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20

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 ...


17

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 ...


13

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 ...


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 ...


11

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

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 (...


11

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 ...


11

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 ...


11

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 ...


10

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 ...


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, ...


9

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 ...


8

Perhaps what your teacher meant was not so much a difference in Leydig cell morphology, but in interstitial tissue morphology, ie. tissue which occupies the space in between seminiferous tubules. Leydig cells are its most interesting component, others being small blood vessels (a lot of them), nerves and connective tissue (mostly fibroblasts, mastocytes, ...


8

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 ...


8

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-...


7

I'd suggest that one factor could be the difficulties of breathing water - it is highly inefficient. The following is a summary of this response to a TREE article. Fish spend 10-30% of their energy on breathing because it is so inefficient to use gills because diffusion rates are much slower and oxygen is less abundant. Gills grow in proportionate scales to ...


7

Males of the carnivorous Jackson's chameleon (Trioceros jacksonii) have three hornlike structures on their heads (also found in a couple of related species). From what I know, these are true horns, which means that they are pointed keratin-covered protrusions that have a core of living bone, see e.g. this website from Toronto Zoo (this page also claims that ...


6

I would argue that the orbiculares do have antagonists. To some extent, levator palpebrae superiorus antagonizes orbicularis oculi, and zygomaticus major/minor as well as risorius antagonize orbicularis oris. I can think of three muscle that don't have obvious antagonists: Stapedius Tensor tympani Articularis genu 1 and 2 essentially perform the same ...


6

I have studied all available (via University library) literature on the Leydig cells and I think your teacher might have this article summarizing the morphological studies on Leydig cells in different animal models in mind. To put it straight, the most common animal models for studying Leydig cells are rats, mice and pigs. The development of Leydig ...


6

Oxygen levels have changed greatly throughout Earths's history starting with very little atmospheric oxygen and gradually increasing as photosynthetic activity appeared until all the oxygen sinks such as minerals had been filled, then taking off, reaching as high as 35% of the atmosphere during the carboniferous period 3. The reasons behind this sudden ...


6

The reason why cats are superb jumpers has not received much attention, but one article in Nature was entirely devoted to just this (Diamond, 1988). Here follows a partly quoted and partly adapted text from this article: First , because mass increases as the cube, but surface area as the square of linear dimensions, falling large animals are in general more ...


6

Citing your source would help to answer. In order to test an elastic modulus, you need to apply some non-zero strain. If the strain is infinitesimal, then compression and stretch modulus will be equal. But if it is finite, then there can be a difference due to the structure of the material. In bone, I believe porosity will provide most of the dissymetry ...


6

The white circle in the center is a bone, probably the femur. The muscles are covered with an external later of fascia, the epimysium (here's a question with some information). These layers separate muscles from each other, and often fat will also deposit in the spaces between muscles, which is what the white lines between muscle bellies are. There is also ...


6

Short answer The number of eyes are not always even-numbered. In fact, many present-day lower vertebrates feature an uneven number of eyes. Background Eyes can be generally defined as photosensitive organs. Besides the regular paired eyes, lower vertebrates such as fish, amphibians and reptiles feature an unpaired (single) brain structure called the pineal ...


5

Mantis shrimp use their first maxillipeds for grooming (maxilliped=modified appendage), which is specialized for this purpose. Details and a picture of the organ can be found in the link. The second maxilliped is their famous specialized organ for striking or spearing prey with enormous force. More about their raptoral appendage, with links to further ...


5

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 ...


5

As part of your question, you ask if other animals can create sound without continuous airflow. Many insects (e.g. cicadas and moths) do exactly this by using tymbals. A tymbal/timbal is an external membrane organ that is controlled by muscles or wing movements, that cause the membrane to flip back and forth, creating clicks or other sounds. So in many ways ...


5

Short answer Echolocating bats have relatively large sensory epithelia in their inner ear, that may correlate with their high upper frequency limit of up to 200 kHz. The basilar membrane is thinner and stiffer, possibly allowing it to decode higher frequencies. Background In terms of the place theory of hearing, the cochlea acts as a frequency transformer, ...


5

The other part of the question has already been amply answered, so I will concentrate on the part of the question which asks about "doesn't divide into odd numbers" The assumption that organisms must have even numbers of cells merely because cells divide into two each time is false. For example, the nematode Caenorhabditis elegans has either 959 or 1031 ...


5

Dogs get benign prostatic hypertrophy, so, yes, other animals (in fact most old male dogs, so that would mean a whole lot of other animals) do get BPH. The most common conditions affecting the canine prostate include benign prostatic hyperplasia, prostatitis, prostatic cysts, and prostatic neoplasia. However, you're almost correct. [T]here is no ...



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