The study of the internal structure of organisms.

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Why Is Most Life Symmetrical Externally But Not Internally?

Mammals, reptiles, arachnids, insects, etc are all as far as I am aware symmetrical in appearance. Take a human for instance, make a line from the top of our head right down the middle. However, ...
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Why are there no known animals with an odd number of legs?

In my 6th grade science book it is said that there are no three legged animals, and that no animal with an odd number of limbs exists. I checked Wikipedia and could confirm this: There are no ...
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1answer
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Evolutionarily, why do male rats and horses lack nipples?

Developmentally male rats don't have nipples because (reddit) Testosterone release in the fetal male rat happens before the stage of mammogenesis where the teat is formed whereas other species ...
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2answers
579 views

What is the morphological difference between Leydig cell in human and pig?

The pig is only an example, just an animal. Leydig cells have protein inclusions (Reinke crystals) that are mostly made of crystallised lipofuscin. They are secretory inclusions i.e. cells formed in ...
13
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1answer
303 views

Evolution of long necks in giraffes

In this question, the OP uses giraffe necks as a supportive example of evolution. Is the mechanism described in this post accurate? At some point, I thought I remember hearing that giraffes did not ...
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1answer
415 views

Do the majority of fish lack scales on their heads?

Based on pictures, it seems to me that a vast majority of fish species that have scales do NOT have scales on their heads. Is that fact true? To make this properly answerable: lets' define a ...
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How many cells does the smallest animal have?

Note: question rewritten to prevent misunderstanding and make it more answerable I know that some small animals like C. elegans display surprising sophistication with a very small number of cells. ...
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109 views

Skull mystery on beach

The skull was found on a beach on the Sechelt inlet on Canada's pacific coast. A marine environment. What species is this skull from?
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Is there a correlation between muscle fibers and body types?

I have been told the number of muscle fibers a particular muscle has varies from person to person. Unfortunately, the person who told me this did not know much more than this. Initial searching on the ...
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1answer
928 views

Why are there nail growth differences between humans and other mammals?

When a cat is growing, his nails are growing with him to some extent. A grown cat has a fixed nail length that is not extending. By contrast, human finger nails just keep on growing, so we have to ...
8
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1answer
372 views

Do antigens protrude through the capsule/slime layer in prokaryotic organisms where these features are present?

In prokaryotic organisms that have a slime layer or capsule, do intrinsic/extrinsic proteins and other molecules that could be used as antigens protrude through the capsule? I assume that they ...
7
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646 views

evolutionary reason behind sharing the same path for food and air?

What is the evolutionary reason behind sharing the same path for food and air? For example, we have a nose and lungs, yet they are linked by the pharynx, which is shared with the mouth and ...
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556 views

What are the atmospheric requirements for large dinosaurs?

What are the atmospheric requirements for large dinosaurs? and are the atmospheric constituents for supporting large dinosaurs any different from the atmosphere today?
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Why do most mammals have long snouts?

It seems like most mammals, e.g. dogs, have long snouts. My pet dog's snout would seem to me like an evolutionary disadvantage, since her canine teeth are way out at the end of her jaw, which acts as ...
7
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1answer
587 views

Skeletal muscle without antagonist

Is there any skeletal muscle that does not have an antagonist? Excluding circular muscles such as around eye and mouth. The reason why I am wondering is because in biology normally nothing is ever ...
7
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1answer
628 views

What is the name of the appendage that mantis shrimp use to rub their eyes?

In this short clip, you can see the mantis shrimp using some sort of feathery arm/claw to rub its eyes. Looking at a diagram here, it appears possibly that it's using maxillipeds. Someone with ...
7
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1answer
317 views

Can humans live without their right atrium?

The right atrium is one of four chambers (two atria and two ventricles) in the hearts of mammals (including humans) and archosaurs (which include birds and crocodilians). It receives deoxygenated ...
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Do other male mammals except for humans have nipples?

When I grew rabbits, I had a pair, one male and one female. And while the female's nipples were quite prominent, especially after giving birth, I don't remember the male having any nipples at all. Do ...
6
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1answer
408 views

How can bats hear such high frequencies?

I attended a talk that glossed over some biology as it was talking about a certain protein. The speaker mentioned humans can hear up to, often less than 20kHz frequencies, whereas bats can hear up to ...
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756 views

Did dinosaurs have more than one brain? If so, why?

I once remember reading (15 years ago) that dinosaurs had two brains. One for their head and another one for their digestive functions. What is the current opinion on this theory? Has more evidence ...
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1answer
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What is the average Leg-to-Foot Length/Width Ratio?

I'm making a program that estimates a probable size (widest width and longest length) for a human's foot given the length and width of the leg to which it is attached. However, try as I might, I can't ...
6
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1answer
628 views

Why do some internal organs regenerate?

I have been reading about the human liver and zebra fish heart muscle having the ability to regenerate. It seems to me that these organs have very little chance to become damaged or worn out. At the ...
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2answers
979 views

Why do adult insects have 6 legs?

Apparently, there is an advantage to having 6 legs in the insect world. What is that advantage, if anything? Why would such an advantage exist for insects, but not for other, larger land animals? ...
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1answer
279 views

What limits the size of fish compared to marine mammals?

The largest extant fish species is the whale shark (Rhincodon typus). With a length of 13 meters and a mass of about 20-30 metric tons, the whale shark is relatively small compared to the largest ...
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275 views

Symmetry of species [duplicate]

I've got a silly question, sorry for that. I know, that we probably have no the right answer and the current answer could be "that's evolution, external conditions". I'd like to speculate, why most of ...
5
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1answer
307 views

Why did Opabinia have 5 eyes?

I recently read about a fossil called Opabinia. What intrigues me is that it had five eyes. I was under the impression that most features, such as eyes, ears and legs are always even-numbered, i.e., ...
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75 views

Stretching and compressing bones

The Young's modulus of elasticity when a bone is stretched is : 16×109 and when it is compressed, it is 9×109 N/m2. That means, change in length will be more if you compress a bone as compared to ...
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270 views

Why do small organisms make faster movements than big organisms?

I hesitated to ask this question because it seems so obvious and intuitive. However, I am not able to explain this tendency. Background It seems to me that small organisms make faster movements than ...
5
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1answer
128 views

Are CN3, CN7, CN9 and CN10 the only Parasympathetic Cranial Nerves?

It has been my thought for a long time that this is the case, but I am unsure currently, since the parasympathetic tract of colon sigmoideum does not seem to have connection with CN 10. It connects ...
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1answer
2k views

Why does the sacrum fuse together?

Everywhere I look describes how the 5 sacral vertebrae start fusing together during adolescence and are totally fused by the time you're 30 or so. But nobody seems to explain why this happens. It ...
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114 views

Does the hepatic portal system form capillary beds?

Does the hepatic portal system form capillary beds? If so, is the capillary bed coordinated with that of arterial capillaries? What review paper might I look in?
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694 views

Why do we have to exhale (or inhale) in order to speak?

Every time we speak, sing, or make any other kind of advanced noise with our throats, we exhale, or to put it that way, blow air through our throats. Why is this required? After all, speakers do not ...
4
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2answers
451 views

Why do snakes not have eyelids?

Why is it that snakes do not have eyelids? Is it due to that fact that they are not as evolved as we humans and other organisms that have eye lids, or is there any other significance behind it if ...
4
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1answer
89 views

Can defibrillation be done with a car battery?

Can successful defibrillation be done with a car battery? Is it POSSIBLE for this to work? Let's say you are stuck on a very remote place and something happens to someone... Moreover, can you jump ...
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1answer
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Cross section through the right arm question

Is the largest drawn vein on first attached pictue v. brachialis or did they make a mistake and it's actually v. basilica? Picture 1: Cross section through the right arm (source: Thieme, General ...
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1answer
78 views

What causes the opaque green colour in Lepidoptera?

Link here to what I mean by 'opaque' colouration on the insect, the colour intensity remains constant despite changes in light intensity and angle (not shown by the picture but the moth exhibits this ...
4
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1answer
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How does this headless fish still move?

There is a (VIDEO) on Facebook where the fish starts to flail around despite no heart or internal organs. What causes the fish to flail around the way it does? My theories: It is because of ...
4
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1answer
57 views

Comparative leg sizes

When I was a child, my father showed me the classic essay "On Being the Right Size", J. B. S. Haldane. It talks (among other things) about how large animals need stockier legs to support their ...
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87 views

Is there differences in components of saliva among species

Are there differences in the components of saliva among species? Also, do the functions of saliva differ between species?
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Which organs do cats not eat on their prey?

My female cat is a very active hunter and brings me her prey daily. Except for shrews and birds, she always eats them entirely. This afternoon she brought me this: It was quite large (maybe ...
3
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1answer
140 views

How do cats survive falling from great heights?

Among mammals, cats are able to land safely when falling from great heights. For example, when compared to dogs or humans they can survive drops from relative large heights. How do they manage? A ...
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2answers
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Do nematodes have organ-level organisation?

Some introductory biology books state that nematodes have a pseudocoelom. So, they have a false body cavity. So, does it also mean that they have organ level body organisation?
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1answer
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Is the thecal sac ectoderm?

The thecal sac, or dural sac, is the membranous sheath of dura mater that surrounds the spinal cord and the cauda equina. The thecal sac contains the cerebrospinal fluid in which the spinal cord ...
3
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1answer
43 views

Why are vertebrae, specifically, so important in classification?

A fundamental level of animal classification is the distinction between vertebrates and invertebrates. The vertebrata are, arguably, often classified as higher life forms than the invertebrata. But ...
3
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1answer
100 views

Please identify organ

This is the gut area of an Eastern Grey Kangaroo taken of a property in the Southern Highlands of NSW, Australia. The top red organ appears to be the spleen but what is the elongated tubular organ ...
3
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1answer
86 views

What accounts for size variation in breeds of dogs?

My assumption is that dogs vary so much in size because of artificial selection pressures coming from human breeders, but are there genetic loci that roughly determine approximate size? Do other ...
3
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1answer
74 views

Does size matter?

As I compare the morphologic differences between mouse models and the human disease for which they are analogous, I have become interested in what difference interspecies size has on molecular ...
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2answers
424 views

How do other primates treat their fingernails?

Do all primates have to trim their fingernails in some way, or do some primates' fingernails wear off through natural use? Also, is constant nail growth common to all primates?
3
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1answer
82 views

Text/resource with information on all skeletal muscles and their motor units

Something analogous to an encyclopedia on baseball players with a list of all thier stats would be ideal. I'm not looking for just generic muscle names, locations and illustrations. Good answers ...
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1answer
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What does it mean for muscles origins and insertions to be the other way around in the right than left limb?

Herbert Srebnik writes in Concepts in Anatomy: In the right limb, the muscle's origin is proximal to the joint and the insertion is distal to it. In the left limb, the muscle's origin is ...