5.1.4. Class Sporozoa

The fourth class of Protozoa is the Sporozoa, all of which are parasitic. They are characterized by the lack of cilia of flagella in adult forms and by their complex life cycle. A single sporozoan undergoes multiple fission, dividing into numerous smaller cells (the spores) simultaneously.

The sporozoans are parasitic protozoans which were formerly united within one class, the Sporozoa, because of the presence of sporelike infective stages in some members of both groups. The presence of flagellated gametes, the ability to move by gliding or body flexions, and the possession of pseudopodia as feeding organelles in different groups suggest a relationship to the flagellates and to the sarcodines.

The subphylum Sporozoa contains the most familiar of the sporozoan parasites—the gregarines and the coccidians. The coccidians infect the intestinal or blood cells, and include the parasites causing human malaria and the coccidioses of domestic animals. Species of Sporozoa are widespread and parasitize vertebrates and most invertebrate phyla.

The life cycle is complex. It can be divided into three phases: schizogony, an asexual multiplication of the parasite following infection of the host; gamogony, the development of gametes; and sporogony, also a multiplication and an infective stage, typically a spore.

The best known sporozoans are members of the genus Plasmodium, which cause malaria. Plasmodium is passed back and forth between man and the Anopheles mosquito. The female mosquito requires blood for the development of her eggs. If the female draws her blood from a person with malaria, she will pick up Plasmodium cells which multiply within her body, although they do not harm her, and travel in her salivary glands. When she bites her next victim, she injects a droplet of salivary fluid under his skin. This fluid seems to anesthetize the victim against the bite; it also keeps the blood from clotting so that it will flow freely through her fine proboscis. Ultimately, through an inflammatory reaction, it raises the familiar welt. This drop of salivary fluid also carries Plasmodium, which eventually enter the blood cells and begin to multiply. The Plasmodium break out of the blood cells at regular intervals – usually every 48 or 72 hours, depending on the species – that is why malaria is characterized by recurrent bouts of chills and fever.

5.2. Phylum Porifera: Sponges

Sponges are simple invertebrate animals that are made up of many cells. Sponges are made up of two layers of cells around a central cavity (the space in the center). The outside layer is the protective epidermis, which contains many pores (each pore is hollow. It’s like a short piece of pipe connecting the outside and the inside of the animal). The inside layer is made up of collar cells. The whip-like flagella on the collar cells wave back and forth creating currents. Food particles in the incoming water are trapped and digested by the collar cells. Wastes leave the sponge through the opening at the top of the central cavity.

The shape of a sponge is determined by the materials found between epidermis and the collar cells. The skeleton is located primarily in the mesenchyme, but spicules frequently project through the epidermis. Spike-like specula form a hard framework. Some sponges have a material called spongin instead of specula. Rubber-like spongin forms a soft, spongy framework.

Sponges reproduce sexually, and sperm and eggs are shed into the central cavity. They meet and fuse. The young sponge (larva) that develops from a fertilized egg leaves the central cavity through the opening and attaches itself to a surface. Then it begins to grow into an adult sponge.

Also the sponges can reproduce asexually. For example, freshwater sponges may form balls of cells called gemmules when the environment is not right. When the sponge dies, the gemmules are released. They are made up of certain cells and some stored food. Each gemmule may become a new sponge when the environment becomes better.

New sponges may also form in another way. Small pieces sometimes break off of large sponges. When this happens, each of the small pieces attaches itself to a stone or other surface and begins to grow into a new sponge. In fact, people often cut living sponges into small pieces, attach each piece to a cement block, and drop the blocks into the sea. Each piece will grow into a new sponge. After several years, the sponges will be large enough to be harvested and sold.

Sponges have been used since ancient times as bath aids, cleaning tools, and even as drinking vessels. When dried, the skeletons of certain species of sponges are soft and elastic yet still capable of absorbing large amounts of water. Natural sponges are still preferred by artists and craft workers and by hospital surgical teams.