The annelids with which the present memoir is concerned are embraced in collections made by the Albatross during three distinct expeditions in charge of Mr. Alexander Agassiz to contiguous, and in some degree overlapping, areas of the Tropical Pacific Ocean. In the first of these, explorations were made off the west coasts of Mexico, Central, and northern South America, and off the Galapagos Islands from February to May, 1891. So far as concerns the polychaetes collected, the second expedition covered the region from the Marquesas and Paumotus westward to the Ellice, Gilbert, and Marshall Islands and was carried on from September, 1899, to March, 1900. The third expedition, continued from October, 1904, to March, 1905, covered an extensive area principally off the South American coast from Panama southward to Peru and eastward to the Galapagos, Easter Island, and the Paumotus. In addition, thirteen species from earlier expeditions of the Albatross in the Atlantic are listed in this Memoir.
These three expeditions, with the exception in some degree of the second, covered areas essentially untouched by other expeditions, and the annelid forms have proved in large measure new. No fewer than one hundred and eighteen out of a total of one hundred and seventy-five species seem not to have been previously described, these including twenty-three new generic types of which several are of high interest. While the littoral forms are well represented, these coming chiefly from Panama and the Polynesian Islands, chief interest attaches to the abyssal and pelagic species. The collection of pelagic species is exceptionally rich and important. Aside from relatively numerous representatives of strictly pelagic families, such as the Alciopidae, Typhloscolecidae, and Tomopteridae, many forms from other families were secured which are either in the epitokous pelagic phase or are completely pelagic. Mention may be made of the epitokes of the Nereidae, including the peculiar new genus Kainonereis
taken by night light off the Gilbert Islands, and those of the Syllidae, including the new generic type Synelmis
. Considerable additions are made to the pelagic species of the Phyllodocidae, two representing new genera, Mastigethus
. To the six previously known pelagic Polynoidae, four are added, three being types of genera of which attention may be called to Plotolepis
, a form showing peculiar adaptations to life at the surface in its greatly elongate and inflated notocirri and in its vesicular though reduced elytra. Complete lists of the forms taken in the different bathymetrical zones are given (p. 16-18).
Separate lists of the forms secured by the different expeditions are here given. It may be noted that duplication of species is negligible.