Poriferans

Poriferans, commonly known as sponges, are a phylum of simple, sessile animals that primarily inhabit marine environments, with some freshwater species. They lack true tissues and organs and show cellular-level organization, making them one of the earliest-diverging animal lineages (parazoa).

Sponges have bodies perforated by pores and channels through which water flows. Water enters through numerous ostia, moves via a network of canals driven by flagellated choanocytes, and exits through an osculum. The simplest body plan is asconoid, with gradually more complex forms such as syconoid and leuconoid that increase surface area for feeding and growth.

The skeleton of sponges is variable, consisting of spicules made of calcium carbonate or silica, and/or a supporting network of collagen-like spongin. Major cell types include pinacocytes on the outer surface, porocytes that form pores, choanocytes that capture food particles, and amoeboid cells (amebocytes) that transport nutrients and can form gametes or contribute to spicule formation.

Feeding occurs by filter feeding: choanocytes trap bacteria and small plankton as water passes through the canal system, with digestion occurring intracellularly in choanocytes and amoebocytes. Sponges lack a digestive tract and true organs; nutrients are distributed via their mesohyl cellular matrix.

Reproduction can be asexual, through budding, fragmentation, or gemmule formation in some freshwater species. Sexual reproduction often involves hermaphroditic individuals; sperm released into the water are captured by other sponges, eggs develop in the mesohyl, and free-swimming larvae disperse before settling.

Taxonomically, sponges are divided into several classes, including Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha. They fossilize poorly, but sponge spicules appear in the late Precambrian, making sponges among the oldest known animal lineages. In ecosystems, they play roles in nutrient cycling, habitat provisioning, and, in some cases, reef dynamics, while remaining sensitive to water quality.