Description[edit]
The sessile colonial stage of Obelia longissima is the most long-lived and the most easily observed of its life stages. The hydroid looks superficially like fronds of seaweed. It has a basal stolon growing in close proximity with the substrate. Out of this grow fragile, flexible stems up to 35 centimetres (14 in) high each with short side branches. As the stolon grows, new branches develop and the older ones are reabsorbed so the colony moves across the substrate. When the availability of food is high, further upright branches develop but when it is low, most growth takes place by way of stolon elongation and branching.[2] The stems have dark-coloured, usually straight internodes between chitinous chambers called hydrothecae which are shaped like small wine-glasses. These protect the polyps.[3]