Sponges Phylum Parazoa examples Characteristics and Orders

All sponges are aquatic and most of them are marine. Freshwater sponges are found in fair numbers—but belonging to relatively few species—in ponds and lakes, streams and rivers, even in lakes formed in the craters of extinct volcanoes 10,000 feet above sea-level. Marine sponges exist in large numbers from the mid-tide level on the shore down to the greatest depths of the oceans. The smallest are about 1 mm. high when fully grown ; the largest are the size of a medium-sized barrel as in Spheciospongia vesparia, the so-called Loggerhead Sponge of the West Indies. On the shore they tend to encrust rocks where, by the coalescence of adjacent growths, certain species may be found in continuous sheets of varying thickness covering areas of several square yards. In the shallow and deep seas the form is more varied, ranging from a crust on stones, shells and dead coral to spherical, finger-shaped, bushy or tree-like, tubular, cup-shaped and funnel-shaped sponges. In general the greater the depth at which they grow the more their form tends to be regular and symmetrical. Those living in sha).low seas are the more colourful, and colour becomes more noticeable as the mean annual temperature of the water increases, as in the tropics, but usually sponges are monochrome with reds, browns, yellows and purples predominating, green less common and usually due to symbiotic algae, and blue very rare and usually the result of symbiotic blue-green algae or bacteria. The texture varies from soft and readily compressible (as in sponges lacking a skeleton, such as Oscarella) to as hard as stone (as in the polyphyletic —i.e. not descended from a common ancestor—group known as the Lithistida). The characteristic feature of a sponge is that it bears one or more usually conspicuous rounded openings. These used to be called oscula (little mouths) but are better described as vents. Under the microscope the rest of the surface is seen to be punctured by minute openings or pores, for which reason sponges are collectively known as the Porifera or pore-bearers. Aristotle was the first to recognise, 2,000 years ago, the animal nature of sponges, and yet it is only within the last 200 years that this verdict has been generally accepted. In the sixteenth century sponges were believed to be solidified sea foam, and in the seventeenth century, it was suggested that they were the homes of marine worms, made by the worms themselves. Otherwise, the general impression was that sponges belonged to the plant kingdom. Not until 1766, when John Ellis dis-covered that they eject currents of water and thus established that they were animals, was Aristotle’s view accepted, and then not by everyone. Even as late as 1841 John Hogg was still arguing before the British Royal Society that sponges were plants. Hogg based his ideas on freshwater sponges. Some of these have long, slender branches springing from a basal crust, and all are green, but turn yellow when growing in places shielded from daylight, just as green plants do. Moreover, in summer they are filled with small brown seed-like bodies. But we now know that the green colour of

 

 

 

Euplectella aspergillum or Venus’s Flower Basket, from 100 fathoms off the Philippines, a hexactinellid (six-rayed) sponge showing the symmetry that is characteristic of deep-sea sponges.
Colour plate PHYLUM CNIDARIA What is commonly thought of as a coral is in fact the calcareous skeleton, or several of these skeletons fused together. secreted by polyps, forms of coelenterates and polyzoans.
The Organ Pipe Coral (Tubipora musica) grows in long, upright, parallel tubes that are united horizontally at intervals by transverse stolons. What is usually seen is the skeleton, as has been explained above, and the red colour is thought to be caused by the presence of iron salts.

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