Above: a 3D computer model of the brown hydra, Pelmatohydra generated using the Pov-Ray graphical language and
ray-tracer (using sphere-sweeps and also superposition of sine waves for the tentacles).
Hydra is a coelenterate, along with the jellyfish (Scyphozoa), sea anemones and corals and other diverse forms. Hydra
belongs to a group of coelenterates called the Hydrozoa. Hydrozoans come in a fantastic range of forms, most are quite
small, but often they form larger colonies, as in Obelia, and siphonophores. In hydrozoans there is usually an alternation of
generations as seen in the Scyphozoa, in which a tentacled polyp form, usually attached to a surface, generates sexual
medusae which are usually free-swimming bell, dome or saucer-shaped and bear tentacles. In the Scyphozoa, the
medusoid form is dominant and often large and known as a 'jellyfish'. In Hydrozoa, the medusoid is much smaller, typically 1
or 2 cm or less in diameter and more glass-like than jelly-like in appearance. The term jellyfish is sometimes also applied to
the hydrozoan medusa, especially by non-zoologists, but traditionally zoologists reserved the term 'jellyfish' for the large
scyphozoan medusae with their large quantities of jelly-like flesh (mesogloea). Hydra is different in that it has no alternation
of generations and the polyp is dominant. There is no medusoid stage in hydra, the polyp instead produces gametes when
The hydra polyp is aquatic, common in fresh water that is well aerated, and quite varied in form as there are a number of
hydra species. However, all have the same basic plan - a crown of tentacles surrounds the mouth at the oral end and the
other end of the body contains an adhesive pedal disc which sticks onto solid surfaces like aquatic vegetation.
Above: a whole mount (permanent prep) of hydra - left: tentacle crown; right tentacles. Note the batteries of stinging
nematocysts on the tentacles. Nematocysts are vessels contained in cells called nematocytes or nematoblasts. Inside the
vessel is a coiled thread, which is inverted upon itself. When activated, the vessel swells with pressure as water rapidly
enters it from the surrounding nematocyte and this high pressure forces the inverted coiled thread to evert and uncoil, firing
it out like a tethered harpoon. The nematocysts consist of four different types arranged in mixed batteries - type 1 are called
stenoteles or penetrants and are the largest. These have hollow tips and inject toxins into prey. The penetrants discharge in
two phases. In the first phase the coil partly extends and its three-pronged barbed tip, resembling an arrow-head, penetrates
the host cuticle. Following this the rest of the thread everts, shooting out from the embedded arrow-head and penetrating
deeper into prey tissues where it releases toxins from its tip.
The type 2 nematocysts are isorhizas, or small glutinants, which have closed tips and are lack spines and are sticky and so
allow the tentacles to adhere to the substrate. This second type act as anchors, allowing the hydra to bend over, grab hold
of the substrate as the base detaches. The base then moves up to join the tentacle crown which detaches again and the
cycle repeats allowing the hydra to move in a looping fashion like a caterpillar. Alternatively, the hydra may move
end-over-end in a somersaulting motion.
Type 3 nematocysts are also isorhizas, but of a different type, they are larger and their threads have spines. These are also
called large glutinants and like the small glutinants they release a sticky secretion. These nematocysts are used in defense.
The fourth type of nematocyst are called desmonemes or volvants. When discharged the threads of this type are coiled like
springs and they are involved in prey capture. They trigger more easily than the larger penetrants and serve to entangle the
prey. As the prey struggles to free itself, its vigorous movements trigger the penetrants to fire.
Behind the tentacle crown is the column, which makes up the bulk of the body. The column can be retracted when the animal
is disturbed, forming a ball with the tentacles retracted. However, when fully extended, then two distinctive regions are
apparent - the front-most part is thicker and forms the stomach region where digestion of food takes place. Below this is the
more slender stalk which ends in an adhesive disc, called the pedal disc. Glands in the epidermis of the pedal disc secrete a
reversible mucoid adhesive. Amoeboid movements in the cells of the pedal disc can also cause the whole animal to slowly
glide over the substrate.
Above: the end of the stalk with its pedal disc.
Right: a close-up view of the column.
Histology of the Body wall
Hydra has been described as essentially a gut with tentacles! The bulk of the animal is hollow, containing the fluid-filled
gastrovascular cavity (enteron) which functions both as a gut and as a hydrostatic skeleton against which the muscular
cells of the body-wall can operate. The enteron extends into the hollow tentacles. When the tentacles catch food, they
bend toward the mouth which engulfs the food, bringing into the gastrovascular cavity of the stomach. Coelenterates are
often described as animals with two body layers (diploblastic). This feature is most apparent in small hydroids like hydra.
The body comprises two layers of cells, with a sheet of mesogloea sandwiched in between. The mesogloea is the jelly
characteristic of coelenterates, though its nature varies a lot between coelenterate types. In large jellyfish, the mesogloea
is highly thickened and becomes infiltrated by cells, forming a primitive tissue. In hydra, the mesogloea is cell-free (though
the 'wires' or axons of nerve cells penetrate it) and thin and functions as a supporting membrane for the two layers of cells
- the epidermis (ectodermis) on the outside and the gastrodermis (endodermis) on the inside. It is this feature that leads
many zoologists to regard only the Scyphozoa with their thick layers of mesogloea as 'jellyfish' and hydra is not so much
jelly-like as protoplasmic as its mass is composed mostly of cells.
The epidermis is made up of column-like epithelial cells packed side-by-side. The base of these cells contain contractile
fibres (myonemes, part of the cytoskeleton) which extend into tails that run parallel to the surface of the animal. These
muscular tails form fine muscle bands, but the muscle is not a separate tissue, and hence these cells are called
epitheliomuscular cells. Within the epidermis are shorter cells that are part of the epidermis but do not reach the outer
surface and these are called interstitial cells (as they sit in the 'spaces' at the bases between neighbouring
epitheliomuscular cells) and to the casual eye make the epidermis appear to be more than a single cell layer in thickness.
These cells have several functions - they are stem cells that can give rise to other cell types by differentiation and so are
involved in repair and regeneration, in producing new nematocytes, and in producing gametes. Some of the epidermal
cells are sensory, responding to touch and/or to chemicals in the water. The muscle bands of the epidermis abut against
the mesogloeal sheet and are longitudinal - running parallel to the long axis of the animal.
The gastrodermis is also a single layer of cells, though these cells are taller than the epidermal cells and in the stomach
region they are involved in synthesising and secreting digestive enzymes for food digestion. At least some of these cells
bear one or more flagella and the beating of these flagella help to circulate the fluid in the gastrovascular cavity (along
with movements of the body and tentacles). Other cells produce small pseudopods to absorb particles of
partially-digested food by phagocytosis. ingested food particles become enclosed within food vacuoles, inside the cells,
and here digestion is completed. The epithelial cells of the gastrodermis also have muscular tails, but these run
perpendicular to those in the epidermis, and run around the circumference of the column.
Hydra may reproduce asexually by budding. A cylindrical outgrowth extends out from the side of the column, generally
in the posterior half. The enteron extends into this bud and the bud develops a mouth and tentacles and feeds (its
enteron or gut is continuous with that of the parent). Eventually, a constriction appears near its base and it is pinched
off, sometimes assisting itself by grabbing hold of a nearby surface with its tentacles and pulling itself free. Hydra may
increase their numbers thirty or forty-fold in several months by this means. When the young hydra detaches it will float
in the water for a time to allow its dispersal from the parent. Young hydra may also secrete gas which forms bubbles in
secreted mucus, assisting flotation, during which the animal floats upside-down, fishing with its tentacles. Mature
individuals will float in this way if they encounter repeated disturbances or unfavourabel conditions, allowing them to
drift to a new location.
Hydra can also reproduce sexually. Sexual reproduction is seasonal, the season depending on species, but generally
summer or autumn. There is no free sexual medusoid stage and gonads develop on the column of the polyp. These
gonads are not really true organs, but accumulations of gametes produced from interstitial cells that cause swellings in
the epidermis on the sides of the column. Usually, several testes develop on the anterior half of the column and usually
one ovary on the posterior half of the column. A thinning of the epidermis occurs over the testis, developing into a
nipple-like papilla which eventually ruptures, shedding ripe sperm from the testes. The spermatozoa are flagellated and
swim towards ripe ovaries. In a ripe ovary, the epidermis has ruptured to form a pore through which the sperm can
swim to reach the oocyte. The ovary begins as a group of cells, but the larger of these cells eventually engulfs the
others to form a single oocyte or egg cell.
Above: a single ovary, which is a swelling in the epidermis
occupied by a large oocyte filled with dark yolk granules. Right:
a testis (three in total were visible in this cross-section of the
column) filled with hundreds of tiny sperm cells.
Most species are protandrous - meaning that although testes and an ovary develop on the same individual, the testes
ripen first and shed their sperm before the ovary becomes receptive, thus ensuring cross-fertilisation.
Once the egg has been fertilised, the resulting zygote develops a thick spiny shell and falls away, sinking to the
bottom of the water where it will remain for a time, being dispersed by animals, surviving drought and cold and
scattering when the pond dries up, until more favourable conditions are encountered when the shell splits and a
young hydra emerges, floating to the surface for further dispersal in the water.
Hydras have the classic nerve net of coelenterates - a network of neurones that innervate the whole animal and join
together in a network. In hydra there is one nerve net in the epidermis, with the nerve cells sitting just above the
muscular tails of the epidermal or musculoepithelial cells. There is a second network on the other side of the
mesogloea that forms a gastrodermal nerve net. Early workers struggled to find connections between these two
systems, but later workers found nerve fibres crossing the meogloeal sheet (mesolamella) to connect the two nets
together, forming a single double-layered net. The nervous system of the hydra is considered to be one of the
simplest in the animal kingdom, however, it is more complex than it seems to be at first sight.
These nerve nets contain neurones with one axon ('wire'), two axons and a few with many axons. These cells are
called ganglion cells, perhaps a slightly misleading term as there are no true ganglia in hydra. (A ganglion is a mass
of intermediate neurons, intermediate because they are between sensory neurones and motor neurones, which
form computational centres or mini-brains). However, these cells are especially numerous at the bases of the
tentacles, and so there is a degree of ganglion formation (forming proto-ganglions). There is also some tendency
for axons to bundle together, forming proto-nerves. Sensory cells connect to the nerve net. In the epidermis, tall
sensory cells are especially frequent in the 'head' region - the tentacles and hypostome. Tall sensory cells occur
throughout the gastrodermis. These sensory cells synapse to the nerve-net neurones. Each of those in the
epidermis are ensheathed by an epidermal cell, forming a layer of insulation.
In addition to the nerve net, which is designed for rapid long-distance communication, the epidermal cells form a
neuroid system. Gap junctions (closable protein channels) connect neighbouring epidermal cells together and local
electrical signals, generated in one cell, can spread to neighbouring cells. In such a neuroid system, the signal
decays rapidly with distance from the source, but in a nervous system it does not, but instead consists of trains of
spikes of electric charge that maintain their strength over a long distance. Every living cell (or more specifically the
cell membrane) is a capacitor - an electrical component that stores electric charge. Various stimuli, such as a pulse
of light or a mechanical touch activate these capacitors, resulting in the flow of electric current across the cell
membrane. It is this flow of charge that constitutes the electrical signals in neuroid and nervous systems.
In all these hyrozoan polyps, including hydra, the reactions are typically non-exact and not integrated, such that
isolated tentacles or stalks carry on doing what they do in the whole animal.
Behaviour, Feeding and Locomotion
If a hydra is well-fed then small animals, such as water-fleas, brushing past it will not trigger feeding behaviour.
Brushing a hungry hydra with a glass rod produces no response. Adding meat extract to the water will cause a
hungry hydra to open its mouth widely. The feeding response proper is triggered by a tripeptide (a chain of three
amino acid residues), called glutathione, which is almost universal in animal tissue. This chemical, along with
mechanical stimulation (touching the tentacles) will trigger nematocyst discharge.
Chemical irritants or prodding the hydra will cause it to contract and then expand again in a slightly different, though
random direction. If the noxious stimulus persists then it will move by looping: the body is extended and bent over to
one side so that the mouth end touches the substrate, the tentacles adhere by discharging a specific type of
nematocyst (glutinant) and then the base detaches and is drawn up toward the mouth and reattached. This looping
may repeat and appears to be in a random direction. During looping the basal end may reattach behind the mouth,
or off to the side beside the mouth, or it may loop around 180 degrees and be positioned in front of the mouth
('somersaulting'). A hydra can move up a glass wall in this way. Locomotion also occurs when the hydra is not
acquiring sufficient food.
Young hydare can also secrete a raft of gas bubbles, formed by secreting gas into mucus covering the base of the
animal, causing the hydra to float upside-down as the base detaches. In this position it may fish with its tentacles.
older hydrae may also do this when a noxious stimulus persists.
The whole hydra can also slowly glide across a surface by amoeboid movements of the cells in its adherent base
Hydras will also move when oxygen levels become too low, moving from regions of low oxygen concentration to
regions of high oxygen concentration.
Hydras are carnivorous, catching and digesting any organism small enough and which contacts the tentacles.
typical food includes, crustaceans (like water fleas and cyclops), nematodes and other small worms, and eggs and
larvae of various invertebrates. Dead organisms may be ingested if freshly killed.
When hungry a hydra will extend its tentacles further and move them about more and they may locomote, as
described, if hunger persists. When food contacts the tentacles the nematocysts discharge, trapping the prey which
adheres to the tentacles by the nematocyst barbs (the nematocysts act like lots of tiny harpoons, firing into the prey,
piercing it, and each remaining attached to the hydra by a thread). The tentacles then contract until the food
touches the mouth, which opens to grasp the food from the tentacles which detach from it. When at rest, the hydra
is typically spread with tentacles spread in the fishing position, but contractions and extensions of the stalk and
tentacles occur at intervals.
Ingested food enters the gastrovascular cavity and dissolves in proeolytic (protein-digesting) enzymes secreted by
the gastrovascular cells. This takes several minutes to an hour. The particles are then absorbed by gastrovascular
cells and presumably passed onto to other (epidermal) cells.
Each nematocyst is contained inside a cnidoblast cell (or nematoblast) - the nematocyst is an organelle. Each
cnidoblast has a sensory barb or cnidocil. Both mechanical stimulation and chemical stimulation triggers nematocyst
discharge. The cnidocil detects the mechanical stimulus and the cnidoblast cell is required to trigger discharge of
the nematocyst. The nematocyst organelle consists of a capsule, with a lid (operculum), containing the coiled
nematocyst thread or tube which is inside-out. At discharge, the lid springs open and the nematocyst tube turns
inside-out and the base of the tube, which is continuous with the wall of the capsule, emerges first, followed by the
rest of the tube from base to tip as it everts.
There are four types of nematocyst-containing nematocytes in Hydra, which all develop from common interstitial
stem cells (interstitial cells occupy 'spaces' between the other main body cells) - the nematocytes migrating to take
up position in the stinging-batteries as they mature. There are 25 known types of nematocyst in coelenterates, but
only 4 in Hydra, these are:
1. Desmonemes - these are involve din prey capture. These have closed tips and are non-venomous, but are
highly coiled and serve to tangle around prey.
2. Stenoteles -these discharge a little later as the ensnared prey struggles (they are less sensitive to discharge and
require the prey to struggle as a stimulus). These have hollow tips with spines, resembling arrow-heads (three
pointed styleyts form the 'arrowhead') which enter the body of the prey, injecting venom.
3. Spiny Isorhizas - these bear spines are used in defense (they have been found discharged in the single-celled
protozoan Paramecium, which is an irritant or parasite of Hydra).
4. Non-spiny isorhizas - these do not bear spines and are not fired during feeding, functioning to stick the tentacles
to the substrate during looping/somersaulting locomotion.
Each nematocyst battery consists of a small mound ( a battery cell) with the nematocytes embedded in it, with one
or a few large central stenoteles surrounded by a ring of smaller isorhizas and desmonemes. Each battery also
contains sensory cells, neurones and contractile myonemes. The stenoteles discharge at speeds of about 2 m/s.
Discharge is accompanied by an expansion of the nematocyst capsule, probably as water is pumped into it,
increasing the pressure inside, followed by rapid opening of the operculum and then evertion of the thread under
pressure. It is thought that contractile fibres inside the nematocyte also shorten, further increasing the pressure.
hydrae have remarkable powers of regeneration. experiments in which a hydra is diced into 30 or so small cubes,
formed into a pile, will regenerate as the cubes (or the cells within them?) move about to reform the whole animal.
Similarly, if a hydra is turned inside-out (!) then it will turn outside the rightside-out again as the cells migrate across
the mesogloea to reach the correct side! (Such that endodermal or gastrodermal cells are inside, ectodermal or
epidermal cells outside).
As usual, this article draws on a large number of sources and experience, including in-house microscopical
investigations, but the following sources were especially useful:
Kass-simon, G. and Scappaticci, Jr, A.A. 2002. The behavioural and developmental physiology of nematocysts.
Can. J. zool. 80: 1772-1794.
Hyman, L.H. 1940. The Invertebrates, protozoa through ctenophora. McGraw-Hill.
Vines, A.E. and rees, N. 1972. Plant and animal biology, volume 1. 4th ed. Pitman.
Barrington, E.J.W. 1979. Invertebrate structure and function, 2nd ed. Thomas Nelson & Sons Ltd.
Unlike the case in hydra, some hydrozoans do alternate between asexual polyps and sexual medusae
(hydromedusae) also called naked-eye medusae since they have no hood over the eyes (ocelli) as do the
medusae of scyphozoa (scyphomedusae, covered-eye medusa or true jellyfish) one of which typically occurs at the
base of each tentacle. these medusae are generally smaller than true jellyfish and have a thinner layer of
mesogloea and are more glassy than gelatinous.
Above: a 3D computer (Pov-Ray) model of the hydromedusa Aglantha (a trachymedusa).
Article updated: 5/1/2016