FIGURE 29.7
Anatomy of a sponge. The wall of this simple sponge has two layers of cells separated by a gelatinous matrix, the mesohyl (“middle matter”). The outer layer consists of tightly packed epidermal cells. The incurrent pores are channels through porocytes, cells shaped like elongated donuts that span the body wall. The spongocoel is lined mainly by choanocytes, each with a flagellum ringed by a collar of fingerlike projections that trap food particles. Mobile amoebocytes produce materials for skeletal fibers (spicules) and also transport nutrients from the choanocytes to other cells of the body.