Having an anus, on the other hand, is a big plus. It means the animal can eat more or less continuously, since there is one-way traffic in the gut. Everything that enters a cell or an organism has to come through a surface. But as size goes up, volume increases much faster than surface area. Volume grows as the third power cubed of length, while surface only as the second power squared , so as an animal's length increases from 2 to 4 units, its surface area increases from 4 to 16 square units and its volume from 8 to 48 cubic units.
Long thin bodies are very fragile though they certainly do exist in such animals as the horsehair worm, which may be hundreds of times as long as wide.
A better solution might be to coil a long gut up inside a short body; other tubular organs could also be packed in a smaller volume by coiling. The coiling of tubular organs requires that the body be hollow.
Thus the formation of a body cavity, or coelom pronounced see-lum represents an important advance. Technically, there are two basic types of cavity, the so-called pseudocoelom and the eucoelom , or "true coelom," but the distinctions between them are technical and there is little evidence to suggest that one works better than the other.
A fluid-filled coelom of some invertebrates provides other advantages. The fluid, as it sloshes around, can circulate food and oxygen and pick up wastes. Because a fluid cannot be compressed, if an animal clamps down the muscles of its body wall on the coelomic fluid, it becomes hard and rigid—the coelom can function as a hydrostatic skeleton. Many animals that swim or creep have no skeletons at all, not even a hydrostatic one.
A flatworm crawls by using tiny projections of the cells of its underside, called cilia, against the surface. A jellyfish swims by partially closing its bell against the resistance of the water.
Effective movement using limbs to walk or swim, however, requires a skeleton, some hard structure for internal or external support. How a hydrostatic skeleton works in this system will be explained in the next section. Right now we can recognize two kinds of skeletons by their positions— endoskeletons are like ours, and are on the inside of the body forming a structural framework, while exoskeletons are on the outside and look like a suit of armor.
Endoskeletons occur primarily in chordates, where they are composed of living cartilage and bone, and in echinoderms, starfish and their relatives, where they are made of massive crystals of calcite. Exoskeletons are found in arthropods. They are not living and are secreted by the outermost layer of cells. Some exoskeletons contain minerals, primarily calcite, but most are made of tough proteins and a sugar polymer, chitin. Which is best, an endoskeleton or an exoskeleton?
There are advantages to both. An exoskeleton doubles as a protective layer and as waterproofing. However, because an exoskeleton is not alive, if the animal is to grow, it must be periodically shed and re-secreted. During this time an arthropod may be immobile, soft, and very vulnerable.
Chitin and protein are also not terribly strong, so most animals with chitinous exoskeletons are limited to small sizes. Bone, on the other hand, is alive and can grow. It is strengthened by mineral deposits, primarily calcium phosphate, and is quite strong. So it is no surprise that chordates number amongst them the largest animals that ever lived.
And also because it is alive, an endoskeleton of bone can repair itself. Distinguishing between a skeleton and a shell can be a problem. Generally, a shell is only for protection and does not provide either a structural framework or a system of levers which muscles operate as does the skeleton.
The most obvious and important group of animals with shells are the molluscs—clams and oysters, snails, and squids and octopi. In the latter two groups, the confusion typical of biological systems comes in: some squids have brought their shell to the interior of the body, and it really does act as a skeleton!
A segmented animal has a body that consists of repeating identical or similar units, called segments. The division of a fundamentally worm-like, bilateral body into segments may be thought of as the final major adaptation. This major characteristic occurs only in three of the 35 or so animal phyla. Therefore it is likely that segmentation only arose two or three times in the evolutionary past of animals.
In the chordates, segmentation arose as an adaptation to efficient swimming , and is primarily a characteristic of the muscles, skeleton, and nervous system. The muscles are arranged in V-shaped bands, and as a wave of contraction passes down them, alternating on each side, the body sweeps into graceful curves which push against the water and propel it forward.
Quite naturally, the backbone and nervous system followed this plan of muscular units. In the annelid worms such as earthworms segmentation arose as an adaptation to burrowing in soft mud. The coelom hydrostatic skeleton was divided into compartments. Because liquids cannot be compressed, the volume of each compartment must remain the same. Each compartment is wrapped in two layers of muscles, circular and longitudinal.
When the longitudinal muscles are contracted, they squeeze the segment into a broader, shorter form that can anchor a worm in a burrow. The contraction of the circular muscles makes the segment longer and thinner, pushing the whole animal ahead of it. By alternating these two shapes, the worm can anchor itself in the soil or mud or get a push to move further into the substrate.
The muscles require nerves to operate them, so these are segmentally arranged, too. Since each compartment must be sealed, it must have its own set of certain organs. Varoqueaux, M. Wahl, and D. Primordial neurosecretory apparatus identified in the Choanoflaggelate Monosiga brevicolis. Cai, X. Liebeskind, B. Hillis, and H. Convergence of ion channel genome content in early animal evolution.
Meech, R. Evolution of excitability in lower metazoans. North and R. Greenspan eds. Moran, Y. Barzilai, B. Liebeskind, and H. Evolution of voltage-gated ion channels at the emergence of Metazoa. Moroz, L. Convergent evolution of neural systems in ctenophores. Ryan, T. The origin and evolution of synapses. This trait should seem familiar, since humans exemplify it. This trend does offer organisms several advantages over other body shapes in some situations.
Like all evolutionary trends, it happens at the species level, not at the individual level. Cephalization ties in with several other evolutionary trends, including bilateral symmetry. Humans share these characteristics with many other animal phyla. Cephalization has several components.
First, includes the development of a recognizable front end, differentiated from the back end. This process also involves concentrating sensory organs at the front of an organism, as well as feeding organs like a mouth and jaws, and nerve tissue. In some organisms, this nerve tissue gets more complicated, leading to ganglia -- a clump of nerves -- and eventually a brain. An animal with a well-developed head can be described as "highly cephalized. Bilateral Symmetry: Bilateral two-sided symmetry is the most common form of symmetry possible, and it is found throughout the biological and non-biological world.
Animals possessing bilateral symmetry have a dorsal top side, a ventral bottom side, an anterior head end, a posterior tail end, and a distinct left and right side. Associated with bilateral symmetry is the phenomenon of cephalization, which is the evolutionary trend towards the concentration of sensory equipment on the anter ior end; this means that such organisms are directionally sensitive and mobile.
Generally the anterior, or cephalized, end is the first to encounter food, danger, or other stimuli. Before bipedal development common in humans and apes , cephalization wa s an adaptation for movement such as crawling, burrowing, or swimming. Examples of animals that possess bilateral symmetry are: flatworms, common worms "ribbon worms" , clams, snails, octopuses, crustaceans, insects, spiders, brachiopods, sea stars, sea urchins, and vertebrates.
The symmetry of an animal generally fits its lifestyle. For example, many radial animals are sessile forms or plankton and their symmetry equips them to meet their environment equally well from all sides. More active animals are generally bilateral.
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