Australia: The Land Where Time Began
According to Gershwin the group “jellyfish” are not a natural collection of animals, they are from 3 evolutionary lineages that are different from each other, the common features of all these disparate organisms being they are all transparent drifters:
1) The Medusozoa, meducae and siphonophores;
2) The ctenophores
3) The Salps and related organisms.
There are smaller subsets within these larger groups that have radically different features. They share basic elements of their anatomy, though they are quite disparate, these elements being reflected in their taxonomy, or identification and classification.
Features that are common to all jellyfish are:
1) a gelatinous body;
2) a method of catching food;
3) a defence mechanism;
4) a method of reproduction; and
5) a method of moving from point A to point B.
Gershwin descries the basic life functions as being amazing and varied. Actually, these simple animals are in many respects not all that simple.
The 3 Jellyfish lineages
The most familiar of the jellyfish and the most numerous is the meducae of phylum Cnidaria, which also contains the corals, sea anemones and sea fans. The meducae, which are from 4 classes within the subphylum Medusozoa, Scyphozoa, Cubozoa, Staurozoa and Hydrozoa, the meducae usually are of a dome or disc shape, though the occur in a great variety of other shapes on that theme. They have a radial symmetrical body plan, that somewhat resembles a pie that is perfectly circular divided into equal segments that are all identical to each other. Most species are tetraradial, i.e. having 4 equal segments; while others can be 8 identical segments (octoradial), and there are a few that have 6 identical segments (hexaradial).
Meducae are built on the same basic body plan as coral polyps or sea anemones, though their stomach opens downwards whereas the stomach of a coral polyp opens upwards and is surrounded by tentacles. Another difference is that coral polyps and sea anemones are fixed to the sea floor while the meducae drift in the water.
The siphonophores are a peculiar offshoot lineage of the cnidarian class Hydrozoa. They have 3 different body plans:
1) A float and swimming bells,
2) A float with no swimming bells,
3) Swimming bells but no float (see Siphonphore Life History).
Siphonophores are neither predominantly radial nor bilateral, though certain components may be one or the other. As they are composed of parts that do not resemble one another or the animal as a whole, they are some of the most difficult of all organisms to identify.
Comb jellies, phylum Ctenophora, and salps and related forms, are also known as pelagic tunicates, of the phylum Chordata. Both these groups of organisms are bilaterally symmetrical, as are humans, in this case there is no way to divide the organisms up that yields identical segments. There groups are quite distinct from each other in other ways. Salps can be described as a barrel encircled by bands of muscle, though ctenophores occur in a variety of shapes and have 8 rows of large cilia that are longitudinally arranged, which are the “combs” from which they get their name “comb jelly”.
The organisms that are called jellyfish share many obvious features, in spite of their different body plans, symmetries and shapes, as well as other various peculiarities. They live in the water column, rather than the sea floor, i.e. they are pelagic, they drift with the current, and with few exceptions they are not able to swim against the current, therefore they are planktonic. They have gelatinous or jellylike bodies, which helps with buoyancy. Most are transparent, which is believed to help them with defence.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|