VII, Usually considered to be the tergum of the genital somite, but suggested by Pocock to be that of the other [According to the system of numbering explained in the text, if VII is the tergum of the praegenital somite (as is probable) it should be labelled Prg without any number, and the somites VIII to XIII should be lettered 1 to 6, indicating that they are the six normal somites of the mesosoma; whilst XV to XVIII should be replaced by the numbers 7 to 12 - an additional suppressed segment (making up the typical six) being reckoned to the metasomatic fusion.] (From Lankester, Q.
7, excepting that VII is here certainly the tergum of the first somite of the mesosoma - the genital somite - and is not a survival of the embryonic praegenital somite.
The posterior carapace of the Trilobites and of Limulus is probably enough in origin a telsonic carapace - that is to say, is the tergum of the last segment of the body which carries the anus.
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.
7) may be the tergum of the suppressed prae-genital somite.
Prae-gen, Tergum of the praegenital somite.
I, Sternite of the genital or first opisthosomatic somite; the prae-genital somite, though represented by a tergum, has no separate sternal plate.
(I) first antenna; (6) tergum; (2) compound (7) biramous eye; feet; (3) liver; (8) carina; (4) simple eye; (9) cement (5) scutum; gland.
In the Entomobryidae the body is elongate and clearly segmented, but the dorsal region (tergum) of the prothorax is much reduced and the head downwardly directed; the spring is well developed.
In the Achorutidae the head is forwardly directed, the tergum of the prothorax conspicuous, and the spring small or vestigial.
A dorsal and a ventral plate are often distinguished, known respectively as the tergum and the sternum, and the tergum may overhang the insertion of the limb on each side as a free plate called the pleuron.
T, tergum; s, sternum; p1, pleuron.