Indeed, but both skilpadden and skilpadda is acceptable. Use this when you are in doubt of a word's gender: http://www.nob-ordbok.uio.no/perl/ordbok.cgi?OPP=skilpadde&bokmaal=+&ordbok=bokmaal. The m means male, and f female. (it says m1 and f1 thought, dont care about that. Click on the m1 or f1 to see the conjugations)
There should be an audible difference between the -a and the -e endings. That said, you may also get clues from the preceeding article/adjective: It's never 'en liten jenta', nor 'skilpadde mi'; these are grammatically incorrect :)
The plural form is 'skilpadder' and 'jenter' (indefinite), and 'skilpaddene' and 'jentene'. Note that an -a suffix marks the plural for a few neuter plural definite nouns (such as 'barna').
Looks like I had the same issue with this exercise most others did. Hearing the difference between -a and -e, but looking back, I should've slowed down and recognized that there was no article. It could have only been "Skilpadda" "The turtle" since "En" didn't precede it. I am having so much fun. Tusen takk to the lovely people that contributed to this course.