Fig. 17. Transverse section of Ceraurus pleurexanthemus, showing the relation of the coxopodite to the appendifer. Traced from a photographic enlargement of the slice. Specimen 128. × 4/5.
Fig. 18. Slice of Ceraurus pleurexanthemus, showing a nearly continuous section of an endopodite and an exopodite above it. The latter is so cut as to show only the edge of the shaft and the bases of a few setæ. Traced from a photographic enlargement. Specimen in. × 4.
Specimen 92 shows traces of the slender endopodites belonging to the cephalon, but no details. Specimen 22 shows on one side exopodites (epipodites of Walcott) belonging to the third and fourth cephalic appendages. That belonging to the third shows some long setæ and a trace of the shaft, while the one on the fourth appendage (third coxopodite) has a portion of a broad shaft and a number of long setæ. It should again be remembered that the slice does not cut through the plane of the exopodite, but across it at a low angle, so that a part but not all of the shaft is shown. On the other side of this slice there is a fairly good section of one of the thoracic exopodites. It is, however, turned around in the opposite direction from the others, as would be expected in an enrolled specimen.
Specimens 4 and 5 (pl. 1, figs. 4, 5, 1881) are slices cut diagonally through the head of Ceraurus, in front of the posterior tip of the hypostoma. They show fragments of endopodites and exopodites which may be interpreted as practically identical in form with those of the thorax. Due to the diagonal plane in which the section is cut, slice 5 shows the coxopodites of two pairs of appendages, one lying nearer the median cavity than the other. It is extremely difficult to visualize the interpretation of such sections.
A transverse section through a thoracic segment (No. 128, our fig. 17) shows the relation of coxopodite to appendifer to be the same as in Calymene, the upper side of the coxopodite having a notch a little outward from the middle. After seeing that specimen, it is possible to understand slice No. 168, which shows longitudinal sections through a number of coxopodites of the thorax, with fragments of both exopodites and endopodites articulated at the distal ends. These and longitudinal vertical sections like No. 18 (pl. 2, fig. 8, 1881) show that the endobases taper inward, and the general uniformity in width in sections taken at various angles indicates that the coxopodites were not greatly flattened.
A unique slice (No. 111, pl. 2, fig. 2, 1881; pl. 27, fig. 1, 1918; our fig. 18) shows a nearly complete thoracic endopodite, and above it a part of the proximal end of the exopodite of the same segment. When one considers that out of over two thousand sections only this one shows the six successive segments of an endopodite, one realizes how futile it is to expect that dozens of the equally slender "spirals" should be cut so as to show practically all their turns.