Simplify the following text for a Grade 11 native English-speaking person.
Original: Caecilians are the only order of amphibians that use internal insemination exclusively ( although the tailed frog in the U.S. does use a tail-like appendage for internal insemination in its fast flowing water environment ) .
Simplified: Caecilians are the only order of amphibians which only use internal insemination .