Figure 3

A subset of PORO illustrating the three layers of structures comprising a whole organism and some of the cell types that comprise these layers. The mesohyl is a gelatinous layer sandwiched between the external pinacoderm and internal choanocyte-lined surface (choanoderm). We re-use the CARO class ‘portion of tissue’ for these layers, although some debate exists about whether these features constitute true tissues.