled via a large common opening called the osculum. Water enters into the spongocoel through numerous pores, or ostia, that create openings in the body wall. The morphology of the simplest sponges takes the shape of an irregular cylinder with a large central cavity, the spongocoel, occupying the inside of the cylinder (Figure 1). There are at least 5,000 named species of sponges, likely with thousands more yet to be classified.

Describe the organizational features of the simplest multicellular organisms.

How does water move through a sponge?