212 THE APODID^E PART n 



frontal ridge and the dorsal shield, but it is always 

 more or less visible in the Trilobites, which formed no 

 such dorsal fold. Round the glabella is developed the 

 remarkable crescent-shaped ridge which runs round 

 the front of the head, such as we found in Apus as a 

 prolongation of the lateral edges of the shield. In the 

 Trilobites, this ridge is often very pronounced, form- 

 ing a wide margin round the head, with horns some- 

 times stretching back far beyond the posterior end of 

 the body (see Figs. 47 and 57, p. 257). The origin 

 of this ridge is probably to be sought in the folds 



FIG. 46. Longitudinal section through Ceraurus pleurexanthemus (after Walcott), 

 showing the intestinal canal and ventral membrane, and the bend in the head. 

 Cf. Figs, i and 2. 



which would naturally arise ventrally and laterally in 

 the bend of the soft body ; the bend is so sudden that 

 we may well imagine the folds forming projecting 

 angles at each side like the angles formed by the 

 bending of an india-rubber tube. This comparison 

 would be almost exact if we imagine the convex 

 curve of the tube so stretched as not seriously to 

 diminish the size of its lumen, as must have been the 

 case in the bent Annelid to prevent compression of 

 the viscera. That the sides of the angle of the bend 

 did thus project we conclude from the position of the 

 second antennae both in Apus and in Limulus, where 



