Both Beyrich and Barrande have shown that from the posterior end of the axial lobe to the neck-ring on the cephalon, the alimentary canal in Cryptolithus has a nearly uniform diameter of less than half the width of the axial lobe. In front of the neck-ring, it enlarges, and while its original describers state that it extends only about halfway to the front of the glabella, Barrande's figure 39 shows it extending quite to the front, and his figure 38 shows it fully two thirds of the distance to the anterior end, as does Beyrich's figure of 1846.
The Museum of Comparative Zoology contains a single specimen of this species from Wesela, Bohemia, which shows the course of the canal from the middle of the pygidium to the anterior part of the glabella. The enlargement appears to begin about halfway to the front of the glabella and to be greatest at the anterior end. At the anterior end of the glabella, the anterior end of the thorax, and the posterior end of the pygidium, the canal is still packed full of a material somewhat darker in appearance than the matrix, while the remainder of it is open. A well defined constriction is present under the middle of the next to the last thoracic segment, but whether this is accidental or whether it indicates the point where the mesenteron discharges into the proctodæum can not be determined. The inside of the canal has somewhat of a lustre and there are three conical projections into it on the median ventral line, a very small one in front of the neck furrow, a larger one under the anterior part of the second segment, and a third between the fourth and fifth segments.
The specimens of Cryptolithus from Bohemia and of Ceraurus and Calymene from New York seem to substantiate the claim of Bernard and Jaekel that at the anterior end of the canal there was an enlarged organ which occupied the greater part of the cavity of the glabella. It appears that it extended into the thorax, and that above it and the heart was a chitinous dorsal sheath. Behind the enlarged portion, the mesenteron appears to have been of practically uniform diameter in Cryptolithus, but to have tapered posteriorly in Ceraurus and Calymene. The proctodæum can not yet be differentiated from the mesenteron, and only in Cryptolithus has the posterior portion of the alimentary canal been seen. It is, there, merely a continuation of the mesenteron. The stomodæum likewise has not been identified, but was probably a short gullet leading up from the mouth into the enlarged digestive cavity.
Fig. 24. Longitudinal section of Ceraurus pleurexanthemus, showing the probable outline of the alimentary canal and the heart above it. A restoration based on the slices described above.
The principle of the enlargement of the latter and its influence on the dorsal shell once established, the significance of different types of glabellæ becomes apparent. It will be remembered that the glabella of the protaspis of most trilobites is narrow, and that the same is true of the glabellæ of most ancient and all primitive trilobites. The free-swimming larvæ and the free-swimming ancestors of the trilobites were probably strictly carnivorous, lived on concentrated food, and needed but a small digestive tract. As the animals "discovered the ocean bottom" and began to be omnivorous or herbivorous, larger stomachs were required, and so in the later and more specialized trilobites the glabella became expanded latterally or dorsally, or both, to meet the requirement for more space, until, in such Devonian genera as Phacops, the cephalon was nearly all glabella.