Phylum Cnidaria

Class Scyphozoa

Class Scyphozoa ("cup animals") includes all (and only) the marine jellies, with about 200 known species. The medusa is the prominent stage in the life cycle, although there is a polyp stage in the life cycle of most species. Most jellies range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata, can reach a size of two meters in diameter. Scyphozoans display a characteristic bell-like morphology (Figure).

Part a shows a photo of a bright red jellyfish with a dome-shaped body. Long tentacles drift from the bottom edge of the dome, and ribbon-like appendages trail from the middle of the body. Part b shows a cross-section of a jellyfish, which has nematocyst-bearing tentacles hanging from the bottom of the dome. Underneath the middle of the dome is an opening that serves as both a mouth and an anus. The opening leads to a gastrovascular cavity that is lined with a gastrodermis. The outer surface of the body is covered with an epidermis. Between the epidermis and gastrodermis is the mesoglea.
A sea jelly. A jelly is shown (a) photographed and (b) in a diagram illustrating its morphology. (credit a: modification of work by "Jimg944"/Flickr; credit b: modification of work by Mariana Ruiz Villareal)

In the sea jelly, a mouth opening is present on the underside of the animal, surrounded by hollow tentacles bearing nematocysts. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In some species, the digestive system may branch further into radial canals. Like the septa in anthozoans, the branched gastrovascular cells serve two functions: to increase the surface area for nutrient absorption and diffusion, and to support the body of the animal.

In scyphozoans, nerve cells are organized in a nerve net that extends over the entire body, with a nerve ring around the edge of the bell. Clusters of sensory organs called rhopalia may be present in pockets in the edge of the bell. Jellies have a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water, as well as to draw in food from the water as they swim. Scyphozoans have separate sexes. The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form. These polyps may bud to form additional polyps or begin immediately to produce medusa buds. In a few species, the planula larva may develop directly into the medusa. The life cycle (Figure) of most scyphozoans includes both sexual medusoid and asexual polypoid body forms.

The illustration shows the lifecycle of a jellyfish, which begins when sperm fertilizes an egg, forming a zygote. The zygote divides and grows into a planula larva, which looks like a swimming millipede. The planula larva anchors itself to the sea bottom and grows into a tube-shaped polyp. The polyp forms tentacles. Buds break off from the polyp and become dome-shaped ephyra, which resemble small jellyfish. The ephyra grow into medusas, the mature forms of the jellyfish.
Scyphozoan life cycle. The lifecycle of most jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding, and the medusa reproduces sexually. (credit "medusa": modification of work by Francesco Crippa)