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.
| Fig. 26. Copy of Volborth's figure of the heart of Illænus. | 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.
Nothing suggesting a heart has been seen in the sections of Ceraurus and Calymene. The mesenteron and its sheath crowd so closely against the dorsal test in the anterior part of the thorax that there seems to be no room for the heart, but it must have been located beneath the sheath and above the alimentary canal. If the latter were filled with mud, and the animals lay on their backs, as most of them did at death, the canal would drop down into the axial lobe and the soft heart would naturally disappear and leave 110 trace of its presence in the fossils.
The Median "Ocellus" or "Dorsal Organ."
Many trilobites, otherwise smooth, bear on the glabella a median pustule which is usually referred to as a simple eye or median ocellus, but whose function can not be said to have been certainly demonstrated. Ruedemann (1916, p. 127), who has recently made a careful study of this problem, lists about thirty genera, members of ten families, Agnostidæ, Eodiscidæ Trinucleidæ, Harpedidæ, Remopleuridæ, Asaphidæ Illænidæ, Goldiidæ, Cheiruridæ, and Phacopidæ, in which this tubercle is present, and had he wished he might have cited more, for it is of almost universal occurrence in Ordovician trilobites.
I have not especially searched the literature for references to this median tubercle. It is often mentioned by writers in descriptions of species, but apparently few have tried to explain it. Beyrich (1846, p. 30) suggested that it indicated the beginning of the alimentary canal. Barrande mentioned it, but if he gave any explanation, it has escaped me. McCoy (Syn. Pal. Foss. 1856, p. 146) called it an ocular (?) tubercle, and that seems to have been the interpretation which most writers on trilobites have assigned to it, if they suggested any function at all. Beecher (1895 B, p. 309) concurred in this opinion.
Bernard (1894, p. 422) ascribed to this tubercle, as well as to the median tubercle on the nuchal segment, an excretory function, comparing it with the "dorsal organ" in Apus.