In the abdominal exoskeleton the segmental structure is very clearly marked, a series of sclerites - dorsal terga and abdominal sterna - being connected by pale, feebly chitinized cuticle, so that considerable freedom of movement between the segments is possible.
The Staphylinidae, or rove-beetles--a large family of nearly Io,000 species - may be known by their very short elytra, which cover only two of the abdominal segments, leaving the elongate hind-body with seven or eight exposed, firm terga (figs.
In Arachnida the highest forms exhibit a fusion of the tergites of five post-oral somites to form one continuous carapace united with the terga of the two prosthomeres.
Io) the whole dorsal region of the abdomen, but sometimes leaving as many as four terga exposed (fig.
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.
In Crustacea the fourth or mandibular somite never has less than the two following somites associated with it by the adaptation of their appendages as jaws, and the ankylosis of their terga with that of the prosthomeres.
The soft integument and limbs of the mesosoma have been removed as well as all the viscera and muscles, so that the inner surface of the terga of these somites with their entopophyses are seen.
In Diplopoda two opisthomeres - that is to say, one in addition to the buccal somiteare united by a fusion of their terga with the terga of the prosthomeres.