Essays --

Animals have always been evolving and are constantly adapting to their changing environments. All organisms require the intake of food and disposal of waste, the exchange of carbon dioxide and oxygen, and respiratory gases. To determine how much of this each organism needs, it is based on each organism’s volume. Organisms are able to exchange materials the fastest when the surface-to-volume relationship is larger. Because larger organisms have a smaller surface-to-volume relationship, the exchange of materials and the ability to lose heat is more difficult. Organisms have changed from single, to multicellular. A single celled organism has a large surface-to-volume ratio, which means it is able to efficiently exchange and remove materials. As these organisms evolve over time and grow larger, they eventually become multicellular and must increase their surface area. Sea Anemones and Tapeworms are a perfect example of this as they have elongated, flat bodies. Thus, the d iffusion between the organism and its environment only require a short time. Question 2 The various structures of porifera include Asconoid, Syconoid and Leuconoid. Asconoids have the simplest structures, an atrium lined with choanocytes. Incurrent ostia allow water directly into the chamber. Asconoids eventually increased the thickness of their body wall and became Syconoids. Syconoids have choanocyte chambers that extend into the body wall. An even thicker body wall was created and Syconoids evolved into the most complex structure, the Leuconoid. Leuconoids have choanocyte chambers isolated deep within a body wall. Incurrent and excurrent canals lead to them from the outside, which then leads to the atrium. With each organ exchanging specific ... ... development was muscle tissue. The platyhelmiths are the first animal with true musculature. Muscle from the mesoderm is attached to the endoderm and ectoderm. Endoderm muscles line the pharynx and are surrounded by muscles that allow it to be extended, withdrawn and suck in food. There are also longitudinal muscle fibers that allow the body to elongate and perform the various turning and bending activities of the worm. A protonephridium is a network of dead-ending tubules, also known as flame cells, that function is osmoregulation and ionoregulation. Each cell has one or more cilia and their beating creates an outward going current and pressurization. The pressure created drives waste fluids from the inside of the animal to the protonephridium. The terminals are large enough for small molecules to pass through, but larger proteins are kept in the animal.