Check out this sample assignment on the immune system! You can record your own video, use a YouTube video or upload your own video, then add a question or prompt for your students. They record a video and send it back. They can use a laptop with a webcam, or a tablet or smart phone. The easiest way for students to use it is to download the app for an iPhone or android.
It's free for one grid! You can use it for free forever if you delete your used grid and then start a new one. Once you fall in love with it, you may want to upgrade to the paid version for $65 a year.
I'm still using the free version to see how Students like it. :) so this sample assignment may not be around long...
But you can just create an account and try it out for yourself!