In some species, the individual zooids within the colony are connected by stolons. Individuals within a multicellular colonial organism may be called ramets, modules, or zooids. Bryozoans form colonies consisting of clones called zooids that are typically about long. They are colonies of modular units known as zooids. In species with calcareous exoskeletons, these do not mineralize until the zooids are fully grown. Generally, a single zygote begins the formation of a colony of zooids. Pyrosome colonies grow by budding off new zooids near the posterior end of the colony. Most of the zooids here are polyps, specialised for collecting food. Although zooids are microscopic, colonies range in size from to over . These colonies can grow thousands of individual zooids in a relatively short period of time. They are generally found in aggregations of 4 or 8 cell zooids. A few species can creep very slowly by using spiny defensive zooids as legs. The constituent zooids form a rigid tube, which may be pale pink, yellowish, or bluish. This process repeats until a colony of zooids forms around the central stalk. An ascidian colony consists of individual elements called "zooids".