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The Atoll of Funafuti, Ellice group : its zoology, botany, ethnology and general structure based on collections made by Charles Hedley of the Australian Museum, Sydney, N.S.W.
Trunk
Trunk.
| (1.) | Branchio-genital Region.—This region is characterised by the great length of the branchial area, and the absence of genital pleura, the latter however being represented in the genital region proper by genital cushions (cf, infra).
It may be subdivided into a branchio-genital region, co-extensive with the gill area, and into an exclusively genital region behind the point of termination of the gills. In the largest page 208specimen in the collection the gill area has a total length of 3·3 cm. It is thus relatively much longer than in P. minuta and P. sarniensis, and is also of a different shape. In these forms the gill area, when viewed from above, presents, as Spengel describes it, the appearance of an elongated narrow triangle with its apex pointing posteriorly. In P. hedleyi, however, the gill area, viewed from above, appears long and band-like, and is not pointed at its posterior end. The gill pores open on each side into a narrow longitudinal groove, which runs parallel with the deep median groove, marking the position of the dorsal nerve cord. The narrow bands of epidermis lying, one on each side, between the median groove and the branchial grooves, and hardly ·5 mm, in width, are divided up by transverse lines into a definite and fairly regular series of oblong or squarish areas, characteristic for the species. The openings of the gill cavities into the branchial grooves can only be made out in sections. Laterally to the branchial grooves, the epidermis is irregularly, but very markedly annulated, the ambulations being interrupted below by the median ventral groove marking the course of the ventral nerve cord. This ventral groove is much shallower than the dorsal. In the branchial region the trunk is almost quite cylindrical, measuring in greatest breadth 4·75 mm. It is not possible, in this region, to speak of genital cushions, such as Spengel* describes and figures for P. minuta (taf. 2, fig. 10), and P. sarniensis (taf. 6, fig. 7). Indeed, sections through the branchial region of P. hedleyi more closely resemble in general form the section, figured by Spengel, through the branchial region of Glandiceps talaboti (fig. 13, taf. 19), than similar sections of P. minuta and P. sarniensis. Behind the branchial region proper there is a short exclusively genital segment of the trunk, characterised by its greater transverse breadth and the presence of distinct genital cushions, similar to the much more extensive cushions described by Spengel for P. miuuta and P. sarniensis. This region, in a fragment of a large and apparently sexually mature individual, has a length of 15 mm., with a transverse breadth of 6 mm. It not only exceeds the branchial region in breadth but presents in sections a very different outline—ventro-laterally it is rounded, while dorsally it is markedly concave on each side of the median ridge formed by the dorsal nerve cord. The genital cushions are the direct continuations of that portion of the epidermis forming the lateral boundary of the branchial grooves. They form low and thick lateral ridges, extending from the posterior end of the branchial region up to within a short distance of the most anterior liver sacs. page 209Behind the branchial region the dorsal nerve cord no longer lies in the bottom of a groove but forms a median ridge, traceable to the posterior extremity of the tail. Just in front of the anus, however, it becomes much less marked, and may even fade away from view. The ventral cord similarly comes to the surface at the end of the branchial region and passes as a median whitish line up to the extreme posterior end of the tail. The gonads extend throughout the whole extent of the branchio-genital region up to within a short distance of the anterior liver sacs. |
| (2.) | Hepatic Region.—May reach a length of 27 mm., and a breadth of 5·5 mm. The number of liver sacs in the larger specimens varies from fifty to sixty on each side. The sacs are arranged in two distinct and uniform longitudinal rows. An-teriorly, they commence abruptly, just behind the point of fading away of the genital cushions, while posteriorly they gradually become smaller, and pass over without definite limit into the transverse annulations of the dorsal region of the tail. The most anterior and posterior sacs are colourless in the preserved speci-mens, while the remaining sacs, as well as the ventral portion of the body wall in the hepatic region, are of a light slaty brown colour. The three or four pairs of anterior liver sacs are some-what smaller and thicker antero-posteriorly than the succeeding ones. The latter are simple, markedly compressed antero-posteriorly, and situated close together so that the anterior and posterior faces of the adjacent sacs touch. Each sac has a broad base of attachment corresponding in transverse extent with its free part. The outer ends of the sacs thus do not project freely so as to overhang the lateral body wall, though owing to the lesser transverse breadth of the ventral half of the hepatic region it is not visible when the region is viewed from above. The line of attachment of the outer ends of the sacs is marked on each side by a low longitudinal ridge, continuous in front with the genital cushion. |
| (3.) | Tail Region.—In the largest complete specimen this region is about twice as long as the hepatic region, and measures 5·3 cm. in length, with a breadth of 5 mm. In this species, as in P. australiensis, the tail region is characterised by the presence of two dorso-lateral epidermal lines, corresponding to the two underlying ciliated grooves of the intestine. The lines extend from the hepatic region over the anterior two-thirds of the tail, running parallel with the dorsal nerve cord, and about ·5 to ·75 mm. distant from it. They enclose between them a band-like area of the dorsal body wall, with the dorsal cord running along its middle, and appearing like a direct backward prolongation of the hepatic region. On each side of the nerve cord the area often page 210appears slightly depressed, and thus stands out very distinctly. It is crossed by a numerous series of close set epidermal ridges, which may even extend continuously across the dorsal cord. Laterally, the ridges may either stop short at the epidermal lines, or may pass across them to become continuous with the annulations of the ventro-lateral body wall. These latter are invariably interrupted at the ventral nerve cord. |
In P. flava, Willey has also recorded the existence of two dorso-lateral bands in the tail region, but as he describes them, these bands, which are visible externally do not cause any interruption in the annulations or islets of the integument, and in fact are probably only the ciliated bands of the intestine showing through the epidermis by transparency.
Behind the termination of the epidermal lines the tail gradually narrows to its posterior end. In this posterior region the epidermal annulations may, in some specimens, be partly broken up into small islands. The annulations of the tail region are, on the whole, more regular than those of the branchio-genital region.
In Part II. I propose to describe and figure the salient features in the internal anatomy of this species.
page 211page 212* Loc. cit.