Phylum Porifera - Biology Textbook

Animals in the subkingdom Parazoa represent the simplest, multicellular animals and include sponges, or phylum Porifera. All sponges are aquatic and the majority of species are marine. Porifera do not display tissue-level organization, although they do have specialized cells that perform specific fu...

Animals in the subkingdom Parazoa represent the simplest, multicellular animals and include sponges, or phylum Porifera. All sponges are aquatic and the majority of species are marine. Porifera do not display tissue-level organization, although they do have specialized cells that perform specific functions. Sponge larvae are able to swim; however, adults are non-motile and spend their life attached to a substratum. Since water is vital to sponges for excretion, feeding, and gas exchange, their body structure facilitates the movement of water through the sponge. Structures such as canals, chambers, and cavities enable water to move through the sponge to nearly all body cells. Image of a cross-section of a sponge, which is vase-shaped. The central cavity is called the spongocoel. The body is filled with a gel-like substance called mesohyl. Pores within the body, called ostia, allow water to enter the spongocoel. Water exits through a top opening called an osculum. The morphology of the simplest sponges takes the shape of a cylinder with a large central cavity, the **spongocoel**, occupying the inside of the cylinder. Water can enter into the spongocoel from numerous pores in the body wall. Water entering the spongocoel is extruded via a large common opening called the **osculum**. However, sponges exhibit a range of diversity in body forms, including variations in the size of the spongocoel, the number of osculi, and where the cells that filter food from the water are located. While sponges (excluding the hexactinellids) do not exhibit tissue-layer organization, they do have different cell types that perform distinct functions. **Pinacocytes**, which are epithelial-like cells, form the outermost layer of sponges and enclose a jelly-like substance called mesohyl. **Mesohyl** is an extracellular matrix consisting of a collagen-like gel with suspended cells that perform various functions. The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges. In addition to the osculum, sponges have multiple pores called **ostia** on their bodies that allow water to enter 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. In other sponges, ostia are formed by folds in the body wall of the sponge. Lacking a true digestive system, sponges depend on the intracellular digestive processes of their **choanocytes** for their energy intake. Choanocytes are also responsible for creating water flow.