ABSTRACT Although the literature which treats of the anatomy of Arenicolais a large and growing one, there are several structures, such as the nervous system, the nephridia, the “hearts” and “heart-body,” which have received little or no attention from anatomists such as de Quatrefages, Grube, Claparède, Pruvot, Racovitza, who have contributed largely to a knowledge of the comparative morphology of Polychætes. Embryologists have neglected the Arenicolidæ even more. Wilson’s early paper (1882)1 still remains the only account of the development of an Arenicola (A. cristata) from the egg up to the early post-larval stage, and naturally enough only deals with the external features. In 1893 the post-larval stage of the common lugworm was identified by Dr. Benham, and though his decision was soon afterwards questioned by Mesnil (1897), Benham’s statements have since received full confirmation. It is not our purpose to give in this place a full résumé of past or recent work on the Arenicolidæ, but merely to point out that both in morphology and embryology there are lacunæ where our present knowledge is scanty or entirely lacking. This statement applies, moreover, to the groups with which it has been suggested, though on somewhat slender grounds, that the Arenicolidæ are allied, namely, the Maldanidæ on the one hand and the Scalibregmidæ on the other. For all we know to the contrary, these families may represent three more or less parallel but independent modifications. Their relationship can only be adequately discussed when the modifications of structure and development of each family are known as a whole. With reference to the Scalibregmidæ, for example, many of the structural features are Only known through the work of Rathke (1843) or of Danielssen (1859). Even more recent writers, such as Wirén (1887), have been content, when dealing with the vascular system or the subdivision of the body-cavity, to record the results of the examination of one or two genera without even determining the limits of specific differences, which at any rate in the Arenicolidæ extend from obvious external marks to the mode of origin of the gonads, or the grade of development of certain ganglion-cells in the nerve-cord.
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