The Components of the Circulatory System The pump may be a simple tube lined by muscle fibers. Alternate contraction and relaxation of these muscles produces a peristaltic wave that pushes the blood along. Thepumpmay be a heart: a localizable, discrete organ whose muscle layers are the primary generators of the power that propels blood through the blood vessels. The heart can be a simple muscular enlargement, or it can be complex and multichambered, depending on the evolutionary history and the needs of the organism. Some organisms may have more than one heart. If the pumped fluid remains within the blood vessels, it is usually called blood. If it leaves the blood vessels to enter cavities surrounded by tissue cells with which it exchanges materials, it is called hemolymph. In the cavities (called lacunae if small and sinuses if large and lined with a membrane) blood mingles with intercellular fluid. Whatever it is called, the fluid is composed of water, solutes (such as salts, sugars, and other nutrients), and, in some cases, cells and formed elements. The blood may also contain a respiratory pigment that helps deliver oxygen to the cells.
Polarized Light Vision Polarized light plays an important role in the visual world of many animals, including many species of fish and insects. These animals are able to use their visual systems to decode one particular aspect of polarized light, namely the angle of polarization. Light which is polarized has an electric vector which has a specific angle, or orientation. Natural light is unpolarized, but becomes polarized by scattering in air, water, or by reflection off surfaces. Scattering of the ultraviolet (UV) light in sunlight is quite predictable and produces obvious patterns in the sky, invisible to humans without a polarizing filter. These patterns are analyzed by some insects and are used as a kind of map for navigation, for example, to get back to the nesting area after a foraging trip. Similarly, the polarization pattern aids in orientation of the animal. Analysis of the skylight polarization patterns is possible because of the unique positioning of the microvillar photoreceptors within the eye. Pairs of microvillar photoreceptor populations are positioned perpendicular to each other. The light-sensitive molecules within the photoreceptor lie parallel to the axes of the microvilli. Each member of the pair of photoreceptors is maximally sensitive to a different angle of polarized light, and the combined response to a given angle of both members allows the animal to compute the exact angle of polarization. Enhancement of contrast is an advantage of polarized light vision used by fish, although the mechanism of detection of the light is obscure.
