4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



6. The degree of development of the neuropodia, especially those 

 of the first three or four, and of the last few chsetigerous segments. 



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 prostomium is so much withdrawn into the nuchal organ that the 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 insufficient 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. 



The internal organs most useful in aiding specific determina- 

 tions are: 



1. The oesophageal glands or coeca, their number and comparative 

 size. 



