OCR Text |
Show 1904.] MARINE FAUNA OF ZAXZIBAR. 305 pair but very little shorter, while the outer are but half this length. The median pair is inserted at the same level as the middle tentacle,: but at some distance laterally from it and close to the outer pair, the insertion of which is more anterior. Immediately beneath these last are the small but distinct eyes. The first ring of the peristomium is of moderate length, the second very short, and the nuchal cirri are small and smooth. _ The jaw-apparatus is very powerful (text-fig. 53, p. 306), consisting of thick black plates sparingly bordered with wdiite matter. The end-plates of the lower jaws are small and but partially calcareous, being marked by dark chitinous rings, as shown in the figure (text-fig. 53, B, p. 306). The upper plates are of the usual form, the great dentals being broad and bearing sharp closely-set teeth. The formula is 6 - 7:7 + 2 - 9 ; the small number of teeth on the second left crescentic plate is clue to its being toothless over the greater part of its cutting-edge, a condition found less prominently in many species, e. g. E. afra. Outside all are two paragnaths on either side, the anterior and inner pair bearing one triangular tooth each, the posterior being mere elongated chitinous bands. The feet project but little, though the seta? stand out prominently. The dorsal cirri are fairly long, projecting well beyond the seta?, except in the first three feet and those near the hind end of the body. The ventral cirri are highly modified, forming secretory pads, in somewhat the same way as in Diopatra. The first is thick and finger-shaped, but they rapidly become still thicker until the twelfth is a conical knob as large as the setigerous portion of the foot. The breadth continuing to increase dorso-ventrally, at the 24th foot it is nearly three times as wide as this. Since these pads are broader than the feet, they are pressed together fairly closely, forming an almost continuous band down the sides of this region of the body, as shown in PL X X I. fig. 4. Ventrally they end in a free flap, dorsally in a little point, the remnant of the true cirrus. The figures of the feet (text-fig. 54, p. 307) explain the changes of form and arrangement of these organs. The lower border and inner angle of the pads are extremely vascular, and, at about the 120th foot, these surfaces contain a close network of blood-vessels. Posteriorly, at the point wdtei-e the gills become conspicuous, the pads gradually decrease in length, and when the former attain their full size about segment 120, the latter become rapidly smaller, and for the rest of the body beyond segment 130 are merely little conical points. One lip of the seta-sac is pointed and projects a little beyond the other, which is rounded. The gills begin at foot 35 as a small papilla and do not become at all conspicuous until about the 70th foot, where they consist of two filaments somewhat larger than the dorsal cirrus. From this point they increase uniformly, until, at the 120th foot, five long filaments are found arising from a short rachis, a condition which seems to last to near the anus (PL X X I . figs. 1, 2, & 3). PROC. ZOOL. Soc-1904, VOL. I. No. XX. 20 |