These feeding cells are similar in appearance to unicellular choanoflagellates (Protista). The structure of a choanocyte is critical to its function, which is to generate a directed water current through the sponge and to trap and ingest microscopic food particles by phagocytosis. The feeding chambers inside the sponge are lined by choanocytes ("collar cells"). The gel-like consistency of mesohyl acts like an endoskeleton and maintains the tubular morphology of sponges. Various cell types reside within the mesohyl, including amoebocytes, the “stem cells” of sponges, and sclerocytes, which produce skeletal materials. 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 28.3). There are at least 5,000 named species of sponges, likely with thousands more yet to be classified. (credit: Andrew Turner) Morphology of Sponges Sponges are members of the phylum Porifera, which contains the simplest invertebrates. Various canals, chambers, and cavities enable water to move through the sponge to allow the exchange of food and waste as well as the exchange of gases to nearly all body cells.įigure 28.2 Sponges. Since water is vital to sponges for feeding, excretion, and gas exchange, their body structure facilitates the movement of water through the sponge. Sponge larvae (e.g., parenchymula and amphiblastula) are flagellated and able to swim however, adults are non-motile and spend their life attached to a substratum. Thus, functionally, the poriferans can be said to have tissues however, these tissues are likely not embryologically homologous to our own. But even though they are not considered to have true tissues, they do have specialized cells that perform specific functions like tissues (for example, the external “pinacoderm” of a sponge acts like our epidermis). This is because they do not create a true gastrula during embryogenesis, and as a result do not produce a true endoderm or ectoderm. We should reiterate here that the Porifera do not possess “true” tissues that are embryologically homologous to those of all other derived animal groups such as the insects and mammals. The split between the Parazoa and the Eumetazoa (all animal clades above Parazoa) likely took place over a billion years ago. This clade currently includes only the phylum Placozoa (containing a single species, Trichoplax adhaerens), and the phylum Porifera, containing the more familiar sponges ( Figure 28.2). We will start our investigation with the simplest of all the invertebrates-animals sometimes classified within the clade Parazoa (“beside the animals”). However, one of the most ancestral groups of deuterostome invertebrates, the Echinodermata, do produce tiny skeletal “bones” called ossicles that make up a true endoskeleton, or internal skeleton, covered by an epidermis. ![]() ![]()
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