As central organs we have to note the brain-lobes and the longitudinal lateral cords which form one continuous unsegmented mass of fibrous and cellular nerve-tissue.
The unsegmented dense chitinous sternal plate of the metasoma (XIII to XVIII) is not removed.
- Section through an early embryo of Limulus longispina, showing seven transverse divisions in the region of the unsegmented anterior carapace.
Met, Unsegmented metasoma.
This telson may enlarge, it may possibly even become internally and sternally developed as partially separate somites, and the tergum may remain without trace of somite formation, or, as appears to be the case in Limulus, the telson gives rise to a few well-marked somites (mesosoma and two others) and then enlarges without further trace of segmentation, whilst the chitinous integument which develops in increasing thickness on the terga as growth advances welds together the unsegmented telson and the somites in front of it, which were previ ously marked by separate tergal thickenings.