ALIMENTARY CANAL. 8l 



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 proctodseum 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 proctodeeum 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 stomodasum 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 glabellre 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 glabellae of most ancient and all primitive trilobites. The free-swimming larvae 

 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. 



