The Cnidarian animals are radially_symmetrical, aquatic and mostly marine. They are at the tissue level of organization and have special stinging cells called cnidocytes that contain stingers termed nematocysts. They range in motility from slow-moving to sessile. They are efficient predators and they display various forms of symbiosis. They have two basic body plans: (1) the polyp and (2) medusa.
These organisms have two tissue layers: the epidermis and the gastrodermis, with a jelly-like mesoglea sandwiched between the two layers. They all have either two or three possible germ_layers: all of them have ectoderm and endoderm and some also have mesoderm. Their larva is called the planula_larva. They lack a coelom but they do have a nerve_net as well as a gastrovascular_cavity.
Currently there are four classes:
(1) Hydrozoa: Hydra, Obelia, and Physalia. Solitary or colonial forms. Polyp or medusa.
(2) Scyphozoa: Aurelia. Medusa stage dominates.
(3) Cubozoa: Medusa stage dominates. Medusa is square-shaped when seen in cross section.
(4) Anthozoa: Sea anemones and corals. The polyp stage predominates.
Obelia is a colony of polyps; some have tentacles, which are for feeding and others are reproductive and form medusae. The perisarc is the outer protective membrane of the body of the colony. When the perisarc contacts a reproductive colony, it becomes the gonotheca. When the perisarc reaches a feeding polyp, it becomes the hydrotheca. The coenosarc is the support system (like a stalk) for the polyps that comprise the colony.
Polyps are usually sessile and asexual. Some polyps are designed for feeding and others for reproduction. Feeding polyps have tentacles with cnidocytes that contain nematocysts, and reproductive polyps lack tentacles. There are several supporting membranes in the body design of the polyp, including the hydrotheca. The hydrotheca specifically surrounds the mouth and tentacle region of the polyp.
Caution: Some editions of your Photo Atlas have reversed the micrographs and captions for the feeding and reproductive polyps.