. . would naturally arise ventrally and laterally inthe bend of the soft body ; the bend is so sudden thatwe may well imagine the folds forming projectingansrles at each side like the angles formed by the o c_> y bending of an india-rubber tube. This comparisonwould be almost exact if we imagine the convexcurve of the tube so stretched as not seriously todiminish the size of its lumen, as must have been thecase in the bent Annelid to prevent compression ofthe viscera. That the sides of the angle of the benddid thus project we conc
. . would naturally arise ventrally and laterally inthe bend of the soft body ; the bend is so sudden thatwe may well imagine the folds forming projectingansrles at each side like the angles formed by the o c_> y bending of an india-rubber tube. This comparisonwould be almost exact if we imagine the convexcurve of the tube so stretched as not seriously todiminish the size of its lumen, as must have been thecase in the bent Annelid to prevent compression ofthe viscera. That the sides of the angle of the benddid thus project we conclude from the position of thesecond antenna; both in Apus and in Limulus, where SECT. XIII THE TRILOBITES 213 they lie outside the longitudinal line which joins theother limbs. The development of hard cuticularpoints, and thence of thorns on such lateral projec-tions, would be but a matter of time. From thesepoints also the gradual development of the ridgeround the front of the head can easily be some Trilobites it remains quite inconspicuous, but. FIG. 47.—Dionide formosn (Uarr), showing the glabella and the gradual rudimentary character of the posterior segments. in others, as already stated, it projects as a greatshovel-shaped We here find, then, the originof all forms of the Crustacean shell, which we havedeferred discussing till now ; we may summarise ourconclusions as follows : Round these lateral projections, due to the bendingof the cylindrical body, all the shapes of the Trilobite 214 THE APODID^: PART n head-shield play. We are inclined to think that theformation of the ridge round the front was the primi-tive variation, because of its great use as a belt-likeshield round the unprotected head of the browsinganimal, especially if it went hand in hand with thethickening of the cuticle of the frontal lateral processes and the frontal riclge thusformed the primitive head-shield of this whole groupof Annelidan-Crustacea, and every form of
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