A. An asconoid body plan.  Water is drawn into the incurrent pores (ostia) by the flagellar action of the choanocytes (black) into a common atrium and out through the osculum.  B. Simple synconoid body plan.  Choanocytes occur on the internal folds of the atrium, called flagellated canals.  Water comes in through ostia, the flagellated canals and into the common atrium and leaves through the common osculum.  C. A more complex synconoid body plan.  Water enters through dermal pores and then conveyed to ostia.  D. The leuconoid body plan.  Dermal pores lead to ostia as in C; however, there is no common atrium or common osculum.