The exact position of these muscles has not been previously discussed. The interior of the dorsal test shows no such apodemes as are found on the mesosternites, but, as I have shown in the discussion of the alimentary canal of Calymene and Ceraurus, there is an opening on either side of the axial lobe between the dorsal test and the abdominal sheath, and it is entirely probable that an extensor muscle passed through each of these. The abdominal sheath extends only along the posterior region of the glabella and the anterior part of the thorax, and probably served to protect the soft organs from the strain of the heavy muscles. The extensors (see fig. 29) probably lay along the top of the axial lobe on either side of the median line of the thorax to the pygidium, where they appear to have been attached chiefly on the under side of the anterior ring of the axial lobe, although strands probably continued further back. This is above and slightly in front of the fulcral points on the pleura, and meets the mechanical requirements. Ceraurus (Walcott, 1875, and 1881, p. 222, pl. 4, fig. 5) shows a pair of very distinct scars on the under side of the first ring of the pygidium, and in many other trilobites (Illænus, Goldius, etc.) distinct traces of muscular attachment can be seen in this region, even from the exterior. The anterior ends were probably attached by numerous small strands to the top of the glabella, and, principally, to the neck-ring.
On enrolling, the sternites of all segments are pulled forward and the tergites backward. In straightening out, the reverse process takes place. The areas available for muscular attachment are so disposed as to indicate longitudinal flexor and extensor muscles rather than short muscles extending from segment to segment. Indeed, the tenuity of the ventral membrane is such as to preclude the possibility of enrollment by the use of muscles of that sort, while powerful longitudinal flexors could have been anchored to cephalon and pygidium. The strongly marked character of the neck-ring of trilobites is probably to be explained as due to the attachment of the extensor muscles, rather than to its recent incorporation in the cephalon. The same is true of the anterior ring on the pygidium.
Fig. 29. Restoration of a part of the internal organs of Ceraurus pleurexanthemus as seen from above. At the sides are the extensor muscles, and in the middle, the heart overlying the alimentary canal. Drawn by Doctor Elvira Wood, under the supervision of the author.
Possible preservations of extensors and flexors in Ceraurus.—Among Doctor Walcott's sections are four slices which I should not like to use in proving the presence of longitudinal muscles, but which may be admitted as corroborative evidence. Two of these transverse sections (Nos. 114 and 199) show a dorsal and a ventral pair of dark spots in positions which suggest that they represent the location of the dorsal and ventral muscles, while two others (Nos. 131 and 140) show only the upper pair of spots. The chief objection to this interpretation is that it is difficult to imagine how the muscles could be so replaced that they happen to show in the section. Both the sections showing all four spots are evidently from the anterior part of the thorax, as they show traces of the abdominal sheath, which seems to be squeezed against the inside of the axial lobe, with the muscles (?) forced out to the sides. The ventral pair lie just inside the appendifers, but even if they are sections of muscles, all four are probably somewhat out of place.
The hypostoma fits tightly against the epistoma, or the doublure when the epistoma is absent, but in no trilobite has it ever been seen ankylosed to the dorsal test, and its rather frail connection therewith is evidenced by the relative rarity of specimens found with it in position. That the hypostoma was movable seems very probable, and that it was held in place by muscles, certain. The maculæ were always believed to be muscle scars until Lindstroem (1901, p. 8) announced the discovery by Liljevall of small granules on those of Goldius polyactin (Angelin). These were interpreted as lenses of eyes by Lindstroem, who tried to show that the maculæ of all trilobites were functional or degenerate eyes. Most palæontologists have not accepted this explanation, and since the so called eyes cover only a part of the surface of the maculæ, it is still possible to consider the latter as chiefly muscle-scars.
In Lindstroem's summary (1901, pp. 71, 72) it is admitted that the globular lenses are found only in Bronteus (Goldius) (three Swedish species only) and Cheirurus spinulosus Nieszkowski, while the prismatic structure supposed to represent degenerate eyes was found in eleven genera (Asaphidæ, Illænidæ, Lichadidæ). All of these are forms with well developed eyes, and Lindstroem himself points out that the appearance of actual lenses in the hypostoma was a late development, long after the necessity for them would appear to have passed.