As Reed has said, these lines are not mere ornamentation, but rather represent traces of structures of some functional importance. They probably can not be explained as traces of nerves and more likely represent either traces of the gastric cæca or of the circulatory 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. _Illænus._ Volborth (1863, pl. 1, fig. 12 = our fig. 26) has described the only organ in a trilobite which suggests a heart. A Russian specimen of _Illænus_ 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. 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 _Illænus_ is not stated, but since no _Illænus_ 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 _Calymene_ would lead one to expect the alimentary canal of _Illænus_ to be. Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is 11 mm. wide. [Illustration: Fig. 26. Copy of Volborth's figure of the heart of _Illænus_.] [Illustration: Fig. 27. Heart of _Apus_. Copied from Gerstäcker.] 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 Calymene._