Protozoa and sponges

Protozoa, or “first animals,” are simple, unicellular (single-celled) organisms that make up the kingdom Protista. Although protists are not technically animals, they have a few animallike characteristics, and many scientists believe that higher animals evolved from them.
Protozoa are divided into four subphyla: the Sarcomastigophora (amebas and flagellates), the Ciliophora (ciliates), the Sporozoa, and Cnidospora. Most protozoa are microscopic.
Scientists have identified about 30,000 species of protozoa. They 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 withstand extremes of temperature and humidity.

Stentor is a large ciliate, about one inch (2.5 centimeters) long. The cilia around the rim of its trumpet-shaped body are joined to form membranelies, or plates, which beat to create feeding currents. It is a selective suspension feeder, rejecting those food particles that are of no nutritive value to it

Amebas and flagellates

Amebas have a constantly changing body shape and move by producing pseudopodia (false feet). The cytoplasm of the ameboid cell is pushed out to form the pseudopodium as the animal moves forward. The common Ameba has a naked cell surface, but a variety of shelled forms exists. The genus Difflugia, for example, constructs a case from grains of sand, whereas other amebas secrete shells of calcium carbonate and silica. These shells contain holes through which the pseudopodium can be extruded to collect food particles.

Some amebas are parasitic, such as Entamoeba coli, which lives in the human intestine. Found in up to 30 per cent of the world population, it scavenges bacteria and food particles. This harmless association is termed commensalism. But Entamoeba histolytica is harmful and causes amebic dysentery.

In contrast to amebas, flagellates move using a long, hairlike structure called a flagellum, which beats like a whip to provide propulsion. Most flagellates have a fixed body shape (usually oval), and almost all reproduce asexually by binary fission; but there is considerable diversity between species. Choanoflag-ellates, for example, have a delicate collarlike structure that surrounds the base of the flagellum and helps to collect food particles. Similar collar cells (choanocytes) are found in sponges, which suggests a possible link between these two groups. In some collared flagellates, the individuals do not separate after cell division, but give rise to a colony.

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. The presence of the photosynthetic pigment, chlorophyll, has led many biologists to classify such flagellates as algae (simple plantlike organisms belonging to the kingdom Monera). In all other respects, however, they are identical to flagellate protozoa.
Many flagellates live in association with other animals. Trichonympha, for example, lives in the intestines of termites. The termite relies on the protozoan to digest the wood that it eats a mutually beneficial relationship known as symbiosis. But some flagellates are blood parasites and cause disease. Trypanosoma, for example, causes African sleeping sickness.

The ciliate Paramecium reproduces by conjugation. When two adults with compatible micronuclei come together, the micronucleus of each divides twice. Three of the four new cells disintegrate but the fourth continues to divide. The cell walls joining the two ciliates dissolve, and the male nuclei of the two conjugants are exchanged. The adults then separate, and development of the new cell, called the synkaron, continues in each one. At about this stage the macronucleus starts to disintegrate and is resorbed.

Ciliates

The most complex and diverse species of protozoa owe their name 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. Ciliates reproduce asexually by binary fission, or sexually by conjugation. They differ from other protozoa in that they have two nuclei a macronucleus and a micronucleus instead of one.

Sporoza and Cnidospora

These organisms, the spore-formers, have no distinct adaptations for movement because they are all parasitic. They live in all animals and are often transmitted by insects. Their name comes from the production of spores, or cysts, during the infective stages of their life. The life cycle of this group is complicated—reproduction alternates from asexual to sexual. Asexual reproduction involves the binary fission of spores, usually in the host. The offspring develop into gametes, which mature and fertilize other gametes that eventually produce spores.

Sponges

The simplest group of invertebrates is Porifera (porebearers), the sponges. Scientists estimate that the number of poriferan species ranges from 5,000 to 10,000 species and that 98 per cent of these live in salt water. Most sponges are found in shallow waters, although some live in deep water. Sponges range in length from a quarter of an inch (0.5 centimeter) to more than 4 feet (1.2 meters). They lack muscles and organ systems of any kind and spend their lives cemented to a solid surface, or substrate, filtering tiny food particles from the water.

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 collar cells, or choanocytes, equipped with whiplike structures called flagella, which maintain a flow of water through the sponge. Sponges extract food particles from this current, which they also use for gas exchange and waste removal. The sponge draws water in through small pore cells in its walls, and ejects it from its large mouth, or osculum. More advanced sponges have complex systems of canals and chambers through which water is channeled.

Sponges reproduce both sexually and asexually. Most are hemaphroditic, or have both male and female characteristics, and produce eggs and sperm at different times. Asexual reproduction occurs by budding. In addition, some sponges produce clusters of cells surrounded by a protective covering called gemmules that survive when the parent body disintegrates in winter. In spring the gemmule develops into an adult sponge. Sexual reproduction occurs when sperm is released and enters another sponge in the water current.

Glass sponges, or hexac-tinellids, comprise the class Hexactinellida, and are mainly deepwater sponges. Like other sponges, they have special cells that secrete the skeleton, which is made up of spicules, or tiny needlelike structures. In bath sponges, these spicules are composed of a horny protein called spongin, but in glass sponges they are made of silica. These siliceous spicules are fused to form a six-pointed shape, from which the class name of the glass sponges is derived.