www.biology4kids.com/files/systems_skeletal.html
https://www.britannica.com/science/human-muscle-system
https://kidshealth.org/en/kids/muscles.html
biodifferences.com/difference-between-voluntary-and-involuntary-muscles.html
www.ck12.org/book/CK-12-Life-Science-For-Middle-School/r7/section/19.1/
http://www.webmd.com/heartburn-gerd/your-digestive-system
http://www.e-missions.net/cybersurgeons/?/dig_teacher/
How does it work with other systems?
https://www.aboutkidshealth.ca/Article?contentid=310&language=English
https://www.britannica.com/science/zygote
http://biology.about.com/od/organsystems/ss/integumentary_system.htm http://sciencenetlinks.com/student-teacher-sheets/integumentary-system/
https://www.sciencedaily.com/terms/excretory_system.htm
https://easyscienceforkids.com/the-scoop-on-poop-human-excretory-system-basics/
http://www.livescience.com/22665-nervous-system.html
http://www.innerbody.com/image/nervov.html
askabiologist.asu.edu/parts-of-nervous-system
kidshealth.org/en/parents/brain-nervous-system.html
https://www.ck12.org/biology/nervous-system/lesson/Nervous-System-MS-LS/?referrer=concept_details
http://www.emedicinehealth.com/anatomy_of_the_endocrine_system/article_em.htm
https://www.verywellmind.com/what-is-the-fight-or-flight-response-2795194
https://drive.google.com/open?id=11HS80D-VT_vBX9_b1jil71EvM9L5kHgU