Major Animal Body Plans
Animal body plans can have one of three different forms concerning symmetry: asymmetrical, radial, or bilateral. Asymmetrical animals show no symmetry. A sponge is an example of an animal with an asymmetrical form. An animal that has radial symmetry is symmetrical longitudinally. Many aquatic animals that attach to a base have radial symmetry. If an animal with radial symmetry was cut on any longitudinal plane, it would look the same inside. Bilateral symmetry is when an animal is symmetrical on the left and right. If you were to cut an animal, such as a horse, in half from the nose to the tail, it would be symmetrical on the left and right sides. All true animals have bilateral symmetry, and it is essential for movement.
Animals can also be characterized by whether or not they have a coelom. A coelom is an internal body cavity that comes from the mesoderm. It contains the digestive system, the heart, kidneys, and reproductive system. Some animals do not develop a coelom, and they are called acoelomates. Flatworms are an example of an acoelomate. Mollusks, arthropods, and echinoderms all develop a true coelom and are called eucoelomates. Other animals, such as roundworms, develop a false coelom and are called pseudocoelomates.
During embryonic development, tissues are separated into germ layers. Animals can either have two or three germ layers. Animals with radial symmetry develop two germ layers. They are called diploblasts. Examples of diploblastic animals are jellyfish and corals. Animals that have bilateral symmetry develop three germ layers. They are called triploblasts and are more complex than diploblasts. Worms, arthropods, and vertebrates are all examples of triploblastic animals.
The respiratory system has two parts: the upper respiratory tract and the lower respiratory tract. The nose, mouth, and beginning of the trachea are the upper respiratory system. The lower respiratory system is in the chest cavity and includes the trachea, the bronchi, bronchioli, and the lungs.
During breathing, the nose and mouth inhale air. Air flows through the pharynx, larynx, and trachea into the lungs. Exhaled air leaves through the same path. The larynx is called the voice box, and as air passes across it, it allows us to make sound. The trachea leads to the lungs, where it is split into the right and left bronchi. The bronchi branch out into bronchioles, which then lead to the alveoli. The alveoli are where oxygen is exchanged for carbon dioxide. The diaphragm is a muscle at the bottom of the rib cage responsible for helping the lungs expand to inhale and exhale.
The body uses feedback loops to turn things on and off. Negative feedback loops will stop the stimulus that has triggered them. Negative feedback loops are usually working to keep things in equilibrium. Negative feedback loops usually control hormones in the body. One example of a negative feedback loop is thyroid regulation. The hypothalamus secretes TRH, which works on the pituitary gland to make TSH. The TSH causes the thyroid gland to secrete hormones. When the hormone level is high enough, the negative feedback loop causes the hypothalamus to stop secreting TRH, and the pituitary stops secreting TSH.
Another example of a negative feedback loop in the body is the regulation of blood sugar. As blood sugar increases, the pancreas releases insulin, which allows cells to take in glucose. As blood sugar drops, the pancreas releases glucagon, which releases glucose into the blood. This brings blood sugar levels into equilibrium.
The fight-or-flight response is also a negative feedback loop. The hypothalamus activates both the sympathetic nervous system and the adrenal cortical system. The sympathetic nervous system sends out signals that tell the adrenal medulla to release adrenaline and noradrenaline. These are stress hormones and cause an increase in heart rate and blood pressure. The hypothalamus also releases corticotropin-releasing factor (CRF) into the pituitary gland, causing the release of adrenocorticotropic hormone. This hormone travels to the adrenal cortex and causes the release of 30 different hormones that help the body deal with threats.