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 Calymene and Ceraurus practically always show the cavity there filled with 
clear crystalline calcite. One actual specimen of Ceraurus (Walcott 1881, pi. 4, fig. 1) 
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 plcurcxanthemus , 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 pleurexan- 
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) . 
Traoed from a 
photographic en- 
largement. One 
appendifer (also 
clear white) is 
shown. Specimen 
153- X3-3- 
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 bod}' 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. 
