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Show 1903.] MARINE FAUNA OF ZANZIBAR 131 on feet 14 to 35. The prostomium is of nearly the same size in both specimens, yet the unpaired tentacle, when laid along the back, extends beyond the anterior border of the fourth segment, or only up to that of the second segment. The colour of the two specimens differs markedly, being much duller in no. 2. Similar results were obtained in the case of Biopatra neapoli-tana, in which species such variations are much more conspicuous in accordance with the great development of the organs concerned. The resulting differences of facies have caused some synonymy, and have made some authors content to give quite insufficient descriptions. The ONUPHIDINJE. The genera Rhamphobrachium (Elders), Onuphis (Aud. et Ed.), HyaliiKjecia (Malm), and Diopatra (Aud. et Ed.) form a very well-marked group characterised by modifications for a permanently tubicolous mode of life. The last three genera are differently defined, however, by the principal authorities. All authors but Ehlers agree in separating the genus Diopatra because of its very characteristically formed and distributed gills. In other cases Ehlers would be theoretically right in objecting that the gills of Annelids are too variable to be made a basis for generic distinctions, and in urging that the gill-less species are derivable from either Onuphis or Diopatra, if the distinction between these genera is maintained. Practically, however, we find no form with gills intermediate in structure between those of Diopatra and any other Eunicid whatever; and this distinction is emphasised by the fact that no other member of this group has its largest gills confined to the anterior part of the body. There is no real necessity, therefore, for the confusion introduced by assigning the two gill-less species, fragosa* and glutinatrix *, and the forms with pectinate gills posteriorly, pourtalesii*, magna t, and dorsalist, to the genus Diopatra instead of to the genera Paronuphis and Omiphis, as would have been done by any other writer. The confusion in the definitions of the genera Hyalincecia and Onuphis has its origin simply in the name of the former. The addition of a coating of m u d or larger pieces of foreign material to the foundation of the tube secreted by the body of the worm cannot be regarded as a sufficient cause for generic distinction, however striking the difference in appearance of the tube. The name Hyalincecia may be retained when not literally applicable, the transparency of their abode being characteristic of the great majority of the species. _ The character of the gills forms the basis of a distinction between nearly all the species of the two genera, but, as elsewhere, all stages are found between the typical comb-like gill of * Ehlers " Annelids of the U.S. Survey ship ' Blake,' " Cambridge, Mass., 1887. + Andrews Pr. U.S. Nat. Mus. xiv. p. 277. t Ehlers, ' Hamburger Magal. Sammelreise/ p. 71 (1897). |
