They were named carcass because the circles which pass from one ring, or plate, to the other, were thought to resemble the ribs of a human carcass.
ðeɪ wɜː nˈeɪmd kˈɑːɹkəs bɪkˈʌz ðə sˈɜːkəlz wˌɪtʃ pˈæs fɹʌm wˈʌn ɹˈɪŋ, ɔːɹ plˈeɪt, tə ðɪ ˈʌðɚ, wɜː θˈɔːt tə ɹᵻzˈɛmbəl ðə ɹˈɪbz əvə hjˈuːmən kˈɑːɹkəs.