Protozoa and sponges animals

 

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.

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.

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