The tergites of this region and those of the following region, the metasoma, are fused to form a second or posterior carapace in Limulus, whilst remaining free in Scorpio.
Passing on now from the mesosoma we come in Scorpio to the metasoma of six segments, the first of which is broad whilst the rest are cylindrical.
The somites of the metasoma carry no parapodia.
In Limulus the metasoma is practically suppressed.
It is probable that we have in the metasoma of Limulus a case of the disappearance of once clearly demarcated somites.