"The turtles" is perfectly fine if the sentence is in reference to a larger collection. So a motley group of things might contain ice, marbles, and turtles. In this case saying "The turtles are animals" has semantic meaning. You can still say "Turtles are animals", but it leaves out the implied reference to the overall group.
In English at least, "THE turtles are animals" seems meaningless: if some turtles are animals then all are (toy turtles etc are not turtles), and this requires the non-use of "the" before turtles. Is it the same in Dutch? If so, I would suggest avoiding such grammatically/logically nonsensical sentences in exercises. (Nonsense is fine if it's just reality-nonsense or falsehood, like "elephants drink beer".)
I wouldn't go so far as to call this a "grammatically/logically nonsensical sentence". The turtles may be animals as opposed to plants... or robots... etc. I understand what you mean with "Turtles are animals." being what you'd use as a general statement, but that's not the meaning here.
The Dutch sentence means the exact same thing, but we're not removing it.
My main point is that it's potentially confusing for the student, because in some languages (e.g. French), you would use the "the" in that position to make a statement corresponding to "turtles (i.e. in general) are animals". "We're not removing it" -well, of course, it's your product (and I appreciate it very much by the way), you do it how you want. Nevertheless I do honestly think it would be more effective to use sentences which are sort of "normal" with regard to use of articles. So - picking up on your "as opposed to" clarification, which I get - why not make that type of implied context explicit? Anyway - don't feel you have to respond to this, I've said what I wanted to say, if you're not convinced, so be it. Thanks for answering and for the course material, I find it generally very helpful.
Yeah, I got that. My point was that the sentence "The turtles are animals" referring to a particular group of animals is a misleading sentence from the student's point of view precisely because everyone knows all turtles are animals. In fact Duolingo, highly praiseworthy though it is, has quite a large number of pedagogically counterproductive elements - another example being the use of male voices to utter first-person sentences with feminine forms and vice versa. I've encountered this in the Italian course, where it's not a problem FOR ME because I know enough of the structure to be aware, but if I were learning a language of which I had no prior knowledge, it would be potentially harmful to the learning process.