ALIMENTARY CANAL. 



79 



This idea of an enlarged mesenteron certainly has much to commend it, and such actual 

 evidence as exists seems in favor of rather than against it. The strongest, firmest, best- 

 protected place in the whole body of the trilobite is the cavity between the vaulted glabella 

 and the hypostoma. As Jaekel has said, it is far too large a cavity for the brain, larger 

 than would seem to be required for a heart, and what else could be there but a stomach? 

 As has already been pointed out, Beyrich and Barrande found a pear-shaped enlargement of 

 the alimentary canal under the glabella of Cryptolithus. Longitudinal sections through 

 the glabella of Calymcnc and Ceraurus practically always show the cavity there filled with 

 clear crystalline calcite. One actual specimen of Ceraurus (Walcott 1881, pi. 4, fig. i) 

 shows the cavity between the glabella and hypostoma entirely empty. The vacant spaces in 

 these two classes of specimens do not, however, necessarily mean anything more than im- 

 perfect preservation. 



Fig. 21. Transverse slice through 

 Ceraurus plcurex.anthemus, to show 

 the dorsal sheath above the abdomi- 

 nal cavity. Specimen 118. Traced 

 from a photographic enlargement. 

 X4. 



Fig. 22. -Transverse section through 

 the cephalon of Ceraurus fleurexan- 

 themus, showing the abdominal sheath 

 and the large mud-filled alimentary 

 canal (clear white). Traced from a 

 photographic enlargement. Specimen 

 97- X 3-3- 



Fig. 23. Trans- 

 verse section of 

 the thorax of Ca- 

 lymene senaria, 

 showing the large 

 size of the mud- 

 filled alimentary 

 canal(clear white). 

 Traced from a 

 photographic en- 

 largement One 

 appendifer (also 

 clear white) is 

 shown. Specimen 

 153- X 3-3- 



Ceraurus pleurexanthemus. 



This species is taken up first, as it is the one shown in Walcott's often-copied figure 

 (1881, pi. 4, fig. 6). It is to be feared that too many have looked at this figure without 

 reading the accompanying explanation, and have taken it for a copy of an actual specimen 

 and not a mere diagram, which it admittedly is. The evidence on which it is based is com- 

 prised in eight transverse slices, one through the glabella and seven through the thorax. 

 Three of these have been figured by Walcott: No. 27, 1881, pi. 3, fig. 7; No. 13, 1881, pi. 2, 

 fig. 3, 1918, pi. 26, fig. 14; No. 202, 1918, pi. 27, fig. 8. In all, as can be seen by reference to 

 the figures, the canal is partially collapsed, and is much larger than is indicated in Walcott's 

 restoration. The other sections bear out the testimony of those figured. One of these figured 

 specimens (No. 27) and another figured herewith (No. 118, see fig. 21) show an exceedingly 

 interesting structure which has previously escaped notice. The body cavity seems to have 

 had, in this region at least, a chitinous sheath on the dorsal side. As shown especially in 

 figure 21, this sheath impinges dorsally and laterally against the axial lobe and thus fur- 

 nishes a special protection for the soft organs beneath, probably protecting them from the 

 strain of the dorsal muscles. 



