Hot answers tagged

127

First of all, let me make it clear that the heart is at the vertical centre of the body -- it is not shifted towards left (or right). However, it is slightly tilted towards the left in most cases. In some cases, it is tilted towards the right, and the condition is called Dextrocardia. For why it is so, lets look at what the heart does. Below is a diagram ...


79

Elephant, rhinoceros, &c all have much thicker legs in proportion. The answer, I think, lies in the fact that the animals you mention all evolved as cursorial animals (that is, they run to escape predators). Less mass in the lower leg means it swings easier, so the animal can run faster. There are two things you're apparently not noticing in that ...


51

This is a mistake in comparative anatomy which is somewhat common. When looking at four-legged animals, people often mistakenly map the parts of the hind legs. Here is an image that shows the different morphology of the same bones in horses and humans: What people often think of as the thigh of a four legged creature is really our calf and shin bone! And ...


30

The anus of Octopus is channeled into its siphon. Image taken from Carina M. Gsottbauer Note: Siphon is a tube that leads from the mantle to the outside. Octopuses use their siphon to force water out in jets for propulsion and to flush waste products from the anus. From Encyclopedia of the Aquatic World, Volume 6 By Marshall Cavendish Corporation


25

First, I think it worthwhile considering 'Why would internal symmetry be beneficial?' Developmental simplicity jumps to mind immediately. You can also consider relationship to external organs; the stomach and esophagus are lined up with the mouth which is symmetrical about the sagittal plane. Or maybe even balance; the lungs are large organs and if put to ...


24

According to the highly respected WORMATLAS: A Database of Behavioral and Structural Anatomy of Caenorhabditis elegans, the number is invariable in this animal, one of the most studied in the world. There are 302 neurons in the nervous system of C. elegans; this number is invariant between animals. Each neuron has a unique combination of properties, such ...


23

It depends what you mean by 'leg'. The instinctive way to define a 'leg' is based on its functional use: we use legs to walk on. But if we adopt that definition of 'leg' then there certainly are animals with odd numbers of legs: kangaroos, for instance, have a five-legged gait, functionally using their tail as a fifth leg. There is no developmental process ...


23

Your best bet is the Terminologia Anatomica, which is the international standard for anatomical terminology. The 1998 edition is freely available. It lists only a few named tendons though, which is consistent with my experience as an anatomist: very few tendons are named separately from the muscles to which they are connected. Central tendon of the ...


20

Welcome to Biology.SE! I think you are talking about plantigrade, digitigrade and unguligrade. Please note that the number of joints in mammals does NOT vary, but only the relative length (and shape) of the different parts of the leg. A Plantigrade walks on the sole of the foot. 'Sole' translates to 'planta' in Latin hence the name. Examples: Human, ...


20

How come large herbivores have such thin legs? They don't. The following book does an extensive comparison of the bones of humans with other animals: Adams BJ, Crabtree PJ. 2008. Human vs Horse. In: Comparative Skeletal Anatomy. Humana Press. Here are some images of human bones (left) next to horse bones (right): Radius/Ulna (fused in horses) Humerus ...


15

There seems to consensus that it is not competition for tall food. Giraffes actually often feed on resources that are lower than their maximum possible height. See: Simmons, R. E. & Scheepers, L. 1996. Winning by a Neck: Sexual Selection in the Evolution of Giraffe. The American Naturalist 148: 771–786 This paper put forth the idea that sexual ...


15

Anatomical terms must be able to fit a wide variety of organisms, from insects to fish, dogs, horses, chimpanzees to humans. That's why the terms are sometimes confusing to people who are thinking only of bipedal humans. In anatomy, the dorsum is the upper side of animals that typically run fly, swim or crawl in a horizontal position. In vertebrates the ...


15

Male mice lack nipples too. Mice are frequently used for embryonic research as they are small and reproduce quickly. It is thought that male mice do develop nipples, but that they regress during development (Wysolmerski, 1998). In general, it is thought that mammalian organisms develop as females by default when there is no male (Y) chromosome present (...


15

Short answer Six legs allow for locomotion, while maintaining a supportive tripod at all times. Background There are several million species of insects, all on 6 legs. This implies that any change in this number is promptly selected against. It is generally agreed that insects were derived from many-legged ancestors, e.g. centipedes. One explanation is ...


15

As a couple of counterexamples, species in the classes Symphyla (Pseudocentipedes) and Pauropoda within Myriapoda have 8-11 and 12 leg pairs respectively, so between 16 to 24 legs (sometimes with one or two leg pair stronlgy reduced in size). (species in Symphyla, from wikipedia) Another common and species-rich group with 14 walking legs (7 leg pairs) is ...


14

First, for reference, see here for a discussion about the difference in directional terms between bipeds and quadrupeds as well as a fairly complete explanation of word meanings/etymology. The etymological meanings of the various anatomical directional terms should help explain their usage in body organs. For example: Ventral -> "belly" side Dorsal -&...


13

Most mammalian males have nipples. The duck-billed platypus does not have nipples but you begin to see development of nipples in marsupials (Park and Lindberg 2004) like the opossum and kangaroo. Development of a complete nipple begins in the eutherian (placental) mammals. The mammary glands develop early in the embryo along a pair of ridges called the ...


13

Short answer The body plan of terrestrial organisms that feature limbs is typically bilaterally symmetric, and hence these organisms have an even number of appendages. Background Terrestrial animals featuring appendices for motility (legs, arms) are typically higher organisms. Typically, the body plan of higher animals such as us mammals is typically ...


13

The FMA lists 705 tendons, but note that it includes separate terms for left and right instances. As @kmm says, many of these simply shadow the list of skeletal muscles (and is likely incomplete). You can browse the list on OLS, or if you want to extract a table you can query this SPARQL endpoint, just type in the query here: SELECT DISTINCT ?x ?v0 WHERE { ...


12

Cat claws are growing all the time, like horse hooves, or human nails. However, cats and horses usually use their claws/hooves, so they get shortened through mechanical action. An indoor cat may need their claws trimmed if it doesn't use them enough (that's why cats will want to scratch everywhere), or if has supernumerary toes that don't normally touch the ...


12

Rotifers are microscopic protostomes with around 1000 cells apiece. This website claims the smallest rotifers have less than 100 cells, but I couldn't confirm this. Most rotifers are eutelic — within a given species, each adult individual has the same number of cells (Encyclopeia of Life: Rotifers). (Source) Edit: Myxozoans might be even smaller. These ...


11

A recent paper called 'Genetic Influences in Sport and Physical Performance'[1] states: "Muscle fibre type determination is complex. Whilst initial composition is likely to be strongly influenced by genetic factors, training has significant effects on fibre shifts." They also go onto say that: "However, the role of genetic variation in determining ...


11

Black herons (Egretta ardesiaca) can overlap their wing tips in front of them while doing what is known as canopy feeding.


10

Building on the answer given by Sean Connolly above, it would be very easy to imagine evolutionary scenarios where organs are more likely to develop asymmetrically than symmetrically. For instance, imagine an organism that has a simple digestive system that consists only of a single undifferentiated intestine that runs directly from mouth to anus in a ...


10

Wootton (1992) reviewed the anatomy and biomechanics of insect wings. Basically the wing is a lightweight but strong scaffolding of veins, supporting a thin membrane. The veins are composed by a sandwich of cuticle with a potential space in between. The membrane is also a double-layer but without the space. In the venous space are is circulating hemolymph ...


10

I don't know about all known animals, or actual cell count. However, males of the wasp Dicopomorpha echmepterygis are considered the smallest known insects with a body size of 139 μm. This should at least be a contender for smallest animal. As a reference point, C. elegans are usually about 1mm long. However, given that we know only about 10% of the ...


10

This depends to some extent on how you define "resting state" (it matters). Innervation of the eye occurs in the brainstem and upper spinal column, so is, like most brainstem functions, on autopilot (like breathing.) If you define resting state as that which the lids would assume without any innervation, then the resting state of the eyelids is semi-open. ...


10

Many Nematodes do not use traditional hox genes instead the have a strange set up which controls cell placement directly. This means individual adults of many nematode species have the exact same number of cells, it can even be used to identify species. So having the exact same number of neurons is what one should expect in nematodes. https://www.ncbi.nlm....


9

At the very least, I know that male primates also have nipples like female, though they are very close relatives to human. On the other hand, in some of my dissection labs, I noticed that male pigs also have nipples just like the female ones. It seems to be the case that most male mammals have nipples, which probably has to do with mammals being breast-...


9

While it might make more logical sense to have separate passageways for air and food/water, this did not happen in evolutionary history due to the peculiarities of lung development. Vertebrate lungs develop as an outpouching of the gut tube, which itself has a very long evolutionary history (probably homologous among all deuterostomes). In the image below, ...


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