LEYS, S.P.: The hexactinellid body plan: syncytial metazoans

Hexactinellids differ from other sponges in possessing mostly syncytial rather than cellular tissues. The major tissue component, termed the trabecular reticulum, forms a cobweb-like continuous network throughout the entire sponge. The trabecular reticulum is bilayered and encloses a thin collagenous mesohyl in which lie several ‘cellular’ components of the sponge – archaeocytes, thesocytes, and spherulous cells – that are connected to the trabecular reticulum by an unusual intrasyncytial junction, the perforate plugged junction. Sclerocytes, that produce the spicule skeleton, form a second syncytial tissue but are not connected to the whole by plugged junctions. Syncytial tissues arise from two different cell populations in the post-gastrula embryo of hexactinellid sponges; the free-swimming trichimella larva is syncytial. A syncytial body plan has two important implications for hexactinellid sponges. First, nutrient transport is intrasyncytial or symplastic, as in plants. Second, the syncytial tissues form a continuous fluid-filled pathway, which allows the propagation of electrical signals that control the cessation of the feeding current. The hexactinellid sponge body plan represents a unique alternative to the cellular organization of all other metazoans.