Volvox is a genus of colonial organisms that secrete a gelatinous substance called mucilage that binds them together into a hollow sphere. In addition to this extracellular matrix, many Volvox species also exhibit cytoplasmic connections between cells. These cellular bridges are generally considered a sign of greater cellular integration than can be found in most other planktonic colonies, especially since breaking them leads to the death of not only the individuals directly involved, but to the entire group. In fact, some scientists believe that Volvox has crossed the evolutionary gap from colonial to multicellular.