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Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles.

As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer such as chitin and is often biomineralized with materials such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton, called apodemes , function as attachment sites for muscles, similar to tendons in more advanced animals ( [link] ). In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually, and may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively small size. The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass.

Illustration shows a crab claw with a small, upper portion that pivots relative to a large, lower portion. The apodemes are located on the large portion, above and below the pivot point.
Apodemes are ingrowths on arthropod exoskeletons to which muscles attach. The apodemes on this crab leg are located above and below the fulcrum of the claw. Contraction of muscles attached to the apodemes pulls the claw closed.

An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement.

Limiting effects of diffusion on size and development

The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective and the center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.

An important concept in understanding how efficient diffusion is as a means of transport is the surface to volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr 2 , and a volume of (4/3)πr 3 . The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface to volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.

Questions & Answers

What's the function of epiglottis
Ugo Reply
What Is The Other Name For Intestinal Juice?
Justin Reply
what is the largestest planet of the universe
rick Reply
what are the types of cell
Bernard Reply
prokaryotic and eukaryotic
prokaryotic cell and eukaryotic cell
what is the protein found in the blood?
Tobias Reply
what is parasitic movement
Emmanuel Reply
Parasitic movement is a problem for all of us. So is its companion, parasitic tension. Parasitic movement is the excess contraction of muscles that you don't actually need to complete an action.
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am OK how a u
absorption may simply mean utilization of food in the body
what are eukaryotic cells
Thiza Reply
eukaryotic cells which posses a true nucleus that is the DNA is enclosed and covered by a nuclear membrane
what is the mean of pair of chromosomes
Kazula Reply
23 haploid and 23diploid
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why did human being need sex?
because he/she have feelings
reproduction...to make more
due to active harmon
One important of sex is to reproduce
to ensure the countinuty of life
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for sexual satisfaction and birth
what is momentum
Asiya Reply
The strength or force that allows something to continue or grow stronger or faster as time pass
What is Centripetal Force?
centrepital force is the inward force required to keep a body moving with constant speed in a circular path
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Takii Reply
List four condition necessary for seed germination
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water, light, oxygen and temperature
water, oxygen, light temperature
water oxygen light and temperature
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importance of boilogy
what is soil
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soil is the upper part of the earth
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soil is the uppermost layer of the earth on which plant grows
soil is defined as the thin surface of the upper most layer of the earth crust on which plants grow
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soil is the uppermost layer of the earth in which most of the necessary nutrient are found and serve as habitats for some organisms
soil is the uppermost layer of the earth in which most of the necessary nutrients are found and serve as habitats for some organisms
soil is the uppermost layer of the earth in which most of the necessary nutrient are found and serve as habitats for some organisms
soil is the uppermost layer of the earth in which most of the necessary nutrients are found and serve as habitats for some organisms
soil is the accumulation of lose weathered materials which covers much of the land surface of the earth

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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