jellyfish Class Scyphozoa

In the large jellyfishes or Scyphozoa the medusoid phase is dominant, the polyp, known as the hydra tuba or scyphistoma, being a small tubular organism resembling Hydra. The class comprises five orders as follows: Semaeostomeae, Rhizostomeae, Coronatae. Cubomedusae and Stauromedusae The most noticeable external feature of the class is its four-rayed symmetry.

The bell of the scypho medusae, the medusoid phase, varies in shape according to the order and is generally gelatinous due to the extensive formation of mesogloea. There is a varying number of tentacles round the bell margin and a four-cornered mouth at the end of a manubrium. The mouth may be drawn out at the corners into lobes or oral arms, which in some jellyfishes may be extensive.

Both the arms and ectoderm of the bell bear many nematocysts. The mouth leads into a central cavity, off which are four gastric pouches from which run radial canals leading to a circular canal at the bell margin, and from which other canals lead back to pores at the bases of the oral arms.

The canals are ciliated and by their action food materials, subjected to preliminary digestion, are circulated through the bell. The gonads are found in gastric pouches. Round the perimeter of the bell is a well-developed muscle band that, like that of hydrozoan medusae, contracts to close the bell and eject water, propelling the animal forwards. To coordinate swimming movements there is an elaborate nervous system similar to that of the other medusac. On the bell, margin are sense-organs responsible for maintaining equilibrium.

The polyp stage, the scyphistoma or hydratuba, when present, develops from the ciliated planula. The zygote often remains in brood pouches formed from the oral arms until it reaches the planula stage. The larva settles and becomes a cylindrical scyphistoma with a basal disc and a tentacled oral disc.

Although superficially like Hydra, internally it differs: the enteron is incompletely divided into four by septa which project from the wall into the center. The septa bear longitudinal muscles and have a curious feature, a peristomial pit, which runs from the oral disc to near the base of the polyp.

During the summer months the scyphistoma feeds and buds off another scyphistoma, but during the winter a process known as strobilation occurs: the polyp becomes multiply constricted in the transverse plane so that it resembles a pile of plates. From each disc, eight lobes or lappets grow out. One by one the discs are constricted off from the parent by contraction of the longitudinal muscles in the septa.

These discs are tiny medusae known as ephyra larvae. Each newly liberated ephyra is about one mm. in diameter and swims in the plankton, feeding and growing until it reaches adult size. The parent scyphistoma produces more oral tentacles and feeds again, living for several years and under favorable conditions producing ephyrae every winter.

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