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In some colonial polyps, a chitinous epidermis gives support and some protection to the connecting sections and to the lower parts of individual polyps. A few polyps collect materials such as sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with sediment particles. [12]
The polyps are connected by epidermis which surrounds a gastrovascular cavity. The epidermis secretes a chitinous skeleton which supports the stem and in some hydroids, the skeleton extends into a cup shape surrounding the polyp. Most of the polyps are gastrozooids or feeding polyps, but some are specialised reproductive structures known as ...
Membranipora membranacea colonies consist of individual organisms called zooids, each with a chitinous exoskeleton which is secreted by the epidermis. [6] This exoskeleton, hardened with calcium carbonate, is known as the zooecium, which not only serves to protect the internal structures of the organism, but also keeps the individual permanently attached to the substrate and neighboring zooids ...
Colonial hydrozoans include siphonophore colonies, Hydractinia, Obelia, and many others. [3] In hydrozoan species with both polyp and medusa generations, the medusa stage is the sexually reproductive phase. Medusae of these species of Hydrozoa are known as "hydromedusae". Most hydromedusae have shorter lifespans than the larger scyphozoan ...
Byronia is thought to represent the polyp stage of a scyphozoan cnidarian that built narrow, chitinous tubes that were attached to the substrate by a disc. How this animal fed is unknown, but compared to other coexisting genera like Cambrorhytium , it is thought to have been a carnivore or a suspension feeder.
The bodies of all types have two main parts. The cystid consists of the body wall and whatever type of exoskeleton is secreted by the epidermis. The exoskeleton may be organic (chitin, polysaccharide or protein) or made of the mineral calcium carbonate. The latter is always absent in freshwater species. [30]
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.
Chondroplon bilobatum is a medusoid Ediacaran fossil.It has sand-filled tubes, although these may not have been sand-filled in life. It has a shield-like shape, with one end different from the other, and bilateral symmetry, and although it has been suggested that it possesses glide reflection symmetry, such suggestions are based upon a taphonomic effect deforming some specimens.