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.
Following the metasoma in Limulus, we have as in Scorpio the post-anal spine - in this case not a sting, but a powerful and important organ of locomotion, serving to turn the animal over when it has fallen upon its back.
Perhaps the most important general agreement of Scorpio compared with Limulus and the Eurypterines is the division of the body into the three regions (or tagmata) - prosoma, mesosoma and metasoma - each consisting of six segments, the prosoma having leg-like appendages, the mesosoma having foliaceous appendages, and the metasoma being destitute of appendages.
It is probable that we have in the metasoma of Limulus a case of the disappearance of once clearly demarcated somites.
In Limulus the metasoma is practically suppressed.
The somites of the metasoma carry no parapodia.
The unsegmented dense chitinous sternal plate of the metasoma (XIII to XVIII) is not removed.
In the most highly developed forms the two anterior divisions (tagmata) of the body, prosoma and mesosoma, each exhibit six pairs of limbs, pediform and plate-like respectively, whilst the metasoma consists of six limbless somites and a post-anal spine.