A collection of tissues which work together to accomplish a function. The organisational unit above tissues and below systems.

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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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Is the appendix a vestigial structure in all vertebrates?

In humans the Appendix is a vestigial organ. Does it serve no apparent purpose in all the vertebrates that have one?
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Why does organ transplant work although it seems organ's motor neuron isn't connected to recipient's CNS

Although it seems impossible with current technology to connect organ's capillaries to the recipient's capillaries and its motor neuron to the CNS, transplanted organ works for years. But why?