Analogous to acrorhagi, functioning in aggressive encounters among individuals.
Anthozoan tissues contain both circular and longitudinal muscle fibers (Fig. 6.20). Provided that the animal keeps its mouth closed by contracting appropriate sphincter muscles, the seawater in the gastrovascular cavity can serve as a hydrostatic skeleton (Chapter 5). For example, by closing the mouth, relaxing the longitudinal musculature, and contracting the circular muscles of the body wall, the animal becomes long and thin as the longitudinal muscles are stretched by the elevated pressure within the gastrovascular cavity (Fig. 6.21). Then, by contracting the longitudinal muscles on one side of the body and relaxing those on the other side, the animal can bend to one side, provided that the circular muscles are not permitted to stretch. In an emergency all the longitudinal muscles can be contracted while mouth the mouth is open, causing the animal to flatten considerably as the fluid of the gastrovascular cavity is expelled. The resulting shape has been referred to as the “bubble gum on the rock” disguises (Fig. 6.21c). Reinflation is rather slow, being dependent upon the activity of the cilia lining the siphonoglyphs; these cilia “pump” water back into the gastrovascular cavity.