Proceedings of the United States National Museum, 1911, Vol. 39 (Classic Reprint) - Softcover

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Excerpt from Proceedings of the United States National Museum, 1911, Vol. 39

The setae of several of the species of Arenicola present such close resemblance to each other that these structures are of comparatively little service in specific work and are not employed in diagnosis in the present paper; more reliable and more practicable characters are available.

It may be readily understood that in a considerable number of the specimens which have been collected and preserved some, at least, Of the above-mentioned external characters are not available for reference. For instance, some features may have been damaged or destroyed at the time of capture or can no longer be seen, owing to the unsuitable mode of preservation adopted, or to the defective con dition of the specimen brought about by its having been long kept in a medium which has partially lost its conserving powers. In speci mens which have died contracted, owing to having been at once plunged into strong alcohol, the apertures of the nephridia and of the statocysts (if the latter pores are present) are invisible, the prostomium is SO much withdrawn into the nuchal organ that the relative size Of its lobes can not be ascertained, and the gills may be so fully contracted that the nature of the branching of their axes is indeterminable. In such cases the number of segments and the num ber of pairs of gills present are the only characters which can be seen, and they are insu'icient to provide the basis for a reliable diagnosis. Even in some cases in which the external characters are moderately preserved the specific differences presented by these features are so small that great care is requisite if a safe diagnosis is to be reached. In all these cases it is necessary to have recourse to an examination of the internal Organs before a definite determination Of the species to which the specimen belongs can be made. If specimens from a new region are concerned it is particularly advisable not to depend upon the external characters alone, but to make an incision along the mid-dorsal line of the worm, extending from about the eleventh segment forward nearly to the prostomium, so as to permit an examination of all the important internal organs to be made. The making of such an incision and examination does not destroy any external feature or impair the value of the specimen for future study. The neglect of the examination of the internal organs has been responsible for many errors of diagnosis, some of which could scarcely have been made had these organs been even casually inspected.

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