Jelly fish or Aurelia is marine medusa. It is free living and free swimming form.
Jelly fish swimming:
Aurelia (jelly fish) freely swims in the sea water. Its highly developed musculature is responsible for bringing swimming movements in Aurelia. Contractions of the muscles will bring contractions of the medusa. Because of these contractions water is expelled forcible from subumbrellar surface and the body is propelled forwards. Such type of movement is called hydro propulsion. While it is swimming, the balance of the medusa is maintained by tentaculocysts.
In Aurelia(jelly fish) medusa there are no specific respiratory organs. Oxygen dissolved in the water will diffuse into the body. C02 present in the body parts will diffuse out into the water through general body surface. Thus respiration is carried on. Sub genital pits may take up respiration.
In Aurelia medusa specific excretory organs are absent. The nitrogenous wastes formed in the body will be diffused out through the general body surface. Thus Aurelia medusa will show locomotion, respiration and excretion.
Jellyfish Nervous System and Sense Organs:
In the animal kingdom nervous system is developed for the first time in Cnidaria. In Aurelia (jelly fish) diffused nervous system is seen.
In the ectoderm many nerve cells are present. They are called proto neurons. From the corners of these cells, nerves will arise. All the nerves of the nerve cells of ectoderm will form a network. Such nerve net will show a concentration as a nerve ring on the margin of the umbrella.
1) The nerve ring is thickened at the regions of rhopalia. These thickenings are called rhopalial ganglia.
2) The nerve net is thick on the sub-umbreliar surface" because of the well developed musculature on that side.
3) In the endoderm also nerve cells are present. Their nerves will also form a nerve net.
Sense Organs : Aurelia medusa shows 8 tentaculocysts on the margin at 4 per-radial and 4 adradial positions.
Tentaculocyst : This is also called rhopalia. Each rhopaiium contains statocyst, ocellus and 2 olfactory pits.
In the marginal lappet of the umbrella's margin club shaped tentaculocyst is present. It shows a hood. Into this tentaculocyst circular canal will extend. At the base of the hood one olfactory depression is present. It is called superior olfactory pit. On the inner side of the tentaculocyst another pit is present. It is called inner olfactory pit. These two olfactory pits will work as chemo receptors.
Below the hood region an ocellus is present. This ocellus works as photoreceptor.
At the distal part of the lentaculocyst statolith cells are present. Each cell contains a statolith composed of calcium sulphate and phosphates. Below this group of statolith cells sensory epidermis is present. It shows sensory cells which are connected with subumbrellar nerve net. The group of statolith cells and sensary epidermis will form the statocyst. It maintains the equilibrium of the animal.