Hydrozoa

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

sexually mature.

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.

Left: 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.

Reproduction

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.

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.

Nervous System

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.