The phylum Cnidaria is predominantly marine and contains many common shore animals such as sea anemones and sea firs (hydroids), as well as deeper water forms such as the Portuguese Man o’ War. Freshwater forms are less common, but one group, the hydras, is probably the best-known member of the phylum.
Classes of Cnidaria
There are three classes:
- Hydrozoa (hydroids or sea firs)
- Scyphozoa (large jellyfishes)
- Anthozoa (sea anemones and corals).
All are animals with a low grade of organisation, simpler structures being found only in sponges, mesozoans and protozoans. The body wall is composed of two layers of cells, ectoderm and endoderm, separated by a layer of non-cellular material, the mesogloca.
There is a single internal cavity, the coelenteron (gastrovascular cavity or, simply, enteron) which has both digestive and circulatory functions.
The coelenteron has only one opening to the exterior, the mouth, which is generally surrounded by ten tacles. Both tentacles and general ectoderm bear nematocysts (cnidae or thread capsules) specialised for stinging or otherwise capturing prey. Cnidarians may be solitary or colonial, many of the latter
producing a calcareous exoskeleton, as in corals. The most obvious external feature is radial symmetry; a plane taken through any lengthwise direction gives identical halves. The main axis of symmetry is from mouth to base (oral/aboral) with the parts arranged concentrically around this axis.
There is no definite head and the nervous system is not centralised. There are no defined respiratory, circulatory or excretory organs.
Polymorphism in Cnidarians
A feature of the Cnidaria is polymorphism, there being two main types of individual, the polyp and the medusa. The polyp or hydroid is the sessile form resembling a cylinder, closed at one end and with mouth and tentacles at the open end. It may occur in a variety of forms even within the same species.
The medusa or jellyfish is a free-swimming form in which the cylindrical part of the polyp is shortened, the mesogloea thickened and the diameter increased, giving a bell- or saucer-shaped animal with a central mouth and tentacles round the bell margin.
When both polyp and medusa are present in the same species, as in many hydroids, they occur in alternate generations. A sedentary polyp genera tion asexually produces a medusa capable of sexual reproduction, the fertilised ovum produc ing a larva which transforms into a polyp.
In scyphozoan jellyfishes the medusa is the dominant stage, the polyp generation often being omitted, whereas hydras, sea anemones and corals are exclusively polypoid.
When both polyp and medusa are present in the same species, as in many hydroids, they occur in alternate generations. A sedentary polyp genera tion asexually produces a medusa capable of sexual reproduction, the fertilised ovum produc ing a larva which transforms into a polyp.
In scyphozoan jellyfishes the medusa is the dominant stage, the polyp generation often being omitted, whereas hydras, sea anemones and corals are exclusively polypoid.
The polyp can be divided into three regions: basal, oral and column. In solitary forms the basal region or pedal disc adheres to the substratum and in colonial cnidarians it is connected by a root-like stolon to the base of the next polyp. The oral region bears solid or hollow tentacles commonly arranged round the mouth, which is usually circular.
The third region, the column, lies between the basal and oral regions. The body wall encloses the sole cavity, the coelenteron, which in hydrozoans is simple and tubular, but in scyphozoan and anthozoan polyps is divided by ridges or septa extending from the body wall.
This generalised form of the polyp may be modi fied. In some colonies polyps occur whose only function is to bud off medusae (Obelia), or they may
have a solely defensive role (Hydractinia), in which case there is no mouth but only tentacles bearing considerable numbers of nematocysts.
The free-swimming medusa, which can be likened to a deep or shallow bowl of gelatin, swims with the convex, aboral surface (the exumbrella) uppermost and the concave, oral or subumbrella surface beneath.
A short projection or manubrium from the middle of the subumbrella surface con tains the mouth. Leading internally from the mouth there is a small central ‘stomach’ region from which radial canals extend towards the bell margin where they connect into a ring or circular canal.
The mesogloea is much more developed than in the polypoid form so that ectoderm and endoderm are separated by some distance, the ectoderm forming a thin layer on the exumbrella and subumbrella surfaces and the endoderm lining the manubrium and canal system. The bell margin bears a varying number of tentacles and sense-organs, the latter being generally concerned with maintaining balance.
The medusa swims by a form of jet propulsion, alternately contracting a ring of muscle fibers round the bell margin and passively expanding via the elasticity of the mesogloea.
Medusae are usually produced by budding from the polypoid colony. The parent may be a modified polyp, a feeding polyp, or some other part of the colony, e.g. the stolon.
The medusa is generally the sexually mature form producing gametes, but in certain cases may be modified by loss of structures such as tentacles, mouth, and reproductive organs to become a float or swimming organelle, or it may lose mouth and tentacles to function as a reproductive body only, often on the parent colony.
Topics covered:
Classes of Cnidaria, Example of Cnidaria, medusa in Cnidarians, mesogloea in Cnidarians, polype in Cnidarians