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Jellyfish Body Parts and Functions
Even though they are cnidarians, the appearance of jellyfish differs immensely from many in the same group, such as corals and sea anemones.
This page explains how the internal and external anatomical characteristics of jellyfish work, with extra information about some special adaptations that help them survive in the ocean.
Body Parts of a Jellyfish and How they Work
So... how does a marine animal with no bones, no brains, and no heart survive for several years in seawater?
In fact, the answer is quite simple! The gelatinous, bell-shaped body of a jellyfish is already more than 95% water with radial symmetry.
There are three (3) main jellyfish layers made of cells and thin tissue:
- Epidermis (outermost layer containing cnidocytes and nerve cells)
- Mesoglea (thick non-living jelly-like substance for shape and buoyancy)
- Gastrodermis (inner layer that surrounds the gastrovascular cavity)
In "true jellyfish species", the bell of a jellyfish is the part that protects the internal organs and it gives them their "domed" body shape (like an umbrella hood).
As the bell contracts and relaxes in rhythmical pulses, it helps to provide these marine invertebrates with limited "jet propulsion" through water.
Adult Jellyfish Anatomy Diagram
There is no centralised nervous system in jellyfishes (e.g., a brain) and the absence of any specialised workings for osmoregulation means containing constant pressure in fluids is a challenge because they do not have a skeleton.
Jellies do not have an efficient respiratory and circulatory function. There are no lungs or gills. Plus, "incomplete" digestion is achieved inside a single gastric cavity or stomach pouch (the gastrodermis gut called a coelenteron).
Here's the thing:
The vast majority of Scyphozoa jellyfish (the marine class of the phylum Cnidaria) display similar characteristics in body construction and in formation.
They are commonly identified by their gelatinous mass and the shape of the protruding manubrium (the stalk-like structure that hangs from the underside of the bell).
The stalk supports the oral arms (frequently called tendrils) and radial canals.
This body part connects with the mouth, at the bottom and underside of the hood, and it also functions as the anus for removing waste products.
The most venomous jellyfish (such as box jellyfish species) contain stinging nematocysts, toxic capsules covering the surface of the oral arms.
When Cnidarians hunt for food, thousands of microscopic toxins known as 'cnidocytes' are triggered and fired into their prey (mostly small fish and animal zooplankton). These stinging structures paralyse the victim, resulting in an immediate death.
Fun Fact: Some jellyfish are more dangerous to humans than others. For example, a lack of potent tentacles in the moon jelly (Aurelia aurita) means it's one of the weakest stings in the entire jellyfish classification.
Jellyfish Brain and Nervous System
Some facts about jellyfish body parts remain unclear, such as how they interpret visual stimuli and see things. Yes, some have eye spots (up to 24), but they do not have sensory organs or a brain to process the information.
Scientists think that image processing involves a nerve ring found in the epidermis. This is the closest resemblance to a central nervous system and it appears as a ganglion-like feature.
It's referred to as the "nerve network" and it's capable of detecting touch as well as changes in water temperature.
The rhopalia (present in many jellyfish species) is a sensory structure connected to the edge of the bell. It contains the balance sensors (statocysts) and the ocelli (simple eyes for detecting changes in lightness and darkness).
Jellyfish Mouth and Eyes
Jellyfish eyes are situated in the center of the underside of the bell. This central part of jellyfish anatomy is one that also doubles as the anus, so they use the same opening for food intake and for waste expulsion.
You're unlikely to see their eyes, and it may be better if you don't. But, jellyfish have light-sensitive eyes. They lack any ears, a nose, or any recognisable characteristics of a head.
Nevertheless, some species have two different forms of sight which actually total 24 eyes. One set of rhopalium is capable of processing colour and form images with a unique 360-degree all-round view of its environment.
Jellyfish Vision
The box jellyfish (Cubozoa) is one of the most developed types of jellyfish for advanced vision and odour detection.
Furthermore, the Australian Box Jelly (Chironex fleckeri) is a species known for causing severe and painful allergic reactions in humans from the toxicity of their sting.
Interesting Fact: The Immortal jellyfish (Turritopsis dohrnii) can revert back to the polyp stage (sessile) from the adult stage (medusa), an essential phenomenon for restarting their life cycle.
Jellyfish Tentacles and Oral Arms
Tentacles extend from the rim of the bell and are covered in cnidocytes (specialised stinging structures). They can release nematocysts (tiny harpoons with venom) to capture prey or as a defencive mechanism against their main predators in the wild (sea turtles).
The elongated frilly appendages, known as oral arms, hang from the scalloped rim and help move food towards the mouth.
The bell also provides axis support for its stinging tentacles or lappets. They have between four (4) and eight (8) oral arms, used to capture and maneuver food to their mouth orifice.
Jellyfish Movement in Water
In spite of having limited control over their movement in water, most jellyfishes use their hydrostatic skeleton to create a type of jet propulsion and move around.
These passive animal drifters achieve locomotion by creating a pulsating rhythmical movement through the water column.
Therefore, ocean currents greatly influence their motion. Even so, there are some species that travel by physical attachment to other floating materials, such as seaweed.
In a nutshell, a mass of trailing tentacles dangling beneath a pulsing mesoglea, a bell-shaped hood, differentiates jellyfish anatomy and body parts from other marine creatures.
Jellyfish Digestive System
It is difficult to digest food without the proper organs needed for digestion, such as pancreas, intestines, and liver.
In fact jellyies possess a simplified and 'incomplete digestive system' - meaning they use the same single opening, called the gastrovascular cavity, for food intake as well as for depositing waste substances.
Cells produce digestive enzymes which break down food substances inside the stomach pouch and help them to absorb the nutrients.
These carnivores dispose of their waste products relatively quickly through this cavity to avoid carrying around too much bulk and risk problems staying afloat in the water column.
Jellyfish Size and Weight
The height of a jellyfish bell ranges from as little as one (1) millimeter up to two (2) metres (6 feet). Many of the biggest species of jellyfish can also reach the same size in diameter.
The largest of all is highly toxic and commonly known as the lion's mane jellyfish (Cyanea capillata), with trailing tentacles up to 120 feet long.
Found most at Japan dive sites, the Nemopilema nomurai is understood to be the heaviest of the species and can reach colossal body weights in excess of 200 kilograms.
What Animals Eat Jellyfish?
Jellyfish cannot survive out of water and they rarely live longer than one (1) year, even in a natural environment. Sea turtles and large fish, such as the ocean sunfish (mola mola) prey on sea jellies, including some fishing birds.
Yet, one of the biggest predators of the species, and one which devours the contents of jellyfish anatomy the most, is other jellyfish. Yes they really are cannibalistic creatures!