HEART. 
85 
system. While they are known chiefly in Cambrian and Lower Ordovician trilobites, there 
is no evidence that the organs represented were not present in later forms, even if the shell 
may not have been affected by them. While they indicate very fine, thread-like canals, the 
present evidence seems to be in favor of assigning to them the function of lodging the glands 
which secreted the principal digestive fluids. 
HEART. 
Illcenus. 
Volborth (1863, pi. 1, fig. 12 = our fig. 26) has described the only organ in a trilobite 
which suggests a heart. A Russian specimen of Illcenus with the shell removed shows a 
somewhat flattened, tubular, chambered organ extending from under the posterior end of 
the cephalon to the anterior end of the pygidium. The posterior nine chambers were each 
1.5 mm. long and 1.5. mm. wide, while the two anterior chambers were respectively 2.5 mm. 
Fig. 26. — Copy of Vol- 
borth's f.gure of the heart 
of Illcenus. 
Fig. 27. — Heart 
of Apus. Copied 
from Gerstacker. 
and 3 mm. wide. These were all under the thorax, and at least two more chambers are 
shown under the cephalon, but rather obscurely. The species of the Illcenus is not stated, 
but since no Illcenus has more than ten segments in the thorax, and this tube has at least 
thirteen chambers, it is evident that its constrictions are inherent in it, and are not due to 
the segmentation of the thorax. Beecher has made a passing allusion to this organ as an 
alimentary canal. This was the original opinion of Volborth. Pander, however, suggested 
to him that it might be a heart. The alimentary canal of Cryptolithus does not show any 
constrictions, while the heart of Apus (see fig. 27) and other branchiopods does show them. 
It should be noted, further, that while this heart enlarges toward the' front, it is everywhere 
very small as compared with the width of the axial lobe, and much narrower than sections 
of Ceraurus and Calymcne would lead one to expect the alimentary canal of Illcenus to be. 
Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is 11 mm. wide. 
While this may be merely a cast of the alimentary canal it is sufficiently like a heart to 
deserve consideration as such an organ. 
Ceraurus and Calymcne. 
Nothing suggesting a heart has been seen in the sections of Ceraurus and Calymcne. 
The mesenteron and its sheath crowd so closely against the dorsal test in the anterior part 
