Sea pens are named for their resemblance to old-fashioned quill pens. The typical sea pen shape is a long stalk (peduncle) with feather-like extensions in the upper part. A bulbous structure at the bottom anchors the sea pen to sandy ocean floors. A central rod made of calcium carbonate, plus spiny skeletal parts (sclerites) support its body, but also make a sea pen a bumpy meal for predators. If attacked, a sea pen can bury itself in the sand. It releases lots of water from small canals that permeate its body, causing it to deflate like an emptying water balloon. The feathery, top part withdraws under the sand. Some predators, such as sea stars, will dig a sea pen out. In response to disturbance, a sea pen may uproot itself and re-anchor in another spot. At least some species of sea pen can tell predators from non-predators and control their response.

Anthozoans (anemones and corals) have stem-like bodies that anchor to the ocean body or other surface. Tentacles radiate like petals around the top of the stem (anthozoa flower animals). Both their bodies and tentacles are hollow, opening into a mouth in the center of the flower. Anemones and corals capture food with their tentacles and push it into their mouths. While they mostly eat small, floating food (plankton), stingers (nematocytes) on their tentacles allow some anthozoans to consume larger prey (even fish or crabs) by stunning them first. Food travels from the mouth to the hollow space inside their bodies, where it is digested. Since they have only one opening, waste products are expelled back out their mouths. Some anthozoans benefit from a relationship with unicellular algae (mutualism). While the coral provides a safe place for the algae to live, the algae makes food by photosynthesizing.