Phylum Porifera

Phylum Porifera

Sponges are the simplest multicellular animals, almost entirely aquatic (mostly marine) and sessile — fixed in place as adults, resembling plants more than animals in outward appearance. Their body shows cellular-level organisation rather than true tissues, and their whole physiology is built around drawing water through the body.

The canal system is the defining feature of the phylum. Water carrying oxygen and food particles is drawn in through numerous small pores (ostia) in the body wall, driven by the beating of flagella on specialised collar cells (choanocytes) that line internal chambers, and expelled through one or more larger openings (oscula). This water-canal system performs several essential functions at once: it brings in a continuous supply of oxygen and suspended food particles (which the choanocytes trap and phagocytose — sponges are filter feeders), it carries away metabolic wastes and carbon dioxide, and, in many sponges, it also carries out gametes and larvae for reproduction and dispersal. Because sponges lack a circulatory system, gut, or specialised respiratory organs, the canal system single-handedly performs the roles that feeding, respiration and excretion play in more complex animals. …