Each "layer" of the dissection contains body parts that belong to specific systems (digestive, circulatory, etc).
(Each "layer" of the dissection; contains; body parts that belong to specific systems (digestive, circulatory, etc)) (body parts; belong; to specific systems)
Each "layer" of the dissection body parts
contains belong
body parts that belong to specific systems (digestive, circulatory, etc)
to specific systems