rachis vs skeletal structure

rachis and skeletal structure both are nouns.

Word	Noun	Adjective	Verb	Adverb
rachis	Yes	No	No	No
skeletal structure	Yes	No	No	No

As nouns, skeletal structure is a hypernym of rachis; that is, skeletal structure is a word with a broader meaning than rachis:

rachis: the series of vertebrae forming the axis of the skeleton and protecting the spinal cord
skeletal structure: any structure created by the skeleton of an organism

rachis (noun)	skeletal structure (noun)
the series of vertebrae forming the axis of the skeleton and protecting the spinal cord	any structure created by the skeleton of an organism
axis of a compound leaf or compound inflorescence