Contributions towards the knowledge of the Annelida Polychaeta

Some time ago a finely preserved collection of Annelids was procured by our Museum from the Zoological Station at Naples; among them I observed some specimens of Arenicola, which in their characters do not agree with the species, hitherto known from the European coast. Going over the litterature about this genus, I saw that not only there reigns a good deal of confusion in the description of the different species, but that we want even an accurate knowledge of the feature of the bristles and branchiae of our European species. Almost all the authors have given an erroneous account of the appearance of the branchiae of the common A. marina; nevertheless Williams 2) in the year 1851 already stated: »they are commonly described as forming an arborescent tuft; the division of the vessels is however regulated by a fixed principle. When fully injected with blood, the vessels of each branchia form a single plane etc.” Even a superficial examination is able to convince us of the correctness of Williams’ assertion, though his drawing is not very accurate. Indeed each branchia of the common lug-worm consists of about 12 secundary branchiae, connected by a membrane at their base and situated next to each other in the same plane, like the divisions of a palmated leaf; these secondary branchiae are not irregularly branched, but possess a middle axis, furnished on each side with 3 or 4 tufts of filaments (fig. 2). If the number of those lateral tufts much increases, as is to be observed in some species of Arenicola, and the middle axis bears on each side, in stead of 4, 12 or more racemous tufts, the secondary branchiae resemble a feather (fig. 6). Therefore a South-American species, provided with such plumous branchiae, was named by Lütken Pteroscolex, which he wishes to be considered as a sub-genus of Arenicola 1). However if we would retain this name, as proposed by Levinsen 2), then also our A. marina ought to be ranged in that sub-genus, because there is only a gradual difference between the branchiae of A. antillensis and marina. Claparède seems not to have rightly understood Lütken’s description; otherwise he would not have written 3): »toutes les branchies sont disposées dans un même plan, comme les nervures d’une feuille, et ne forment pas de buisson touifu. Ce caractère a été relevé chez une Arénicole des Antilles par M. Lütken etc.”