Sponges are the most primary phylum of Kingdom Animalia. They also called Poriferans because they belong to Phylum Porifera. The Latin name “Porifera” stands for “pore-bearers”. Sponges are the simplest of Kingdom Animalia, they don’t have either germ layers or typical organ systems such as respiratory, circulatory, digestive, or excretory. They are consisting of a jelly-like layer calls mesohyl, between two thin cell layers. The outermost cell layer is called as Pinacoderm while the inner cell layer is called choanoderm. There are pores and channels allowing water to circulate through them in the body. Poriferans are multicellular animals that only live in water(aquatic). 98% of species are marine while the other 2% are freshwater species. They are found in shallow ocean environments to depths as great as five kilometres, from polar regions (Antarctic & Arctic) to tropical regions, in seagrass lands, coral reefs, and mangroves. Adults are attached to the substratum(sessile). Still, they are simplest of invertebrates in the current world, sponges are relatively well diverse and distributed animals.
What is the secret behind the success of Phylum Porifera?
The shortest answer is having an aquiferous system. What is this aquiferous system? Aquiferous system can be found only in Sponges. It means it is a unique feature to the sponges. As mentioned above, they don’t have a mouth, germ layers, or organ systems. But this aquiferous system acts as a multi-purpose system. In other hand, Sponges have another unique feature called “totipotency“. It means their cells can change into another type of cells structurally and functionally. And also they are suspension feeders (filter feeders). All of these features help them with success.
Poriferans have 3 types of body designs.
Asconoid is the simplest type of organization. Small and tube-shaped, water enters the sponge through dermal pores and flows into the atrium. Choanocyte flagella create the current to expel it through a single osculum. smallest species of phylum Porifera do have this type of body organization. They have a tubular body with a single osculum, but the body wall is thicker and more complex than that of Asconoids and contains choanocyte-lined radial canals that empty into the spongocoel. Leuconoid is the most complex body design of all. Water flowing in through incurrent canals is selectively pumped through those chambers which are, and expelled via one of a series of oscula. They are highly adapted to larger size bodies. Some of Leuconoids are around 2m long.
As mentioned above sponge body consists of two cell layers call pinacoderm and choanoderm. The inner layer (choanoderm) is made up of Choanocytes (Collor cells) while the outermost cell layer (Pinacoderm) is made up of Pinacocytes. Choanocytes have flagellum on each cell. It helps to form water currents and trap food particles. Sponges have pores call ostia all over the body and a single osculum on the top of each branch. Ostia are formed of Porocytes. Water coming through ostia into the cavity call spongecoel and getting out through the osculum. Zooplankton and other food particles are trapped in flagellates of Choanocytes. Those cells call Amoebocytes do phagocytosis thus, here Aquiferous system acts as the digestive system. Cells excrete waste into water current in the water canals system (Excretory). The water flow through the complex aquiferous system oxygenates the sponge body interior, allowing oxygen to diffuse into the interior sponge cells, in addition to oxygen diffusing across the sponge surface, a process that may be enhanced by increasing ambient water flow. Sponges have a skeleton which is made up of spicules.
Sponges show both asexual and sexual reproduction. Budding, the formation of gemmules, and fragmentation. The formation of gemmules is another unique feature to Phylum Porifera. Gemmules resist drying, freezing, and oxygen depletion. Even if a sponge doesn’t survive, its gemmules do. When conditions improve, they create another sponge.
There are four classes within the phylum Porifera: Calcarea, Desmospongia, Homoscleromorpha, and Hexactinellida. Sponges are divided into these classes based primarily on the composition of their spicules and skeletal fibres.