I'm reading Franz Kafka's Die Verwandlung. The second sentence is: 'Er lag auf seinem panzerartig harten Rücken [...]'. I think this sentence means 'He lied on his shell-like hard back [...]". In particular, 'panzerartig' seems to me as an adjective meaning shell-like ('panzer' + '-artig'). But if it's an adjective why doesn't it get an -en suffix like 'hart' does in order to agree with 'dem Rücken'? I would write this sentence as 'Er lag auf seinem panzerartigen harten Rücken [...]'; why is this wrong?
Sometimes it's small things that make a difference:
Sie wusch ihr künstliches blondes Haar.
She washed her artificial blonde hair.
Sie wusch ihr künstlich blondes Haar.
She washed her artificially blonde hair.
Similarily, in your example:
Er lag auf seinem panzerartig harten Rücken.
panzerartig describes hart, not the Rücken.