Example: Form and Function and Natural Selection (for Zoology)
5. Form and Function: The earthworm exhibits bilateral symmetry, a segmented body, and a high degree of cephalization (a cerebral ganglion), but lacks a typical annelid head with specialized sensory organs. Each segment bears four pairs of chitinous setae, which aid the earthworm in locomotion and burrowing by preventing slipping. The earthworm conducts locomotion by way of a peristaltic motion in which the worm first contracts circular muscles to lengthen the body, and then contracts longitudinal muscles to shorten the body, pulling the anterior end forward. Instead of the more complex sensory organs found in other annelids, earthworms are suited with simple sensory receptors which are sensitive to touch, light, and chemicals. Chemical receptors are especially important, since they aid the worm in finding food, and in mating. The worm is also equipped giant nerve fibers (giant axons) which allow it to quickly withdraw into the ground if it senses a threat. This escape mechanism is further improved by a dorsal pore through which an earthworm may expel coelom fluid, thereby decreasing its turgidity so its body will be easier to pull into the ground. The earthworm’s mouth is found at the anterior end, and is overhung by a fleshy prostomium; the anus is on the posterior end. Food is moistened by secretions from the mouth, and drawn in by the sucking action of the muscular pharynx. From there food passes through a crop, and a gizzard where it is ground up, and later to the intestine where digestion and absorption occur. Finally, unused material is expelled through the anus. Since the earthworm ingests calcium from the soil along with its food, calciferous glands are used to regulate the calcium concentration in the worm’s blood. The earthworm’s circulatory system is closed, and has five pairs of aortic arches to maintain steady blood pressure. Respiration takes place across the worm’s moist skin. The excretory system consists of a pair of metanephridia in every segment except for the first three and the last one. Earthworms are monoecious; all animals bear both male and female reproductive organs. Spermatozoa from testes mature in seminal vesicles, and are expelled through male pores. These fertilize ova from small ovaries and female pores inside a cocoon which is secreted by the clitellum.
6. Natural Selection: Earthworms are relatively defenseless animals with a simple sensory system, so their survival depends largely on other mechanisms. The complex sense organs present in other annelids (such as the clamworm which is predatory) aren’t necessary in the earthworm, which spends its time burrowing through the soil and scavenging for food. Of course, the earthworm’s chemical receptors are necessary, since the worm relies on them to find food by sampling the organic content of its surroundings. The worm is further adapted for a foraging and burrowing lifestyle with its system of locomotion using setae and longitudinal and circular muscles as described above. With this system, the worm may crawl or burrow until it encounters decayed organic material, which it may eat. The earthworm solves its problem of defenselessness through a system of response to strong vibrations. When the earthworm senses danger, its giant axons allow it to quickly respond by pulling itself back into its burrow, where it then becomes difficult to remove due to the anti-slipping action of its setae. Thus, although the worm has weak senses and no obvious defense mechanism, it may evade predators like robins, that would otherwise make prey of it. Many other annelids, such as the clamworm, also evade predators by retreating into a burrow, and securing themselves inside with setae. Earthworms may gain an additional advantage in general survival by being hermaphroditic. In order for earthworms to reproduce, they must encounter another worm (sought out using their chemical receptors), so that they may engage in copulation. Because they are hermaphroditic, they only need to encounter another worm in order to do this, rather than having to specifically encounter another worm of the opposite gender. Some other annelids share this characteristic, while other do not (Polychaetes, for example, are mostly dioecious).
6. Natural Selection: Earthworms are relatively defenseless animals with a simple sensory system, so their survival depends largely on other mechanisms. The complex sense organs present in other annelids (such as the clamworm which is predatory) aren’t necessary in the earthworm, which spends its time burrowing through the soil and scavenging for food. Of course, the earthworm’s chemical receptors are necessary, since the worm relies on them to find food by sampling the organic content of its surroundings. The worm is further adapted for a foraging and burrowing lifestyle with its system of locomotion using setae and longitudinal and circular muscles as described above. With this system, the worm may crawl or burrow until it encounters decayed organic material, which it may eat. The earthworm solves its problem of defenselessness through a system of response to strong vibrations. When the earthworm senses danger, its giant axons allow it to quickly respond by pulling itself back into its burrow, where it then becomes difficult to remove due to the anti-slipping action of its setae. Thus, although the worm has weak senses and no obvious defense mechanism, it may evade predators like robins, that would otherwise make prey of it. Many other annelids, such as the clamworm, also evade predators by retreating into a burrow, and securing themselves inside with setae. Earthworms may gain an additional advantage in general survival by being hermaphroditic. In order for earthworms to reproduce, they must encounter another worm (sought out using their chemical receptors), so that they may engage in copulation. Because they are hermaphroditic, they only need to encounter another worm in order to do this, rather than having to specifically encounter another worm of the opposite gender. Some other annelids share this characteristic, while other do not (Polychaetes, for example, are mostly dioecious).