Phylum: Porifera

• Sponges are the simplest example of multicellular animals
• Have no tissue layers and therefore have a cellular level of organisation
• Different cells have become specialised to perform different functions
• The cells are not attached to each other, however, but organised in a losse arrangement, so if separated, they can reorganise themselves again
• Sponges have no symmetry- asymmetrical
• Sponges have no muscles of nerves, therefore cannot move around
• They are sessile (sedentary)
• Sponges are aquatic animals attached to rocks, branches or other underwater substrates
• Bodies of simple sponges are in the shape of a vase, with one large opening, the osculum, on top and a large number of pores in the side walls
• Sponge diagrams

  
• The sponge body has a large central cavity, known as the spongocoel
• Sponges do not have a coelom, and are therefore acoelomate
• The sponge body consists of two cell layers
• These cell layers are not called tissues because the cells are not attached to each other to perform a particular function
• The outer cell layer consists of flat cells, with small pores between some of the cells
• The inner cell layer lines the spongocoel and consists of specialised cells that are known as collar cells
• A collar cell has a single flagellum that performs sweeping movements to circulate water through the sponge and out vie the osculum
• Collar cells have a funnel-shaped collar that surrounds the flagellum
• The collar consists of fine fibrils that form a net and function as a micro-fine filter
• As the water is drawn in from the outside through the small pores, the collar cells filter it and retain the small food particles (even bacteria) that are suspended in the water
• The collar cells take up food particles by phagocytosis and digest the food while water flows out via the osculum
• They are highly specialised for filter feeding
• Between two cell layers is a jelly-like layer that produces the skeleton or the sponge. The skeleton supports the sponge and consists of hard needle-like spines, the spicules as well as protein fibres known as spongin