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Claparède, in his ‘Beobachtungen über Anatomie und Entwicklungsgeschichte wirbelloser Thiere an der Küste von Normandie Angestellt,’ published in 1863, describes and figures (pp. 77—80, Taf. vi, figs. 1—11) several stages in the development of an annelid larva, which he was unable at the time to assign to any known genus. This larva was very common in the plankton at St. Vaast, and the same, or a very similar one, had previously been found (in 1855) by Claparède on the coast of Norway. He surmised that the larva must belong to some common worm at that time still undescribed.
No further advance seems to have been made in the knowledge of this form until the appearance in 1874 of a report by Claparède on the annelids collected by the “Lightning “Expedition. This report is contained in Ehler’s paper, “Beitrage zur Kenntniss der Verticalverbreitung der Borstenwürmer in Meere” (Ehlers, 1874). Amongst the material collected by the “Lightning,” Claparède found a number of fragments of a worm, which he considered must be the adult form of the larva he had previously described. He states that the species is represented in the “Lightning” material “par un fragment dans les preparations Nr. 15 et Nr. 24, et par tous les fragments inclus dans la préparation Nr. 22.” The localities from which these specimens were obtained are not mentioned. In the same paper Ehlers refers to two fragments of the worm described by Claparède, which he found amongst the material dredged by the “Porcupine.” According to the table given (loc. cit., p. 25), these were dredged on July 21st, 1869, at 48°51'N., 11°7'W. (11°9'W.) in 725 fathoms, on a bottom of muddy sand.
From the fragments at his disposal Claparède was able to give a fair account of the general external features of the worm, and to convince himself that it was the adult form of the larva which he had previously described, or at any rate closely allied to the adult of that larva. He gives to the worm the name
Pœcilochætus fulgoris, both the generic and the specific name being new. He was still unable to include it in any known family, and thought it not improbable that a special family would have to be made to receive it. Figures are given (loc. cit., Taf.i, fig. 1, A, B, C, and D) of the head end from the dorsal and ventral surfaces, of several chætæ, of a parapodium, and of the external opening of one of the epithelial glands, the latter being described as “petits tubercules granuleux.”
Levinsen (1883, p. 106) gives some further details of the structure of late larval stages of
Pœcilochætus from observations upon specimens which had been taken by the “Hauch” Expedition in the Skager Rack. He also discusses the relations of
Pœcilochætus with
Disoma multisetosum, Oersted, and points out that the two genera are closely allied. He places both genera in the family Spionidæ.
