|Colony of Sertularia argentea|
These dried hydroid colonies are commonly sold as a curiosity, as a decorative "indoor plant", or as underwater decorations for aquaria in stores. They are sometimes labeled as "Neptune plants". Despite a superficial resemblance to plants, they are actually animal skeletons or shells. The dried colonies are often dyed green, but, when soaked in water, the coloring will dissolve.
The fernlike branches of S. argentea are composed of many small, chitinous chambers where individual animals once lived. When the colony was alive, a polyp with numerous tentacles occupied each of the chambers, called hydrotheca.