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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

a person with blood group A can not donate blood to a person with blood group B
KHUNGA Reply
differentiate between cytoplasm and protoplasm
Annet Reply
cell as a unit of life
Bhrah Reply
differentiate between cytoplasm and protoplasm
Annet
forms which cell exist
Zainab Reply
Wats a molecules in terms of biology?
Ayomide Reply
Important of kreb's cycle
yeboah
what conversation
Sally Reply
is agurment btn two or more people
Okello
what is biology?
KINEI Reply
the scientific study of the life and structure of plants and animals.
Boame
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Guor
After finishing school
Hellen
study of living things n non living things
Benjamin
sex is sweet at anytime
Okello
study of living organsm
Hellen Reply
which element is the largest
Haja Reply
francium
Izzati
francium
Paul
how many groups do we have in the periodic table
Theola Reply
seven groups
Moses
the one that makes eight is the noble gases
Moses
okay thank you
Theola
we have eight groups in the periodic table
Ramsey
there are actually 18 groups or numbered columns
Izzati
so it will be like the s- p-and d block elements of the periodic table
Izzati
8 groups
Boame
how many times can a man release before the lady get pregnant
Smart Reply
nearly a hundred million
Izzati
one
Moses
what do you by one?
Izzati
two times
Theola
just only one time
Okello
what is a function of cell sap
Martin
it repair worn out tissue
Moses
when is the right time for playing sex to prevent pregnancy?
Guor
at anytime
Okello
what is biology
Darkoa Reply
it's study of all living organisms and their characteristics
Ice
study of living organism
Hellen
by what proces do ameaba reproduce?
Okello Reply
hi dear
Janet
hi my sist gudmorning
Okello
Hi
Muah
hello
Bob
yes
Mathew
hi
promise
what is biology
promise
amoeba reproduce asexually
Chidi
gudevening member
Okello
good chidi
Okello
hello
Felix
hello
Felix
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Mary Reply
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Mary
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Mary
same here
Rachel
lol I am just learning idk how to do assignment
Izzati
lol idk how I am just learning lol
Izzati
lol I think I understand why we repeat this
Izzati
what is active transport
Amona Reply
is the movement's of water & mineral salts from the reason of highly concentrated to the reason of low concentration across semi permeable membrane & it take place in plant
Okello

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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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