The cells of higher plants are encased in a network of chambers formed by the interlocking cell walls surrounding the cells and are connected by cytoplasmic bridges called plasmodesmata (see Figure 1-12a). Animal cells are often “glued” together into a chain, a ball, or a sheet by cell-adhesion proteins on their surfaces, often called cell-adhesion molecules, or CAMs (see Figure 1-4d). Some CAMs bind cells to one another; other types bind cells to the extracellular matrix, forming a cohesive unit. In animals, the matrix cushions cells and allows nutrients to diffuse toward them and waste products to diffuse away. A specialized, especially tough matrix called the basal lamina, made up of polysaccharides and multiple proteins such as collagen, forms a supporting layer underlying cell sheets and prevents the cell aggregates from ripping apart (see Figure 1-4). Many CAMs and extracellular-matrix proteins found in humans also occur in invertebrates, indicating their importance during metazoan evolution. Similarly, many of the proteins and small molecules used by metazoans as signaling molecules are conserved in humans and many invertebrates, as are their receptors, the cellular proteins that bind to these signaling molecules and trigger an effect in the receiving cell. As one example, the signaling protein Wnt, discussed in Chapter 16, was discovered simultaneously as the gene mutated in the Drosophila Wingless mutation and as the site of integration of a cancer-causing virus in mice.