Protozoa and Sponges

The Protozoa are divided into 4 subphyla:

1. Sarcomastigophora (amebae and flagellates)
2. the Ciliophora (ciliates)
3. Sporozoa
4. Cnidospora

They are acellular (having a body which is not composed of cells) organisms and constitute the simplest animal group. They comprise about 50,000 species and are found in almost every habitat where moisture is present, and also as parasites in most animals. Because of their resistant spores, some protozoa can resist extremes of temperatures and humidity. It's not surprising that they are among the most numerous animals in the world. Some protozoa share similar features with sponges, which suggests that these two groups may be related.

Amebae and Flagellates
Amebae have a constantly changing body shape and more by producing pseudopodia (false feet). The cytoplasm of the ameboid cell is extruded at one point to form the pseudopodium as the animal moves forward. The common Amoeba has a naked cell surface, but a variety of shell forms exist; the genus Difflugia, for example, constructs as a case from minute grains of sand, whereas other amebae secrete intricate shells of calcium carbonate and silica.

Some amebae are parasitic, such as Entamoeba coli which lives in the human large intestine. It does not transmit disease, but scavenges bacteria and food detritus (organic debris). Such an association is termed commensalism. But the related Entamoeba histolytica is harmful and causes amebic disentery.

<==This electron micrograph depicts an amoeba, Hartmannella vermiformis (orange) as it entraps a Legionella pneumophila bacterium (green) with an extended pseudopod After it is ingested, the Legionella pneumophila bacterium can survive as a symbiont within what then becomes its protozoan host. The amoeba then becomes what is referred to as a "Trojan horse," for by harboring the pathogenic bacteria, the amoeba can afford them protection, and in fact, in times of adverse environmental conditions, are able to metamorphose into a cystic stage enabling it, and its symbiotic resident pathogens to withstand such environmental stresses.

In contrast to amebae, flagellates move using a long hairlike structure called flagellum, which beats like a whip to provide propulsion. Most flagellates have a fixed body shape (usually oval) and almost reproduce asexually, but there is considerable diversity between the species. Most flagellates feed on small particles and organic materials dissolved in the surrounding water. But some, such as the green Euglena commonly found in ponds, are able to produce their own food by photosynthesis. May flagellates live in association with other animals. Trichonympha, for example, lives in the intestine of termites. The termite relies on the protozoon to digest the wood that it eats, a mutually beneficial relationship known as symbiosis. But some flagellates are blood parasites, such as Trypanosoma, and cause disease (in this case sleeping sickness).

Ciliates,
The most complex and diverse species of protozoa owe their names to the cilia (short hairlike fibers similar in structure to flagella) which grow in orderly rows on the body and beat rhythmically to propel the animal. In many ciliates, the cilia occur only on parts of the body, whereas in others they form plates or "membranelles." Still others have cilia that are fused into stiff cirri and used as legs for crawling. Ciliates reproduce asexually by binary fission, or sexually by conjugation. They differ from other ptotozoa in that they have two nuclei, a macro- and micronucleus (other protozoa have only one).

Sporozoa and Cnidospora
These organisms, the spore-formers, have no distinct locomotory adaptations because they are all parasitic. They live in all animals and are often transmitted by insect vectors. Their name comes from the production of spores, or cysts, during the infective stage of their life. The life cycle of this group is complicates, reproduction alternate from sexual to asexual.

Sponges
Sponges comprise the phylum Porifera (pore-bearers) and represent the simplest level of multicellular . Most of the 10,000 species are found in shallow waters, although some live in deep water. They range in length from one-quarter of an inch to more than 3 feet.
Photo courtesy of Eugene Weber © California Academy of Sciences

In contrast to the colonial protozoa, sponges consist of several cell types, each of which performs a specific function and is independent. This feature means that regeneration is easy—if a sponge is fragmented, the cells simply organize themselves into a new sponge.

The simplest sponges are tubular with an external layer of epithelial, or lining, cells. The internal surface is covered with flagellated collar cells (choanocytes) which maintain a water current through the sponge. Food particles are extracted fron this current which is also used for gas exchange and waste removal. Water is drawn in through small pore cells in the walls of the sponge, and ejected from the large mouth (osculum). More advanced sponges have complex sytems of canals and chambers, through which water is channeled.

Sponges reproduce sexually and asexually. Most are hermaphroditic, and produce eggs and sperm at different times. In addition some sponges produce gemmules (buds or internal groups of cells) which survive when the parent disintegrates in winter. In spring, the gemmule develops into an adult sponge.

References:
1. Mike Janson and Joyce Pope (consultant editors). The Animal World
2. U.S. Fish and Wildlife Service Multimedia database
3. Public Health Image Library (PHIL) Photographs, Illustrations, Multimedia Files '