Zoology

Paramecia are often described as slipper shaped. Paramecium caudatum is 150 to 300 µm in length and is blunt anteriorly and somewhat pointed posteriorly. The organism has an asymmetrical appearance because of the oral groove, a depression that runs obliquely backward on the ventral side. The pellicle is a clear, …

The Euglenozoa is generally considered a monophyletic group, based on the shared persistence of the nucleoli during mitosis, and the presence of discoid mitochondrial cristae. Members of this phylum have a series of longitudinal microtubules just beneath the cell membrane that help to stiffen the membrane into a pellicle. The …

Most animals move to search for food, but a sessile sponge draws food and water into its body instead. The entrance of water through myriads of tiny pores is reflected in the phylum name, Porifera (po-rif´ -er-a) (L. porus, pore, + fera, bearing). The sponge uses a flagellated “collar cell,” …

Reproduction Sexual phenomena occur widely among protozoa, and sexual processes may precede certain phases of asexual reproduction, but embryonic development does not occur; protozoa do not have embryos. The essential features of sexual processes include a reduction division of the chromosome number to half (diploid number to haploid number), the …

Vacuoles can be seen by light microscopy in the cytoplasm of many protozoa. Some of these vacuoles periodically fill with a fluid substance that is then expelled. Evidence is strong that these contractile vacuoles function principally in osmoregulation. They are more prevalent and fill and empty more frequently in freshwater …

Nucleus As in other eukaryotes, the nucleus is a membrane-bound structure whose interior communicates with the cytoplasm by small pores. Within the nucleus the genetic material (DNA) is borne on chromosomes. Except during cell division, chromosomes are not usually condensed in a form that can be distinguished, although during fixation …

A cilium or flagellum has considerable internal structure. Each flagellum or cilium contains nine pairs of longitudinal microtubules arranged in a circle around a central pair and this is true for all motile flagella and cilia in the animal kingdom, with a few notable exceptions. This “9+ 2” tube of …

A protozoan, or unicellular eukaryote, is a complete organism in which all life activities occur within the   limits of a single plasma membrane. Unicellular eukaryotes are found wherever life exists. They are highly adaptable and easily distributed from place to place. They require moisture, whether they live in marine or …

Eukaryotes (“true nucleus”) have cells with membrane-bound nuclei containing chromosomes composed of chromatin. Constituents of eukaryotic chromatin include proteins called histones and RNA, in addition to DNA. Some nonhistone proteins are found associated with both prokaryotic DNA and eukaryotic chromosomes. Eukaryotes are generally larger than prokaryotes and contain much more …

The Oparin-Haldane hypothesis stimulated experimental work to test the hypothesis that organic compounds characteristic of life could be formed from the simpler molecules present in the prebiotic environment. In 1953, Stanley Miller and Harold Urey in Chicago successfully simulated the conditions thought to prevail on the primitive earth. Miller built …