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Jellyfish Anatomy and Glossary Print E-mail

Cnidaria


A jellyfish, like any other cnidarian, consists of two basic layers of cells (an ectodermis and a gastrodermis) that are separated by a jelly-filled space or mesoglea; it is the mesoglea that gives the jelly to jellyfish.

The basic shape of a jellyfish is umbrella-like, with the ectodermis on the outside and the gastrodermis hidden on the inside. The upper surface of the umbrella (ectodermis) is known as the exumbrella surface, whilst the lower surface of the umbrella (also ectodermis) is referred to as the subumbrella surface.

cronodon.com
A diagram showing the typical anatomy of jellyfish


The umbrella of a jellyfish, or bell, can be of variable shape depending on the species: saucer-like or discoid, near-spherical or hemispherical, campanulate or cuboidal. Shape is a useful character in separating species.

The colour, colour pattern and density of colour (opacity and translucence) on the exumbrella (and other) surfaces all vary with the species, as well as with the age of the individual. Colour can provide a useful first clue as to the identity of a jellyfish but it is rarely considered diagnostic.

The thickness of the mesoglea in the bell dictates how “rubbery” or flaccid a jellyfish will be. It is a useful character in separating major jellyfish groups out, but beware - it also varies with individual age.

The edge of the umbrella may be scalloped into what are known as lappets; the number, shape and relative size of which can be diagnostic and can be used to separate species.

ęScotty Kyle
A photo of Crambionella stuhlmanni highlighting the lappet area


ęSimone Neethling
A photo of Crambionella stuhlmanni showing the highlightes lappet area showing where the rhopalium is situated


Tentacles are often found between lappets at the edge of the umbrella. Their number, arrangement and size or length can be diagnostic and they can be used to separate species, although it should be realized that these characters too change with individual age. Tentacles may be pigmented and may also have (especially in the Hydromedusae) basal swellings or bulbs. Tentacles are often very fragile, and will frequently break off or get damaged – so look out for their bases between lappets at the edge of the bell.

Although jellyfish are considered to be fairly simple animals they do possess sense organs. These too are fairly rudimentary and in the case of scyphozoans and cubozoans the most obvious types are known as rhopalia (plural; rhopalium, singular). Typically, an individual animal will have four (Cubozoa) or eight (Scyphozoa) rhopalia (or multiples thereof), which are sunk into pits located equidistantly around the edge of the umbrella. These rhopalia sense gravity and light and provide the individual with information on its orientation. In the case of the Cubozoa, the rhopalia may be relatively sophisticated and individuals can navigate through complex underwater structures without “touching sides”. Because the rhopalia are distributed equidistantly around the edge of the umbrella, they effectively divide the animal up into four, eight (etc) radial sectors. The number and type of rhopalia are useful diagnostic characters, and the lappets that surround them may be different from the others (i.e. those that do not surround rhopalia).

The sensory information from the rhopalia is distributed around the animal through an invisible nerve net that runs through mesoglea, and is usually translated into muscular action of one form or another. Although jellyfish do not possess well defined muscle tissues, circular muscles are arranged as distinct bands on the subumbrella surface and these can contract or relax to cause the bell to pulse. The expulsion of water from the inside of the bell on muscle contraction “pushes” the animal through the sea.

In the case of the Cubozoa and the Hydrozoa, the umbrella margin may have an inwardly projecting shelf – or velum. This is relatively rigid and muscular and serves to restrict the cross-sectional area across which the water flows on leaving the bell during contraction – it gives more oomph to the pulse and increases swimming efficiency.

The gastrodermis opens to the outside via the mouth, which is usually situated mid-ventrally. The lips of the mouth vary in structure and shape and can be diagnostic. Prey items that are captured by the animal are passed through the mouth into the “stomach” or gastric cavity, which is lined by gastrodermal tissue. Extracellular digestion takes place here and the soluble products are then distributed around the body of the jellyfish through a system of canals of generally decreasing diameter – much like arteries and capillaries. This system of canals is diagnostic. Undigested food is simply expelled through the mouth again…..Yuk!

The first canals to come off the central stomach are referred to as the radial canals, and they literally radiate out towards the edge of the bell. There are usually at least twice as many radial canals (in Scyphozoa) as there are rhopalia: some of which will be orientated in the same plane as the rhopalia and some will be scattered in between. Those in the same plane as the rhopalia are referred to as rhopalial canals and those in between as known as inter-rhopalial canals.

ęSimone Neethling
A photo of the complicated canal system of Crambionella stuhlmanni after being injected with a coloured dye latex


Some of the radial canals may branch repeatedly to the edge of the umbrella, others will be straight and others still will fail to reach the edge of the umbrella directly but will instead meet a ring canal first. This is a canal that runs around the umbrella circumference but at some distance from the edge of the bell, and which in turn gives off a branching network of smaller canals towards the edge of the bell. In some cases the ring canal may also give off a branching network inwards – towards the stomach again! Regardless – the radial canals and their various branches eventually meet the edge of the bell where they join a circular canal that runs around the circumference of the animal.

The mouth typically has a collar or manubrium, so that it “hangs” below the bell on a “clapper”. In some hydrozoans, the manubrium may itself be on a long stalk. In the case of Semaeostomeae, the manubrium is developed into what are known as oral arms, four of which hang down and surround the mouth: these can be extremely long and delicate and they aid in prey capture.

ęSimone Neethling
Oral arms of Chrysaora hysoscella, arms are extremely long delicate, typical for a semaestome


ęDr. Andrew Brierley
Oral arms of Chrysaora hysoscella, arms are extremely long delicate, typical for a semaestome


In the Rhizostomeae medusa however, the oral arms are very stout and robust (thick mesoglea); they are fused basally for a variable length and may have a distinct epaulette. The single mouth opening to the outside has sealed up and the gastric cavity opens to the outside via a complex system of mouthlets situated in the oral arms. These mouthlets may be armed with mini-tentacles and other elaborate accessory structures externally. The oral arms may terminate in a “club” of variable (and diagnostic) relative length and shape.

ęSimone Neethling
Oral arms of Crambionella stuhlmanni, arms are very stout, robust and fused basally, typical for a rhizostome


ęSimone Neethling
Oral arms of Crambionella stuhlmanni, arms are very stout, robust and fused basally, typical for a rhizostome


The gonads of a jellyfish are specialized outgrowths of the ectodermal tissue in the case of Hydrozoa but of the gastrodermal tissue in the Scyphozoa and Cubozoa. Mature gonads may be brightly coloured and male gonads may be of a different colour to female gonads: the gonadal colour will vary with the state of individual sexual maturity. The distribution of gonadal tissue in a jellyfish reflects its systematic position. In the case of Leptomedusae, the gonads are wrapped around the radial canals, whilst in Anthomedusae, the gonads are wrapped around the manubrium. In the case of the Scyphozoa, there are generally four distinct gonads, and they are found in special gastric pouches at the base of the manubrium.

For further information, please refer to this site:

http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa.htm