In colonial octocorals, e.g., sea pens, one polyp grows very large and loses its tentacles, forming the central supporting axis. This is the primary polyp. The base of this primary polyp forms a bulb, which anchors it to the substratum. Branching off this primary polyp are various secondary polyps. Some are specialized for feeding (autozoids). Others (siphonozooids) serve as intakes for water, which circulates within the colony and help keep it upright .