Are animals..

Are animals allowed to travel on Airplans?

Yes,but they have to be in a cage carrier.

They travel via cargo in kennels.

no, not all animals are allowed in
customs obviously check them in case there illegal in another country, there illegal pets, they have any diseases, ect
so its not like you can just take an illegal pet monkey or a lizard you have in your country over to another one
usually its just domestic animals like cats and dogs

you have to provide food and water, and holders for that that attach to the cage. Consider the space in the carrier, they need to be able to get up, turn around and lay down without any problems. Only some airlines allow pets on travel. There are websites that tell you what their animal death/ lost/ accident percent is... you should check it out before flying.
I think since 9-11, only specialty companies can board animals, meaning, you have to hire someone to do this for you... when I flew my cat over to AZ, it cost about 2 grand for door to door service, with boarding for the flight... My cat was 13 years old, but I knew he would make it, cause he is a tough SOB, and has been through a lot, this scary plane ride and crossing multiple planes and states would be nothing for him... Make sure you really know you're cat can survive this experience.

I've heard that they can, but I think they are carried in the belly of the plane.

I think they go wiv the suit casses?