Jellyfish are radially symmetrical, with a body arranged in a circular pattern, without a clear left or right side.
Jellyfish have a body consisting of a disc or bell, which can vary in thickness from strongly flattened to almost spherical. This disc houses the jellyfish’s senses, organs, arms and tentacles, so you could compare it to a skeleton. The disc is covered on the outside and inside with a thin layer of skin. On the inside, the disc consists of “mesogloea” consisting of long, stringy proteins for firmness and especially lots of water, around 95%. At the bottom, in the middle of the disc, the skin folds inwards, forming a cavity that serves as a stomach. This cavity has only one entrance and exit, so a jellyfish poops through its mouth. From the stomach run off into the tentacles and into a ring-shaped channel along the edge of the disc, along which nutrients are transported. The muscles of the jellyfish are in a ring-shaped pattern in the disc, and in some species, such as the hair jellyfish, are clearly visible.
Nerves and senses
The nervous system of jellyfish is a lot simpler than ours. The nerves lie in rings around the disc, connected to a net of nerves that runs through the whole jellyfish. The nerves control the muscles and are connected to clusters of senses at the edge of the disc called rhopalia. In these rhopalia there are balance organs, with which the jellyfish can feel whether it is swimming upright or upside down, and light-sensitive structures that serve as eyes, with which the jellyfish can perceive whether it is light or dark. That’s about it, there is no brain, although there may be some kind of nerve nodes in the nervous network.
Along the edge of the bell are the tentacles, which the jellyfish uses to catch its food. The tentacles are covered with stinging cells. Some jellyfish, like the lion’s mane, have tentacles that can become very long, up to several meters. Other species such as the barrel jellyfish have (almost) no tentacles. They catch their food with their oral arms.
The jellyfish’s oral arms, usually four or eight, also have stinging cells and are also used to collect food. The oral arms arms start in the middle of the disc. In species that mainly eat very small animals, such as the barrel jellyfish, the mouth arms are very finely branched and have many small mouths through which the food is absorbed.
In jellyfish, the gonads are in the bell, against the gastric cavity. Most jellyfish are either male or female, although some species can change sex. In many species both eggs and sperm are discharged into the water and fertilized there, but there are also species, such as the moon jellyfish and the lion’s mane jellyfish, where the female broods the fertilized eggs in the mouth arms.