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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 is ecology
Odion Reply
what is cell
Odion
what is cell
Odion
What is a cell
Awal Reply
What are tissues
Awal
A cell is the smallest unit of living organisms.
hamidat
Tissues are group of similar cells performing a particular function.
hamidat
how to pronounce the word cyanobacteria
siva Reply
Cyano then bacteria. look that "cyano" is just like "ciyano".
hamidat
explain the term transpiration pull
Tank Reply
water is mainly " pulled" through the plant and that the driving force for this process is transpiration from the leaves .This is also referred to as the cohesion- tension -transpiration pull model transport.
Vanessa
explain TRANSIOCATION and GUTATION
Tank
what Is corona
INGIEBE Reply
My brother go and ask GOOGLE😷😷😷
hamidat
Google it for the right answer. Corona is a virus which causes Novel Covid-19 disease that has been declared by WHO as a global or world pandemic.
Katlholo
Why is it called Covid-19?
Katlholo
jjwhat is. condinative system
Odion
joy
Odion
condinative system is the system that harmonize all the. different. part. of. the. body and make. them function as a. specific unit
Odion
Describe the process of protein sythesis?
Kizito Reply
Ribosomes are responsible for protein synthesis in animals.
hamidat
What is a ploidy level
Francis Reply
Ploidy refers to the number of chromosomes. We have 23 pairs of chromosomes in somatic cells. Sex cells are haploid thus 23 chromosomes vs. 46 Chromosomes.
Eric
Evolution is evolvement according to one's environment. Let's use humidity as an example. A person from a very cold environment would not be used to hot humid weather. But over time their body would slowly, slowly adapt.
Eric
Give me (3) types of biodiversity
Jay
Hi guys 🇧🇪
Namutebi
hello boy
Tedi
hi
INGIEBE
yes
amani
what do you guys understand about evolution?
Kiana Reply
it's a process or analayzing some program
Fahamia
but am not sure if some know plz let share
Fahamia
is a study of
Sarah
unfolding or unrolling hence the process of development of growth
Sarah
is the periodic change of the structural features of an organism
Rahim
I think
Rahim
process of developing
Nikita
hi guiz
Mohammed
hello guys
Safuna
hi
Kaniki
Evolution ~ Is the gradual change of development over time in which a new species are formed
amani
Evolution is the sum total of adaptive changes that has existed before.
hamidat
odionjimoh
Odion
hi
Odion
why are diagrams not available
Evans Reply
How can nucleotide molecules of DNA be constructed?construct a six molecules of DNA?
Julia Reply
You take a Deoxyribose sugar. The Deoxyribose sugar has a hydrogen on the second carbon vs. an OH group. The nitrogenous base binds to the #1 Carbon and a phosphate binds to the 5'. When the 2nd NBase bonds, it does so on the 3' Carbon. Thus you have 5' to 3' directional growth.
Eric
what is the explanation of local biomes in Nigeria
Kabir
It would be savanna, tropical grassland.
Eric
describe the breathing mechanism in the body fish.
Jackson Reply
what is genetics.
Jackson
the study of hereditary
Julia
state the importance of biodiversity of organisms in an area?
Chris Reply
Let's compare The Brazilian Rain Forest to Antarctica. There many, many types of flora and fona in The Rain Forest. So many that we are still discovering them vs. the variety in Antarctica.
Eric
it boosts the ecosystem where each species has an important tole
Kizito
what is biology
Adam Reply
Biology is the study of living things.
Patrick
what are some of the branches of biology?
Jackson
anatomy,genetics,parasitology
Evans
Botany
Kizito
Botany and zoology.
hamidat
what is meosis
Mabatho Reply
did you mean meiosis? meiosis this is the type of cell division where one diploid parent cell produces two daughter cells that are haploid and genetically different from the parent cell.
Patrick
thankyou
Mabatho
I'm struggling to understand genetics
Mabatho
Am also struggling as well
Patrick
nothing comes on a silver platter, you need to put effort in everything you are doing not forgetting God. study as if there is no tomorrow. "Suffer now and enjoy the future"
Patrick
true bro
Patrick
exactly brother
Odhiambo
Glory to God
Patrick
Thanks Pat🙏
Mabatho
you're welcome
Patrick
thanks
Odion
can anybody tell me what is cell
Odion
what's a nervous system
tessy Reply
what's the human brain
tessy
The human brain consist of the cerebrum, cerebellum and brain stem. The cerebral cortex is composed of neuronal cell bodies, which is the gray matter. The white matter is composed of neuronal axons. The brain is the motherboard of our system.
Eric
The nervous system brings about coordination of body activities.
hamidat
The human brain is protected by the skull and cranium and it is divided into three: the forebrain, the midbrain and the hindbrain.
hamidat
nervous system are the most specialize organs of .the body
Odion

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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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