The individual in the photograph is a U. S. Navy Pharmacist's Mate (as navy medical corpsmen were so designated prior to 1948).
The uniform is the marine corps summer service uniform of post-World War I issue (collar insigina were authorized for marine enlisted men in 1921).
The marine corps is - and always has been - under jurisdiction of the Navy Department and has never had a medical department of its own. Navy doctors and medical personnel have traditionally been attached to marine corps units to provide for their medical and dental needs.
Sometime in the late 1800's, navy doctors and corpsmen serving with marine corps units were authorized to wear the marine corps uniform while so assigned or attached (with navy rank/rating insignia, of course). At first optional, this later became mandatory.