Animals World

Some interesting facts about Animals:

  • Response to Stimuli Reflexes are types of action consisting of relatively simple segments of behavior usually occurring as direct and immediate responses to particular stimuli uniquely correlated with them. In addition to other, more complex stimulus-bound responses such as fixed action patterns, reflexes account for many of the behavioral patterns of invertebrates. In higher animals, such as primates, learned behavior dominates; however, reflexes remain a significant component of total behavior. Reflexes are genetically determined. No effort is needed to acquire reflexes; they simply occur automatically, usually taking place beneath the conscious level. Although heart rate and ventilation rate, for example, are constantly regulated by reflexes, most people are unaware of these modifications. In like manner, pupillary diameter and blood pressure are regulated reflexively, without conscious knowledge. Other reflex responses, however, such as perspiring, shivering, blinking, and maintaining an upright position, are more apparent, although they also occur without conscious intervention. Only one neuron (or even an absence of neurons) is required for the simplest known reflexes. For example, ciliated protozoa, single-celled animals having no neurons, exhibit what appear to be reflex actions. When a paramecium collides with an object, it reverses the stroke of the cilia (hairlike outgrowths), backs away a short distance, turns, and again moves forward. When touched caudally (at or near the tail), the animal moves forward. In this instance, the animal's cell membrane itself serves as the receptor of the stimulus, and the cilia act as effectors for directed movement. Very simple reflexes also occur in higher animals. For example, when human skin is injured sufficiently to stimulate a single pain neuron, an unknown substance is released, causing small local blood vessels to dilate. In contrast to these simple responses, however, most reflexes require a vast sequence of neurons. In most reflexes, the neurons involved are connected by specific synapses to form functional units in the nervous system. Such a sequence begins with sensory neurons and ends with effector cells such as skeletal muscles, smooth muscles, and glands, all controlled by motor neurons. Interneurons, or central neurons, are often interposed between the sensory and motor neurons. This sequence of neurons is called a reflex arc. The sensory aspect of the reflex arc conveys specificity regarding the particular reflex to be activated. That is, the sensory cells themselves determine which environmental change is sensed, either inside or outside the body. The remainder of the reflex response is regulated by the specific synaptic connections that lead to the effector neurons.

  • Zoology Attempts at classification are known fromdocuments in the collection of the Greek physician, Hippocrates, as early as 400 b.c.e. However, the Greek philosopher Aristotle (384-322 b.c.e.) was the first to devise a system of classifying animals that recognized commonalities among diverse organisms. Aristotle arranged groups of animals according to mode of reproduction and habitat. After observing the development of selected animal groups, he noted that general structures appear before specialized ones, and he also distinguished between sexual and asexual reproduction. Aristotle was also interested in form and structure, and concluded that different animals can have similar embryological origins and different structures can have similar functions. In Roman times, Pliny the Elder (23-79 c.e.) compiled four volumes on zoology widely read during the Middle Ages. Some scholars have deemed those volumes little more than a collection of folklore, myth, and superstition. One of the more influential figures in the history of physiology, the Greek physician Galen (c. 130-c. 201 c.e.), dissected farm animals, monkeys, and other mammals and described many features accurately, although scholars have noted that some of these features were then wrongly applied to the human body. His misconceptions, especially with regard to the movement of blood, remained virtually unchanged for hundreds of years. In the seventeenth century, the English physician William Harvey established the true mechanism of blood circulation.

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