Members of phylum Porifera are commonly called sponges. They are generally marine and mostly asymmetrical, and are primitive multicellular animals with a cellular level of organisation. A characteristic feature is the water transport or canal system. Water enters through minute pores called ostia in the body wall into a central cavity, the spongocoel, and leaves through the osculum. This pathway of water helps in food gathering, respiratory exchange and removal of waste. The spongocoel and canals are lined by choanocytes, also called collar cells. Digestion is intracellular. The body is supported by a skeleton made up of spicules or spongin fibres. Sexes are not separate, so sponges are hermaphrodite, producing eggs and sperms in the same individual. Sponges reproduce asexually by fragmentation and sexually by formation of gametes. Fertilisation is internal, and development is indirect, with a larval stage that is morphologically distinct from the adult. Examples include Sycon (Scypha), Spongilla (fresh water sponge) and Euspongia (bath sponge). Because they lack true tissues, organs and a digestive cavity, they are the simplest animal phylum, and questions often ask about the pathway of water, the role of choanocytes and the hermaphrodite condition.