ALIMENTARY CANAL. Si 
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 re- 
mainder 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 proctodaeum 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. . 
Summary. 
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 
Fig. 24. — Longitudinal section of Ceraurus pleurexanthemus, show- 
ing the probable outline of the alimentary canal and the heart above 
it. A restoration based on the slices described above. 
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 proctodeum can not yet be differentiated from the mesen- 
teron, and only in Cryptolithus has the posterior portion of the alimentary canal been seen. 
It is, there, merely a continuation of the mesenteron. The stomodaeum likewise has not been 
identified, but was probably a short gullet leading up from the mouth into the enlarged 
digestive cavity. 
The principle of the enlargement of the latter and its influence on the dorsal shell once 
established, the significance of different types of glabella? becomes apparent. It will be re- 
membered that the glabella of the protaspis of most trilobites is narrow, and that the same 
is true of the glabella; of most ancient and all primitive trilobites. The free-swimming larva? 
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 re- 
quired, and so in the later and more specialized trilobites the glabella became expanded lat- 
terally or dorsally, or both, to meet the requirement for more space, until, in such Devonian 
genera as Phacops, the cephalon was nearly all glabella. 
