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Rats have some anatomical peculiarities which may baffle a vet who is only used to dogs and cats.
They have a large, knobbly caecum (a cul-de-sac at the beginning of the large intestine, containing bacteria used to break down cellulose) which can be mistaken for an internal tumour.
They do not have a gall-bladder.
The right lung of a rat has four lobes, but the left lung has only one. If your vet is not expecting this, it can make chest X-rays hard to interpret.
Males do not have even rudimentary nipples (though they do have some mammary tissue).
Females have a sort of rudimentary penis called the genital papilla. They urinate through this, rather than through an opening at the entrance of the vagina as most mammals do - so a prolapsed uterus can be snipped off from the outside and stitched at the vagina, without blocking the urethra.
Something a vet will know but many rat-lovers won't: rats - and rodents and lagomorphs (rabbits etc.) generally - do not have lips running round the edge of their mouth in the way that most mammals do. Instead the upper lip folds in into a toothless gap called the diastema, behind the top front teeth.
If you open a rat's mouth you will find that there are folds of actual, hairy skin tucked inside the mouth behind the front teeth, covering the front part of the roof of the mouth. The function of these giant, in-folded lips is to create a strong skin barrier between the front teeth and the mouth proper - the tongue and throat - so that the animal can gnaw splintery, inedible and even slightly poisonous materials without the shreds falling onto the tongue and being swallowed.