A tendency is exhibited to the formation of a metasomatic as well as a prosomatic carapace by fusion of the tergal surfaces of the somites.
This gives seven somites to the Hexapod's head, the tergites of which are fused to form a cephalic carapace or box.
In the family Homolidae stands the strange genus Latreillia, Roux, with long slender limbs and triangular carapace after the fashion of oxyrhynch spider-crabs.
The carapace is much more developed in the comparatively sedentary female than in the usually more active male.
In the Phalangiotarbi the appendages resembled those of the Anthracomarti, except that the basal segments of the last four pairs were usually approximated in the middle line leaving a long and narrow sternal area between; and the carapace of the prosoma was unsegmented.
In Homola the carapace is quadrilateral.
Many of the Cambrian trilobites appear to have been blind, and they had not at this period developed that flexibility in the carapace that some forms acquired later.
This is partly due to the fact that many important forms must have escaped fossilization altogether owing to their small size and delicate structure, while very many of those actually preserved are known only from the carapace or shell, the limbs being absent or represented only by indecipherable fragments.
But most important of the evidences presented by the trilobites of affinity with Limulus, and therefore with the Arachnida, is the tendency less marked in some, strongly carried out in others, to form a pygidial or telsonic shield - a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of Limulus.
It may be supposed to have approximated, in general form, to A pus, with an elongated body composed of numerous similar somites and terminating in a caudal furca; with the post-oral appendages all similar and all bearing gnathobasic processes; and with a carapace originating as a shell-fold from the maxillary somite.