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Without segmentation, organisms would lack sophisticated means of movement and complex body structures that enable advanced functions. Segmentation provides the means for an organism to travel and protect its sensitive organs from damage. The ability to divide functions into different portions of the body allows an organism to perform increasingly complex activities and use different segments to perform varying functions.
So the next time you bend down and touch your feet, thank your far-flung, worm-like ancestors for that. |
"To a first approximation, all multicellular species on Earth are insects."
N. E. Stork, 'Biodiversity: World of Insects' (2007)
The concept of segmentation in biology relies upon the ability for organisms to duplicate organs and structural elements, such as arms and legs. Segmentation allows for more variety among species. In biology, the segmentation follows the longitudinal axis (the length of the body from head to tail) and separates the different body functions into separate systems such as the circulatory, digestive, nervous and excretory systems. Each segment plays a significant role that relies upon other functions to work. For example, humans consist of heteromeric segmentation, in which each segment differs from another and fulfills a specific role. Homomeric segmentation refers to segments that contain similar elements, such as with the segments in an Errantia, a type of worm.
Annelids
The first segmented animals to evolve were the annelid worms, phylum Annelida. These advanced coelomates are assembled as a chain of nearly identical segments, like the boxcars of a train. The great advantage of such segmentation is the evolutionary flexibility it offers—a small change in an existing segment can produce a new kind of segment with a different function. Thus, some segments are modified for reproduction, some for feeding, and others for eliminating wastes.
Two-thirds of all annelids live in the sea (about 8,000 species, including the bristle; most of the rest—some 3,100 species—are earthworms. The basic body plan of an annelid is a tube within a tube: The digestive tract, is suspended within the coelom, which is itself a tube running from mouth to anus. There are three characteristics of annelid body organization:
The first segmented animals to evolve were the annelid worms, phylum Annelida. These advanced coelomates are assembled as a chain of nearly identical segments, like the boxcars of a train. The great advantage of such segmentation is the evolutionary flexibility it offers—a small change in an existing segment can produce a new kind of segment with a different function. Thus, some segments are modified for reproduction, some for feeding, and others for eliminating wastes.
Two-thirds of all annelids live in the sea (about 8,000 species, including the bristle; most of the rest—some 3,100 species—are earthworms. The basic body plan of an annelid is a tube within a tube: The digestive tract, is suspended within the coelom, which is itself a tube running from mouth to anus. There are three characteristics of annelid body organization:
- Repeated segments. The body segments of an annelid are visible as a series of ringlike structures running the length of the body, looking like a stack of doughnuts.
The segments are divided internally from one another by partitions, just as walls separate the rooms of a building. In each of the cylindrical segments, the excretory and locomotor organs are repeated. The body fluid within the coelom of each segment creates a hydrostatic (liquid-supported) skeleton that gives the segment rigidity, like an inflated balloon. Muscles within each segment pull against the fluid in the coelom. Because each segment is separate, each can expand or contract independently. This lets the worm’s body move in ways that are quite complex. When an earthworm crawls on a flat surface, for example, it lengthens some parts of its body while shortening others.
- Specialized segments. The anterior (front) segments of annelids contain the sensory organs of the worm. Elaborate eyes with lenses and retinas have evolved in some annelids. One anterior segment contains a well-developed cerebral ganglion, or brain.
- Connections. Because partitions separate the segments, it is necessary to provide ways for materials and information to pass between segments. A circulatory system carries blood from one segment to another, while nerve cords connect the nerve centers located in each segment with each other and the brain. The brain can then coordinate the worm’s activities.
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Arthropods
A profound innovation marks the origin of the body plan characteristic of the most successful of all animal groups, the arthropods, phylum Arthropoda. This innovation was the development of jointed appendages.
The name arthropod comes from two Greek words, arthros, jointed, and podes, feet. All arthropods have jointed appendages. Some are legs, and others may be modified for other uses. To gain some idea of the importance of jointed appendages, imagine yourself without them—no hips, knees, ankles, shoulders, elbows, wrists, or knuckles. Without jointed appendages, you could not walk or grasp an object. Arthropods use jointed appendages as legs and wings for moving, as antennae to sense their environment, and as mouthparts for sucking, ripping, and chewing prey. A scorpion, for example, seizes and tears apart its prey with mouthpart appendages modified as large pincers.
The arthropod body plan has a second great innovation: Arthropods have a rigid external skeleton, or exoskeleton, made of chitin. In any animal, a key function of the skeleton is to provide places for muscle attachment, and in arthropods the muscles attach to the interior surface of the hard chitin shell, which also protects the animal from predators and impedes water loss.
However, while chitin is hard and tough, it is also brittle and cannot support great weight. As a result, the exoskeleton must be much thicker to bear the pull of the muscles in large insects than in small ones, so there is a limit to how big an arthropod body can be. That is why you don’t see beetles as big as birds or crabs the size of a cow—the exoskeleton would be so thick the animal couldn’t move its great weight. Another limitation on size is the fact that in many arthropods, including insects, all parts of the body need to be near a respiratory passage to obtain oxygen. The reason for this is that the respiratory system, not the circulatory system, carries oxygen to the tissues.
In fact, the great majority of arthropod species consist of small animals—mostly about a millimeter in length— but members of the phylum range in adult size from about 80 micrometers long (some parasitic mites) to 3.6 meters across (a gigantic crab found in the sea off Japan). Some lobsters are nearly a meter in length. The largest living insects are about 33 centimeters long, but the giant dragonflies that lived 300 million years ago had wingspans of as much as 60 centimeters!
Arthropod bodies are segmented like those of annelids, from which they almost certainly evolved. Individual segments often exist only during early development, however, and fuse into functional groups as adults. For example, a caterpillar (a larval stage) has many segments, while a butterfly (and other adult insects) has only three functional body regions—head, thorax, and abdomen—each composed of several fused segments. Some of the segmentation can still be seen in the grasshopper, especially in the abdomen.
A profound innovation marks the origin of the body plan characteristic of the most successful of all animal groups, the arthropods, phylum Arthropoda. This innovation was the development of jointed appendages.
The name arthropod comes from two Greek words, arthros, jointed, and podes, feet. All arthropods have jointed appendages. Some are legs, and others may be modified for other uses. To gain some idea of the importance of jointed appendages, imagine yourself without them—no hips, knees, ankles, shoulders, elbows, wrists, or knuckles. Without jointed appendages, you could not walk or grasp an object. Arthropods use jointed appendages as legs and wings for moving, as antennae to sense their environment, and as mouthparts for sucking, ripping, and chewing prey. A scorpion, for example, seizes and tears apart its prey with mouthpart appendages modified as large pincers.
The arthropod body plan has a second great innovation: Arthropods have a rigid external skeleton, or exoskeleton, made of chitin. In any animal, a key function of the skeleton is to provide places for muscle attachment, and in arthropods the muscles attach to the interior surface of the hard chitin shell, which also protects the animal from predators and impedes water loss.
However, while chitin is hard and tough, it is also brittle and cannot support great weight. As a result, the exoskeleton must be much thicker to bear the pull of the muscles in large insects than in small ones, so there is a limit to how big an arthropod body can be. That is why you don’t see beetles as big as birds or crabs the size of a cow—the exoskeleton would be so thick the animal couldn’t move its great weight. Another limitation on size is the fact that in many arthropods, including insects, all parts of the body need to be near a respiratory passage to obtain oxygen. The reason for this is that the respiratory system, not the circulatory system, carries oxygen to the tissues.
In fact, the great majority of arthropod species consist of small animals—mostly about a millimeter in length— but members of the phylum range in adult size from about 80 micrometers long (some parasitic mites) to 3.6 meters across (a gigantic crab found in the sea off Japan). Some lobsters are nearly a meter in length. The largest living insects are about 33 centimeters long, but the giant dragonflies that lived 300 million years ago had wingspans of as much as 60 centimeters!
Arthropod bodies are segmented like those of annelids, from which they almost certainly evolved. Individual segments often exist only during early development, however, and fuse into functional groups as adults. For example, a caterpillar (a larval stage) has many segments, while a butterfly (and other adult insects) has only three functional body regions—head, thorax, and abdomen—each composed of several fused segments. Some of the segmentation can still be seen in the grasshopper, especially in the abdomen.
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Arthropods have proven very successful due to the innovations of jointed appendages and exoskeletons. About two-thirds of all named species on earth are arthropods. Scientists estimate that a quintillion (a billion billion) insects are alive at any one time—200 million insects for each living human!
Ponder this
Why don't annelids develop an exoskeleton and appendages as arthropods do?
Why do some arthropods go through metamorphosis, while others remain unchanged throughout their life cycles?
Discuss
Divide groups to analyze the skeletal, muscular, circulatory, digestive, and any other principle systems of the human body. Discuss about segmentation in those systems. Details may include segmentation in the cellular, tissue, and organs of each bodily system and how the structure contributes to their function.
Further reading
Annelids, also known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant species.
Annelid fossils, from the Virtual Fossil Museum.
Arthropods, are invertebrate animals having an exoskeleton, a segmented body, and paired jointed appendages.
Arthropod fossils, from the Virtual Fossil Museum.
'Segmentation in animals', a technical but readable article by Dr. Seth S. Blair.
